Nat  Grady

Nat Grady

1658879220

A Command Line tool To Generate PDF From URL, HTML Or Markdown File

electron-pdf   

A command line tool to generate PDF from URL, HTML or Markdown files with electron.

Versioning

Starting with version 4.0.x the master branch will always have the latest electron version.

Semantic Versioning is used, and corresponds to electron versions in the following way:

  • electron-pdf 15.0.x (master) => electron=15.1.1, node=16.5.0, chrome=94.0.4606.61
  • electron-pdf 10.0.x => electron=10.1.3, node=12.16.3, chrome=85.0.4183.121
  • electron-pdf 7.0.x => electron 7.x (Chromium 78, Node 12.8.1)
  • electron-pdf 4.0.x => electron 4.x (Chromium 69, Node 10.11.0)
  • electron-pdf 1.3.x => electron 1.6.x (Chromium 56, Node 7.4)
  • electron-pdf 1.2.x => electron 1.4.x (Chromium 53, Node 6.5)

Note: The Chromium versions employed by electron have impacts based on the functionality you may be exporting.
Choose the version you need based on Chromium.

Install

npm install electron-pdf

Note: If you're installing electron-pdf as root using the system level npm (vs a user-level install like with NVM) then you may need to run the following command instead:

sudo npm install electron-pdf -g --unsafe-perm

Please see the npm docs for more information.

For gnu/linux installations without a graphical environment:

$ sudo apt-get install xvfb # or equivalent
$ export DISPLAY=':99.0'
$ Xvfb :99 -screen 0 1024x768x24 > /dev/null 2>&1 &
$ electron-pdf ...

There is also an example docker machine here.

Node Usage

Electron PDF can be used inside of an application, or more commonly as the engine for a pdf rendering service. For instance, to handle http requests using Express. The following snippets show you how you can get started.

The application must run in an Electron process

In package.json

"start": "DEBUG=electronpdf:* electron index.js",
"watch": "DEBUG=electronpdf:* nodemon --exec electron index.js"

You can use the same instance

var ElectronPDF = require('electron-pdf')
var express = require('express')
var bodyParser = require('body-parser')
var app = express()
app.use(bodyParser.json())

var exporter = new ElectronPDF()
exporter.on('charged', () => {
    //Only start the express server once the exporter is ready
    app.listen(port, hostname, function() {
        console.log(`Export Server running at http://${hostname}:${port}`);
    })
})
exporter.start()

And handle multiple export job instances

app.post('/pdfexport', function(req,res){
    // derive job arguments from request here
    // 
    const jobOptions = {
      /**
        r.results[] will contain the following based on inMemory
          false: the fully qualified path to a PDF file on disk
          true: The Buffer Object as returned by Electron
        
        Note: the default is false, this can not be set using the CLI
       */
      inMemory: false 
    }
    const options = {
          pageSize : "A4"
    }
    exporter.createJob(source, target, options, jobOptions).then( job => {
    job.on('job-complete', (r) => {
            console.log('pdf files:', r.results)
            // Process the PDF file(s) here
        })
        job.render()
    })    
})

Using an in memory Buffer

If you set the inMemory setting to true, you must also set closeWindow=false or you will get a segmentation fault anytime the window is closed before the buffer is sent on the response. You then need to invoke job.destroy to close the window.

Sample Code:

const jobOptions = { inMemory: true, closeWindow: false }
exporter.createJob(source, target, options, jobOptions).then( job => {
    job.on('job-complete', (r) => {
      //Send the Buffer here
      process.nextTick(() => {job.destroy()})
    })
})

Events

The API is designed to emit noteworthy events rather than use callbacks. Full documentation of all events is a work in progress.

Environment Variables

  • ELECTRONPDF_RENDERER_MAX_MEMORY : The --max-old-space-size option for each Electron renderer process (browser window); default: 75% of total system memory up to 8GB
  • ELECTRONPDF_WINDOW_CLEANUP_INTERVAL : Interval for which to check for hung windows, in milliseconds; default: 30 seconds
  • ELECTRONPDF_WINDOW_LIFE_THRESHOLD : How long a window can remain open before it is terminated, in milliseconds; default: 5 minutes
  • ELECTRONPDF_PNG_CAPTURE_DELAY : Amount of millis to wait before invoking WebContents.capturePage for PNG exports; default: 100ms

Command Line Usage

For Ad-hoc conversions, Electron PDF comes with support for a CLI.

To generate a PDF from a HTML file

$ electron-pdf index.html ~/Desktop/index.pdf

To generate a PDF from a Markdown file

$ electron-pdf index.md ~/Desktop/index.pdf

To generate a PDF from a Markdown file with custom CSS (defaults to Github markdown style)

$ electron-pdf index.html ~/Desktop/index.pdf -c my-awesome-css.css

To generate a PDF from a URL

$ electron-pdf https://fraserxu.me ~/Desktop/fraserxu.pdf

Rendering Options

Electron PDF gives you complete control of how the BrowserWindow should be configured, and when the window contents should be captured.

To specify browser options

The BrowserWindow supports many options which you may define by passing a JSON Object to the --browserConfig option.

Some common use cases may include:

  • height and width - electron-pdf calculates the browser height and width based off of the dimensions of PDF page size multiplied by the HTML standard of 96 pixels/inch. So only set these values if you need to override this behavior
  • show - to display the browser window during generation
$ electron-pdf https://fraserxu.me ~/Desktop/fraserxu.pdf --browserConfig '{"show":true}'

To generate a PDF after the an async task in the HTML

electron-pdf ./index.html ~/Desktop/README.pdf -e

In your application, at the point which the view is ready for rendering

document.body.dispatchEvent(new Event('view-ready'))

Warning: It is possible that your application will be ready and emit the event before the main electron process has had a chance execute the javascript in the renderer process which listens for this event.

If you are finding that the event is not effective and your page waits until the full timeout has occurred, then you should use setInterval to emit the event until it is acknowledged like so:

  var eventEmitInterval = setInterval(function () {
    document.body.dispatchEvent(new Event('view-ready'))
  }, 25)

  document.body.addEventListener('view-ready-acknowledged', function(){
    clearInterval(eventEmitInterval)
  })

When the main process first receives your ready event it will emit a single acknowlegement on document.body with whatever event name you are using suffixed with -acknowledged. So the default would be view-ready-acknowledged

Observing your own event

If the page you are rending is under your control, and you wish to modify the behavior of the rendering process you can use a CustomEvent and an observer that will be triggered after the view is ready but before it is captured.

your-page.html

document.body.dispatchEvent(new CustomEvent('view-ready', { detail: {layout: landscape} }))

your-exporter.js

You are required to provide a function that accepts the detail object from the CustomEvent and returns a Promise. You may optionally fulfill the promise with and object that will amend/override any of the contextual attributes assigned to resource (url) currently being exported.

As an example, suppose you wanted to change the orientation of the PDF, and capture the output as PNG instead of a PDF.

job.observeReadyEvent( (detail) => {
    return new Promise( (resolve,reject) => {
      const context = {}
      if( detail && detail.landscape ){
        job.changeArgValue('landscape', true)
        context.type = 'png'
      }
      resolve(context)
    })
})

Note: Future versions of the library will only allow you to provide context overrides, and not allow you to change job level attributes.

All Available Options

Electron PDF exposes the printToPDF settings (i.e. pageSize, orientation, margins, etc.) available from the Electron API. See the following options for usage.


  A command line tool to generate PDF from URL, HTML or Markdown files

  Options
    --help                     Show this help
    --version                  Current version of package
    
    -i | --input               String - The path to the HTML file or url
    -o | --output              String - The path of the output PDF
    
    -b | --printBackground     Boolean - Whether to print CSS backgrounds.
    
    --acceptLanguage           String - A valid value for the 'Accept-Language' http request header
    
    --browserConfig            String - A valid JSON String that will be parsed into the options passed to electron.BrowserWindow
    
    -c | --css                 String - The path to custom CSS (can be specified more than once)
    
    -d | --disableCache        Boolean - Disable HTTP caching
                                 false - default
    
    -e | --waitForJSEvent      String - The name of the event to wait before PDF creation
                                 'view-ready' - default
    
    -l | --landscape           Boolean - true for landscape, false for portrait (don't pass a string on the CLI, just the `-l` flag)
                                 false - default
    
    -m | --marginsType         Integer - Specify the type of margins to use
                                 0 - default margins
                                 1 - no margins (electron-pdf default setting)
                                 2 - minimum margins
    
    --noprint                  Boolean - Do not run printToPDF, useful if the page downloads a file that needs captured instead of a PDF.  
                                         The Electron `win.webContents.session.on('will-download')` event will be implemented 
                                         and the file saved to the location provided in `--output`.
                                         Currently only supports a single import url.
                                         The page is responsible for initiating the download itself.
    
    -p | --pageSize            String - Can be A3, A4, A5, Legal, Letter, Tabloid or an Object containing height and width in microns
                                 "A4" - default
    
    -r | --requestHeaders      String - A valid JSON String that will be parsed into an Object where each key/value pair is: <headerName>: <headerValue>
                                 Example: '{"Authorization": "Bearer token", "X-Custom-Header": "Hello World"}'  
    
    -s | --printSelectionOnly  Boolean - Whether to print selection only
                                 false - default
                                 
    -t | --trustRemoteContent  Boolean - Whether to trust remote content loaded in the Electron webview.  False by default.
    --type                     String - The type of export, will dictate the output file type.  'png': PNG image, anything else: PDF File
    
    -w | --outputWait          Integer โ€“ Time to wait (in MS) between page load and PDF creation.  
                                         If used in conjunction with -e this will override the default timeout of 10 seconds    
    --ignoreCertificateErrors  Boolean - If true, all certificate errors thrown by Electron will be ignored.  This can be used to accept self-signed and untrusted certificates.  You should be aware of the security implications of setting this flag.
                             false - default

Find more information on Electron Security here.

Debugging

Sentry

If you have a Sentry account and setup a new app to get a new DSN, you can set a SENTRY_DSN environment variable which will activate sentry logs. See lib/sentry.js for implementation details.

This will allow you to easily see/monitor errors that are occuring inside of the Chromium renderer (browser window). It also automatically integrates with Electron's Crash Reporter

CLI Usage

You can see some additional logging (if you're getting errors or unexpected output) by setting DEBUG=electron* For example: DEBUG=electron* electron-pdf <input> <output> -l

  Usage
    $ electron-pdf <input> <output>
    $ electron-pdf <input> <output> -l

  Examples
    $ electron-pdf http://fraserxu.me ~/Desktop/fraserxu.pdf
    $ electron-pdf ./index.html ~/Desktop/index.pdf
    $ electron-pdf ./README.md ~/Desktop/README.pdf -l
    $ electron-pdf ./README.md ~/Desktop/README.pdf -l -c my-awesome-css.css

Inspired by electron-mocha

Other Formats

Want to use the same options, but export to PNG or snapshot the rendered HTML? Just set the output filename to end in .png or .html instead!

  Examples
    $ electron-pdf http://fraserxu.me ~/Desktop/fraserxu.pdf
    $ electron-pdf http://fraserxu.me ~/Desktop/fraserxu.html
    $ electron-pdf http://fraserxu.me ~/Desktop/fraserxu.png

Extensions

If you need powerpoint support, pdf-powerpoint picks up where Electron PDF leaves off by converting each page in the PDF to a PNG and placing them on individual slides.

Author: Fraserxu
Source Code: https://github.com/fraserxu/electron-pdf 
License: MIT license

#electron #pdf #url #html #markdown 

What is GEEK

Buddha Community

A Command Line tool To Generate PDF From URL, HTML Or Markdown File

Swift Tips: A Collection Useful Tips for The Swift Language

SwiftTips

The following is a collection of tips I find to be useful when working with the Swift language. More content is available on my Twitter account!

Property Wrappers as Debugging Tools

Property Wrappers allow developers to wrap properties with specific behaviors, that will be seamlessly triggered whenever the properties are accessed.

While their primary use case is to implement business logic within our apps, it's also possible to use Property Wrappers as debugging tools!

For example, we could build a wrapper called @History, that would be added to a property while debugging and would keep track of all the values set to this property.

import Foundation

@propertyWrapper
struct History<Value> {
    private var value: Value
    private(set) var history: [Value] = []

    init(wrappedValue: Value) {
        self.value = wrappedValue
    }
    
    var wrappedValue: Value {
        get { value }

        set {
            history.append(value)
            value = newValue
        }
    }
    
    var projectedValue: Self {
        return self
    }
}

// We can then decorate our business code
// with the `@History` wrapper
struct User {
    @History var name: String = ""
}

var user = User()

// All the existing call sites will still
// compile, without the need for any change
user.name = "John"
user.name = "Jane"

// But now we can also access an history of
// all the previous values!
user.$name.history // ["", "John"]

Localization through String interpolation

Swift 5 gave us the possibility to define our own custom String interpolation methods.

This feature can be used to power many use cases, but there is one that is guaranteed to make sense in most projects: localizing user-facing strings.

import Foundation

extension String.StringInterpolation {
    mutating func appendInterpolation(localized key: String, _ args: CVarArg...) {
        let localized = String(format: NSLocalizedString(key, comment: ""), arguments: args)
        appendLiteral(localized)
    }
}


/*
 Let's assume that this is the content of our Localizable.strings:
 
 "welcome.screen.greetings" = "Hello %@!";
 */

let userName = "John"
print("\(localized: "welcome.screen.greetings", userName)") // Hello John!

Implementing pseudo-inheritance between structs

If youโ€™ve always wanted to use some kind of inheritance mechanism for your structs, Swift 5.1 is going to make you very happy!

Using the new KeyPath-based dynamic member lookup, you can implement some pseudo-inheritance, where a type inherits the API of another one ๐ŸŽ‰

(However, be careful, Iโ€™m definitely not advocating inheritance as a go-to solution ๐Ÿ™ƒ)

import Foundation

protocol Inherits {
    associatedtype SuperType
    
    var `super`: SuperType { get }
}

extension Inherits {
    subscript<T>(dynamicMember keyPath: KeyPath<SuperType, T>) -> T {
        return self.`super`[keyPath: keyPath]
    }
}

struct Person {
    let name: String
}

@dynamicMemberLookup
struct User: Inherits {
    let `super`: Person
    
    let login: String
    let password: String
}

let user = User(super: Person(name: "John Appleseed"), login: "Johnny", password: "1234")

user.name // "John Appleseed"
user.login // "Johnny"

Composing NSAttributedString through a Function Builder

Swift 5.1 introduced Function Builders: a great tool for building custom DSL syntaxes, like SwiftUI. However, one doesn't need to be building a full-fledged DSL in order to leverage them.

For example, it's possible to write a simple Function Builder, whose job will be to compose together individual instances of NSAttributedString through a nicer syntax than the standard API.

import UIKit

@_functionBuilder
class NSAttributedStringBuilder {
    static func buildBlock(_ components: NSAttributedString...) -> NSAttributedString {
        let result = NSMutableAttributedString(string: "")
        
        return components.reduce(into: result) { (result, current) in result.append(current) }
    }
}

extension NSAttributedString {
    class func composing(@NSAttributedStringBuilder _ parts: () -> NSAttributedString) -> NSAttributedString {
        return parts()
    }
}

let result = NSAttributedString.composing {
    NSAttributedString(string: "Hello",
                       attributes: [.font: UIFont.systemFont(ofSize: 24),
                                    .foregroundColor: UIColor.red])
    NSAttributedString(string: " world!",
                       attributes: [.font: UIFont.systemFont(ofSize: 20),
                                    .foregroundColor: UIColor.orange])
}

Using switch and if as expressions

Contrary to other languages, like Kotlin, Swift does not allow switch and if to be used as expressions. Meaning that the following code is not valid Swift:

let constant = if condition {
                  someValue
               } else {
                  someOtherValue
               }

A common solution to this problem is to wrap the if or switch statement within a closure, that will then be immediately called. While this approach does manage to achieve the desired goal, it makes for a rather poor syntax.

To avoid the ugly trailing () and improve on the readability, you can define a resultOf function, that will serve the exact same purpose, in a more elegant way.

import Foundation

func resultOf<T>(_ code: () -> T) -> T {
    return code()
}

let randomInt = Int.random(in: 0...3)

let spelledOut: String = resultOf {
    switch randomInt {
    case 0:
        return "Zero"
    case 1:
        return "One"
    case 2:
        return "Two"
    case 3:
        return "Three"
    default:
        return "Out of range"
    }
}

print(spelledOut)

Avoiding double negatives within guard statements

A guard statement is a very convenient way for the developer to assert that a condition is met, in order for the execution of the program to keep going.

However, since the body of a guard statement is meant to be executed when the condition evaluates to false, the use of the negation (!) operator within the condition of a guard statement can make the code hard to read, as it becomes a double negative.

A nice trick to avoid such double negatives is to encapsulate the use of the ! operator within a new property or function, whose name does not include a negative.

import Foundation

extension Collection {
    var hasElements: Bool {
        return !isEmpty
    }
}

let array = Bool.random() ? [1, 2, 3] : []

guard array.hasElements else { fatalError("array was empty") }

print(array)

Defining a custom init without loosing the compiler-generated one

It's common knowledge for Swift developers that, when you define a struct, the compiler is going to automatically generate a memberwise init for you. That is, unless you also define an init of your own. Because then, the compiler won't generate any memberwise init.

Yet, there are many instances where we might enjoy the opportunity to get both. As it turns out, this goal is quite easy to achieve: you just need to define your own init in an extension rather than inside the type definition itself.

import Foundation

struct Point {
    let x: Int
    let y: Int
}

extension Point {
    init() {
        x = 0
        y = 0
    }
}

let usingDefaultInit = Point(x: 4, y: 3)
let usingCustomInit = Point()

Implementing a namespace through an empty enum

Swift does not really have an out-of-the-box support of namespaces. One could argue that a Swift module can be seen as a namespace, but creating a dedicated Framework for this sole purpose can legitimately be regarded as overkill.

Some developers have taken the habit to use a struct which only contains static fields to implement a namespace. While this does the job, it requires us to remember to implement an empty private init(), because it wouldn't make sense for such a struct to be instantiated.

It's actually possible to take this approach one step further, by replacing the struct with an enum. While it might seem weird to have an enum with no case, it's actually a very idiomatic way to declare a type that cannot be instantiated.

import Foundation

enum NumberFormatterProvider {
    static var currencyFormatter: NumberFormatter {
        let formatter = NumberFormatter()
        formatter.numberStyle = .currency
        formatter.roundingIncrement = 0.01
        return formatter
    }
    
    static var decimalFormatter: NumberFormatter {
        let formatter = NumberFormatter()
        formatter.numberStyle = .decimal
        formatter.decimalSeparator = ","
        return formatter
    }
}

NumberFormatterProvider() // โŒ impossible to instantiate by mistake

NumberFormatterProvider.currencyFormatter.string(from: 2.456) // $2.46
NumberFormatterProvider.decimalFormatter.string(from: 2.456) // 2,456

Using Never to represent impossible code paths

Never is quite a peculiar type in the Swift Standard Library: it is defined as an empty enum enum Never { }.

While this might seem odd at first glance, it actually yields a very interesting property: it makes it a type that cannot be constructed (i.e. it possesses no instances).

This way, Never can be used as a generic parameter to let the compiler know that a particular feature will not be used.

import Foundation

enum Result<Value, Error> {
    case success(value: Value)
    case failure(error: Error)
}

func willAlwaysSucceed(_ completion: @escaping ((Result<String, Never>) -> Void)) {
    completion(.success(value: "Call was successful"))
}

willAlwaysSucceed( { result in
    switch result {
    case .success(let value):
        print(value)
    // the compiler knows that the `failure` case cannot happen
    // so it doesn't require us to handle it.
    }
})

Providing a default value to a Decodable enum

Swift's Codable framework does a great job at seamlessly decoding entities from a JSON stream. However, when we integrate web-services, we are sometimes left to deal with JSONs that require behaviors that Codable does not provide out-of-the-box.

For instance, we might have a string-based or integer-based enum, and be required to set it to a default value when the data found in the JSON does not match any of its cases.

We might be tempted to implement this via an extensive switch statement over all the possible cases, but there is a much shorter alternative through the initializer init?(rawValue:):

import Foundation

enum State: String, Decodable {
    case active
    case inactive
    case undefined
    
    init(from decoder: Decoder) throws {
        let container = try decoder.singleValueContainer()
        let decodedString = try container.decode(String.self)
        
        self = State(rawValue: decodedString) ?? .undefined
    }
}

let data = """
["active", "inactive", "foo"]
""".data(using: .utf8)!

let decoded = try! JSONDecoder().decode([State].self, from: data)

print(decoded) // [State.active, State.inactive, State.undefined]

Another lightweight dependency injection through default values for function parameters

Dependency injection boils down to a simple idea: when an object requires a dependency, it shouldn't create it by itself, but instead it should be given a function that does it for him.

Now the great thing with Swift is that, not only can a function take another function as a parameter, but that parameter can also be given a default value.

When you combine both those features, you can end up with a dependency injection pattern that is both lightweight on boilerplate, but also type safe.

import Foundation

protocol Service {
    func call() -> String
}

class ProductionService: Service {
    func call() -> String {
        return "This is the production"
    }
}

class MockService: Service {
    func call() -> String {
        return "This is a mock"
    }
}

typealias Provider<T> = () -> T

class Controller {
    
    let service: Service
    
    init(serviceProvider: Provider<Service> = { return ProductionService() }) {
        self.service = serviceProvider()
    }
    
    func work() {
        print(service.call())
    }
}

let productionController = Controller()
productionController.work() // prints "This is the production"

let mockedController = Controller(serviceProvider: { return MockService() })
mockedController.work() // prints "This is a mock"

Lightweight dependency injection through protocol-oriented programming

Singletons are pretty bad. They make your architecture rigid and tightly coupled, which then results in your code being hard to test and refactor. Instead of using singletons, your code should rely on dependency injection, which is a much more architecturally sound approach.

But singletons are so easy to use, and dependency injection requires us to do extra-work. So maybe, for simple situations, we could find an in-between solution?

One possible solution is to rely on one of Swift's most know features: protocol-oriented programming. Using a protocol, we declare and access our dependency. We then store it in a private singleton, and perform the injection through an extension of said protocol.

This way, our code will indeed be decoupled from its dependency, while at the same time keeping the boilerplate to a minimum.

import Foundation

protocol Formatting {
    var formatter: NumberFormatter { get }
}

private let sharedFormatter: NumberFormatter = {
    let sharedFormatter = NumberFormatter()
    sharedFormatter.numberStyle = .currency
    return sharedFormatter
}()

extension Formatting {
    var formatter: NumberFormatter { return sharedFormatter }
}

class ViewModel: Formatting {
    var displayableAmount: String?
    
    func updateDisplay(to amount: Double) {
        displayableAmount = formatter.string(for: amount)
    }
}

let viewModel = ViewModel()

viewModel.updateDisplay(to: 42000.45)
viewModel.displayableAmount // "$42,000.45"

Getting rid of overabundant [weak self] and guard

Callbacks are a part of almost all iOS apps, and as frameworks such as RxSwift keep gaining in popularity, they become ever more present in our codebase.

Seasoned Swift developers are aware of the potential memory leaks that @escaping callbacks can produce, so they make real sure to always use [weak self], whenever they need to use self inside such a context. And when they need to have self be non-optional, they then add a guard statement along.

Consequently, this syntax of a [weak self] followed by a guard rapidly tends to appear everywhere in the codebase. The good thing is that, through a little protocol-oriented trick, it's actually possible to get rid of this tedious syntax, without loosing any of its benefits!

import Foundation
import PlaygroundSupport

PlaygroundPage.current.needsIndefiniteExecution = true

protocol Weakifiable: class { }

extension Weakifiable {
    func weakify(_ code: @escaping (Self) -> Void) -> () -> Void {
        return { [weak self] in
            guard let self = self else { return }
            
            code(self)
        }
    }
    
    func weakify<T>(_ code: @escaping (T, Self) -> Void) -> (T) -> Void {
        return { [weak self] arg in
            guard let self = self else { return }
            
            code(arg, self)
        }
    }
}

extension NSObject: Weakifiable { }

class Producer: NSObject {
    
    deinit {
        print("deinit Producer")
    }
    
    private var handler: (Int) -> Void = { _ in }
    
    func register(handler: @escaping (Int) -> Void) {
        self.handler = handler
        
        DispatchQueue.main.asyncAfter(deadline: .now() + 1.0, execute: { self.handler(42) })
    }
}

class Consumer: NSObject {
    
    deinit {
        print("deinit Consumer")
    }
    
    let producer = Producer()
    
    func consume() {
        producer.register(handler: weakify { result, strongSelf in
            strongSelf.handle(result)
        })
    }
    
    private func handle(_ result: Int) {
        print("๐ŸŽ‰ \(result)")
    }
}

var consumer: Consumer? = Consumer()

consumer?.consume()

DispatchQueue.main.asyncAfter(deadline: .now() + 2.0, execute: { consumer = nil })

// This code prints:
// ๐ŸŽ‰ 42
// deinit Consumer
// deinit Producer

Solving callback hell with function composition

Asynchronous functions are a big part of iOS APIs, and most developers are familiar with the challenge they pose when one needs to sequentially call several asynchronous APIs.

This often results in callbacks being nested into one another, a predicament often referred to as callback hell.

Many third-party frameworks are able to tackle this issue, for instance RxSwift or PromiseKit. Yet, for simple instances of the problem, there is no need to use such big guns, as it can actually be solved with simple function composition.

import Foundation

typealias CompletionHandler<Result> = (Result?, Error?) -> Void

infix operator ~>: MultiplicationPrecedence

func ~> <T, U>(_ first: @escaping (CompletionHandler<T>) -> Void, _ second: @escaping (T, CompletionHandler<U>) -> Void) -> (CompletionHandler<U>) -> Void {
    return { completion in
        first({ firstResult, error in
            guard let firstResult = firstResult else { completion(nil, error); return }
            
            second(firstResult, { (secondResult, error) in
                completion(secondResult, error)
            })
        })
    }
}

func ~> <T, U>(_ first: @escaping (CompletionHandler<T>) -> Void, _ transform: @escaping (T) -> U) -> (CompletionHandler<U>) -> Void {
    return { completion in
        first({ result, error in
            guard let result = result else { completion(nil, error); return }
            
            completion(transform(result), nil)
        })
    }
}

func service1(_ completionHandler: CompletionHandler<Int>) {
    completionHandler(42, nil)
}

func service2(arg: String, _ completionHandler: CompletionHandler<String>) {
    completionHandler("๐ŸŽ‰ \(arg)", nil)
}

let chainedServices = service1
    ~> { int in return String(int / 2) }
    ~> service2

chainedServices({ result, _ in
    guard let result = result else { return }
    
    print(result) // Prints: ๐ŸŽ‰ 21
})

Transform an asynchronous function into a synchronous one

Asynchronous functions are a great way to deal with future events without blocking a thread. Yet, there are times where we would like them to behave in exactly such a blocking way.

Think about writing unit tests and using mocked network calls. You will need to add complexity to your test in order to deal with asynchronous functions, whereas synchronous ones would be much easier to manage.

Thanks to Swift proficiency in the functional paradigm, it is possible to write a function whose job is to take an asynchronous function and transform it into a synchronous one.

import Foundation

func makeSynchrone<A, B>(_ asyncFunction: @escaping (A, (B) -> Void) -> Void) -> (A) -> B {
    return { arg in
        let lock = NSRecursiveLock()
        
        var result: B? = nil
        
        asyncFunction(arg) {
            result = $0
            lock.unlock()
        }
        
        lock.lock()
        
        return result!
    }
}

func myAsyncFunction(arg: Int, completionHandler: (String) -> Void) {
    completionHandler("๐ŸŽ‰ \(arg)")
}

let syncFunction = makeSynchrone(myAsyncFunction)

print(syncFunction(42)) // prints ๐ŸŽ‰ 42

Using KeyPaths instead of closures

Closures are a great way to interact with generic APIs, for instance APIs that allow to manipulate data structures through the use of generic functions, such as filter() or sorted().

The annoying part is that closures tend to clutter your code with many instances of {, } and $0, which can quickly undermine its readably.

A nice alternative for a cleaner syntax is to use a KeyPath instead of a closure, along with an operator that will deal with transforming the provided KeyPath in a closure.

import Foundation

prefix operator ^

prefix func ^ <Element, Attribute>(_ keyPath: KeyPath<Element, Attribute>) -> (Element) -> Attribute {
    return { element in element[keyPath: keyPath] }
}

struct MyData {
    let int: Int
    let string: String
}

let data = [MyData(int: 2, string: "Foo"), MyData(int: 4, string: "Bar")]

data.map(^\.int) // [2, 4]
data.map(^\.string) // ["Foo", "Bar"]

Bringing some type-safety to a userInfo Dictionary

Many iOS APIs still rely on a userInfo Dictionary to handle use-case specific data. This Dictionary usually stores untyped values, and is declared as follows: [String: Any] (or sometimes [AnyHashable: Any].

Retrieving data from such a structure will involve some conditional casting (via the as? operator), which is prone to both errors and repetitions. Yet, by introducing a custom subscript, it's possible to encapsulate all the tedious logic, and end-up with an easier and more robust API.

import Foundation

typealias TypedUserInfoKey<T> = (key: String, type: T.Type)

extension Dictionary where Key == String, Value == Any {
    subscript<T>(_ typedKey: TypedUserInfoKey<T>) -> T? {
        return self[typedKey.key] as? T
    }
}

let userInfo: [String : Any] = ["Foo": 4, "Bar": "forty-two"]

let integerTypedKey = TypedUserInfoKey(key: "Foo", type: Int.self)
let intValue = userInfo[integerTypedKey] // returns 4
type(of: intValue) // returns Int?

let stringTypedKey = TypedUserInfoKey(key: "Bar", type: String.self)
let stringValue = userInfo[stringTypedKey] // returns "forty-two"
type(of: stringValue) // returns String?

Lightweight data-binding for an MVVM implementation

MVVM is a great pattern to separate business logic from presentation logic. The main challenge to make it work, is to define a mechanism for the presentation layer to be notified of model updates.

RxSwift is a perfect choice to solve such a problem. Yet, some developers don't feel confortable with leveraging a third-party library for such a central part of their architecture.

For those situation, it's possible to define a lightweight Variable type, that will make the MVVM pattern very easy to use!

import Foundation

class Variable<Value> {
    var value: Value {
        didSet {
            onUpdate?(value)
        }
    }
    
    var onUpdate: ((Value) -> Void)? {
        didSet {
            onUpdate?(value)
        }
    }
    
    init(_ value: Value, _ onUpdate: ((Value) -> Void)? = nil) {
        self.value = value
        self.onUpdate = onUpdate
        self.onUpdate?(value)
    }
}

let variable: Variable<String?> = Variable(nil)

variable.onUpdate = { data in
    if let data = data {
        print(data)
    }
}

variable.value = "Foo"
variable.value = "Bar"

// prints:
// Foo
// Bar

Using typealias to its fullest

The keyword typealias allows developers to give a new name to an already existing type. For instance, Swift defines Void as a typealias of (), the empty tuple.

But a less known feature of this mechanism is that it allows to assign concrete types for generic parameters, or to rename them. This can help make the semantics of generic types much clearer, when used in specific use cases.

import Foundation

enum Either<Left, Right> {
    case left(Left)
    case right(Right)
}

typealias Result<Value> = Either<Value, Error>

typealias IntOrString = Either<Int, String>

Writing an interruptible overload of forEach

Iterating through objects via the forEach(_:) method is a great alternative to the classic for loop, as it allows our code to be completely oblivious of the iteration logic. One limitation, however, is that forEach(_:) does not allow to stop the iteration midway.

Taking inspiration from the Objective-C implementation, we can write an overload that will allow the developer to stop the iteration, if needed.

import Foundation

extension Sequence {
    func forEach(_ body: (Element, _ stop: inout Bool) throws -> Void) rethrows {
        var stop = false
        for element in self {
            try body(element, &stop)
            
            if stop {
                return
            }
        }
    }
}

["Foo", "Bar", "FooBar"].forEach { element, stop in
    print(element)
    stop = (element == "Bar")
}

// Prints:
// Foo
// Bar

Optimizing the use of reduce()

Functional programing is a great way to simplify a codebase. For instance, reduce is an alternative to the classic for loop, without most the boilerplate. Unfortunately, simplicity often comes at the price of performance.

Consider that you want to remove duplicate values from a Sequence. While reduce() is a perfectly fine way to express this computation, the performance will be sub optimal, because of all the unnecessary Array copying that will happen every time its closure gets called.

That's when reduce(into:_:) comes into play. This version of reduce leverages the capacities of copy-on-write type (such as Array or Dictionnary) in order to avoid unnecessary copying, which results in a great performance boost.

import Foundation

func time(averagedExecutions: Int = 1, _ code: () -> Void) {
    let start = Date()
    for _ in 0..<averagedExecutions { code() }
    let end = Date()
    
    let duration = end.timeIntervalSince(start) / Double(averagedExecutions)
    
    print("time: \(duration)")
}

let data = (1...1_000).map { _ in Int(arc4random_uniform(256)) }


// runs in 0.63s
time {
    let noDuplicates: [Int] = data.reduce([], { $0.contains($1) ? $0 : $0 + [$1] })
}

// runs in 0.15s
time {
    let noDuplicates: [Int] = data.reduce(into: [], { if !$0.contains($1) { $0.append($1) } } )
}

Avoiding hardcoded reuse identifiers

UI components such as UITableView and UICollectionView rely on reuse identifiers in order to efficiently recycle the views they display. Often, those reuse identifiers take the form of a static hardcoded String, that will be used for every instance of their class.

Through protocol-oriented programing, it's possible to avoid those hardcoded values, and instead use the name of the type as a reuse identifier.

import Foundation
import UIKit

protocol Reusable {
    static var reuseIdentifier: String { get }
}

extension Reusable {
    static var reuseIdentifier: String {
        return String(describing: self)
    }
}

extension UITableViewCell: Reusable { }

extension UITableView {
    func register<T: UITableViewCell>(_ class: T.Type) {
        register(`class`, forCellReuseIdentifier: T.reuseIdentifier)
    }
    func dequeueReusableCell<T: UITableViewCell>(for indexPath: IndexPath) -> T {
        return dequeueReusableCell(withIdentifier: T.reuseIdentifier, for: indexPath) as! T
    }
}

class MyCell: UITableViewCell { }

let tableView = UITableView()

tableView.register(MyCell.self)
let myCell: MyCell = tableView.dequeueReusableCell(for: [0, 0])

Defining a union type

The C language has a construct called union, that allows a single variable to hold values from different types. While Swift does not provide such a construct, it provides enums with associated values, which allows us to define a type called Either that implements a union of two types.

import Foundation

enum Either<A, B> {
    case left(A)
    case right(B)
    
    func either(ifLeft: ((A) -> Void)? = nil, ifRight: ((B) -> Void)? = nil) {
        switch self {
        case let .left(a):
            ifLeft?(a)
        case let .right(b):
            ifRight?(b)
        }
    }
}

extension Bool { static func random() -> Bool { return arc4random_uniform(2) == 0 } }

var intOrString: Either<Int, String> = Bool.random() ? .left(2) : .right("Foo")

intOrString.either(ifLeft: { print($0 + 1) }, ifRight: { print($0 + "Bar") })

If you're interested by this kind of data structure, I strongly recommend that you learn more about Algebraic Data Types.

Asserting that classes have associated NIBs and vice-versa

Most of the time, when we create a .xib file, we give it the same name as its associated class. From that, if we later refactor our code and rename such a class, we run the risk of forgetting to rename the associated .xib.

While the error will often be easy to catch, if the .xib is used in a remote section of its app, it might go unnoticed for sometime. Fortunately it's possible to build custom test predicates that will assert that 1) for a given class, there exists a .nib with the same name in a given Bundle, 2) for all the .nib in a given Bundle, there exists a class with the same name.

import XCTest

public func XCTAssertClassHasNib(_ class: AnyClass, bundle: Bundle, file: StaticString = #file, line: UInt = #line) {
    let associatedNibURL = bundle.url(forResource: String(describing: `class`), withExtension: "nib")
    
    XCTAssertNotNil(associatedNibURL, "Class \"\(`class`)\" has no associated nib file", file: file, line: line)
}

public func XCTAssertNibHaveClasses(_ bundle: Bundle, file: StaticString = #file, line: UInt = #line) {
    guard let bundleName = bundle.infoDictionary?["CFBundleName"] as? String,
        let basePath = bundle.resourcePath,
        let enumerator = FileManager.default.enumerator(at: URL(fileURLWithPath: basePath),
                                                    includingPropertiesForKeys: nil,
                                                    options: [.skipsHiddenFiles, .skipsSubdirectoryDescendants]) else { return }
    
    var nibFilesURLs = [URL]()
    
    for case let fileURL as URL in enumerator {
        if fileURL.pathExtension.uppercased() == "NIB" {
            nibFilesURLs.append(fileURL)
        }
    }
    
    nibFilesURLs.map { $0.lastPathComponent }
        .compactMap { $0.split(separator: ".").first }
        .map { String($0) }
        .forEach {
            let associatedClass: AnyClass? = bundle.classNamed("\(bundleName).\($0)")
            
            XCTAssertNotNil(associatedClass, "File \"\($0).nib\" has no associated class", file: file, line: line)
        }
}

XCTAssertClassHasNib(MyFirstTableViewCell.self, bundle: Bundle(for: AppDelegate.self))
XCTAssertClassHasNib(MySecondTableViewCell.self, bundle: Bundle(for: AppDelegate.self))
        
XCTAssertNibHaveClasses(Bundle(for: AppDelegate.self))

Many thanks Benjamin Lavialle for coming up with the idea behind the second test predicate.

Small footprint type-erasing with functions

Seasoned Swift developers know it: a protocol with associated type (PAT) "can only be used as a generic constraint because it has Self or associated type requirements". When we really need to use a PAT to type a variable, the goto workaround is to use a type-erased wrapper.

While this solution works perfectly, it requires a fair amount of boilerplate code. In instances where we are only interested in exposing one particular function of the PAT, a shorter approach using function types is possible.

import Foundation
import UIKit

protocol Configurable {
    associatedtype Model
    
    func configure(with model: Model)
}

typealias Configurator<Model> = (Model) -> ()

extension UILabel: Configurable {
    func configure(with model: String) {
        self.text = model
    }
}

let label = UILabel()
let configurator: Configurator<String> = label.configure

configurator("Foo")

label.text // "Foo"

Performing animations sequentially

UIKit exposes a very powerful and simple API to perform view animations. However, this API can become a little bit quirky to use when we want to perform animations sequentially, because it involves nesting closure within one another, which produces notoriously hard to maintain code.

Nonetheless, it's possible to define a rather simple class, that will expose a really nicer API for this particular use case ๐Ÿ‘Œ

import Foundation
import UIKit

class AnimationSequence {
    typealias Animations = () -> Void
    
    private let current: Animations
    private let duration: TimeInterval
    private var next: AnimationSequence? = nil
    
    init(animations: @escaping Animations, duration: TimeInterval) {
        self.current = animations
        self.duration = duration
    }
    
    @discardableResult func append(animations: @escaping Animations, duration: TimeInterval) -> AnimationSequence {
        var lastAnimation = self
        while let nextAnimation = lastAnimation.next {
            lastAnimation = nextAnimation
        }
        lastAnimation.next = AnimationSequence(animations: animations, duration: duration)
        return self
    }
    
    func run() {
        UIView.animate(withDuration: duration, animations: current, completion: { finished in
            if finished, let next = self.next {
                next.run()
            }
        })
    }
}

var firstView = UIView()
var secondView = UIView()

firstView.alpha = 0
secondView.alpha = 0

AnimationSequence(animations: { firstView.alpha = 1.0 }, duration: 1)
            .append(animations: { secondView.alpha = 1.0 }, duration: 0.5)
            .append(animations: { firstView.alpha = 0.0 }, duration: 2.0)
            .run()

Debouncing a function call

Debouncing is a very useful tool when dealing with UI inputs. Consider a search bar, whose content is used to query an API. It wouldn't make sense to perform a request for every character the user is typing, because as soon as a new character is entered, the result of the previous request has become irrelevant.

Instead, our code will perform much better if we "debounce" the API call, meaning that we will wait until some delay has passed, without the input being modified, before actually performing the call.

import Foundation

func debounced(delay: TimeInterval, queue: DispatchQueue = .main, action: @escaping (() -> Void)) -> () -> Void {
    var workItem: DispatchWorkItem?
    
    return {
        workItem?.cancel()
        workItem = DispatchWorkItem(block: action)
        queue.asyncAfter(deadline: .now() + delay, execute: workItem!)
    }
}

let debouncedPrint = debounced(delay: 1.0) { print("Action performed!") }

debouncedPrint()
debouncedPrint()
debouncedPrint()

// After a 1 second delay, this gets
// printed only once to the console:

// Action performed!

Providing useful operators for Optional booleans

When we need to apply the standard boolean operators to Optional booleans, we often end up with a syntax unnecessarily crowded with unwrapping operations. By taking a cue from the world of three-valued logics, we can define a couple operators that make working with Bool? values much nicer.

import Foundation

func && (lhs: Bool?, rhs: Bool?) -> Bool? {
    switch (lhs, rhs) {
    case (false, _), (_, false):
        return false
    case let (unwrapLhs?, unwrapRhs?):
        return unwrapLhs && unwrapRhs
    default:
        return nil
    }
}

func || (lhs: Bool?, rhs: Bool?) -> Bool? {
    switch (lhs, rhs) {
    case (true, _), (_, true):
        return true
    case let (unwrapLhs?, unwrapRhs?):
        return unwrapLhs || unwrapRhs
    default:
        return nil
    }
}

false && nil // false
true && nil // nil
[true, nil, false].reduce(true, &&) // false

nil || true // true
nil || false // nil
[true, nil, false].reduce(false, ||) // true

Removing duplicate values from a Sequence

Transforming a Sequence in order to remove all the duplicate values it contains is a classic use case. To implement it, one could be tempted to transform the Sequence into a Set, then back to an Array. The downside with this approach is that it will not preserve the order of the sequence, which can definitely be a dealbreaker. Using reduce() it is possible to provide a concise implementation that preserves ordering:

import Foundation

extension Sequence where Element: Equatable {
    func duplicatesRemoved() -> [Element] {
        return reduce([], { $0.contains($1) ? $0 : $0 + [$1] })
    }
}

let data = [2, 5, 2, 3, 6, 5, 2]

data.duplicatesRemoved() // [2, 5, 3, 6]

Shorter syntax to deal with optional strings

Optional strings are very common in Swift code, for instance many objects from UIKit expose the text they display as a String?. Many times you will need to manipulate this data as an unwrapped String, with a default value set to the empty string for nil cases.

While the nil-coalescing operator (e.g. ??) is a perfectly fine way to a achieve this goal, defining a computed variable like orEmpty can help a lot in cleaning the syntax.

import Foundation
import UIKit

extension Optional where Wrapped == String {
    var orEmpty: String {
        switch self {
        case .some(let value):
            return value
        case .none:
            return ""
        }
    }
}

func doesNotWorkWithOptionalString(_ param: String) {
    // do something with `param`
}

let label = UILabel()
label.text = "This is some text."

doesNotWorkWithOptionalString(label.text.orEmpty)

Encapsulating background computation and UI update

Every seasoned iOS developers knows it: objects from UIKit can only be accessed from the main thread. Any attempt to access them from a background thread is a guaranteed crash.

Still, running a costly computation on the background, and then using it to update the UI can be a common pattern.

In such cases you can rely on asyncUI to encapsulate all the boilerplate code.

import Foundation
import UIKit

func asyncUI<T>(_ computation: @autoclosure @escaping () -> T, qos: DispatchQoS.QoSClass = .userInitiated, _ completion: @escaping (T) -> Void) {
    DispatchQueue.global(qos: qos).async {
        let value = computation()
        DispatchQueue.main.async {
            completion(value)
        }
    }
}

let label = UILabel()

func costlyComputation() -> Int { return (0..<10_000).reduce(0, +) }

asyncUI(costlyComputation()) { value in
    label.text = "\(value)"
}

Retrieving all the necessary data to build a debug view

A debug view, from which any controller of an app can be instantiated and pushed on the navigation stack, has the potential to bring some real value to a development process. A requirement to build such a view is to have a list of all the classes from a given Bundle that inherit from UIViewController. With the following extension, retrieving this list becomes a piece of cake ๐Ÿฐ

import Foundation
import UIKit
import ObjectiveC

extension Bundle {
    func viewControllerTypes() -> [UIViewController.Type] {
        guard let bundlePath = self.executablePath else { return [] }
        
        var size: UInt32 = 0
        var rawClassNames: UnsafeMutablePointer<UnsafePointer<Int8>>!
        var parsedClassNames = [String]()
        
        rawClassNames = objc_copyClassNamesForImage(bundlePath, &size)
        
        for index in 0..<size {
            let className = rawClassNames[Int(index)]
            
            if let name = NSString.init(utf8String:className) as String?,
                NSClassFromString(name) is UIViewController.Type {
                parsedClassNames.append(name)
            }
        }
        
        return parsedClassNames
            .sorted()
            .compactMap { NSClassFromString($0) as? UIViewController.Type }
    }
}

// Fetch all view controller types in UIKit
Bundle(for: UIViewController.self).viewControllerTypes()

I share the credit for this tip with Benoรฎt Caron.

Defining a function to map over dictionaries

Update As it turns out, map is actually a really bad name for this function, because it does not preserve composition of transformations, a property that is required to fit the definition of a real map function.

Surprisingly enough, the standard library doesn't define a map() function for dictionaries that allows to map both keys and values into a new Dictionary. Nevertheless, such a function can be helpful, for instance when converting data across different frameworks.

import Foundation

extension Dictionary {
    func map<T: Hashable, U>(_ transform: (Key, Value) throws -> (T, U)) rethrows -> [T: U] {
        var result: [T: U] = [:]
        
        for (key, value) in self {
            let (transformedKey, transformedValue) = try transform(key, value)
            result[transformedKey] = transformedValue
        }
        
        return result
    }
}

let data = [0: 5, 1: 6, 2: 7]
data.map { ("\($0)", $1 * $1) } // ["2": 49, "0": 25, "1": 36]

A shorter syntax to remove nil values

Swift provides the function compactMap(), that can be used to remove nil values from a Sequence of optionals when calling it with an argument that just returns its parameter (i.e. compactMap { $0 }). Still, for such use cases it would be nice to get rid of the trailing closure.

The implementation isn't as straightforward as your usual extension, but once it has been written, the call site definitely gets cleaner ๐Ÿ‘Œ

import Foundation

protocol OptionalConvertible {
    associatedtype Wrapped
    func asOptional() -> Wrapped?
}

extension Optional: OptionalConvertible {
    func asOptional() -> Wrapped? {
        return self
    }
}

extension Sequence where Element: OptionalConvertible {
    func compacted() -> [Element.Wrapped] {
        return compactMap { $0.asOptional() }
    }
}

let data = [nil, 1, 2, nil, 3, 5, nil, 8, nil]
data.compacted() // [1, 2, 3, 5, 8]

Dealing with expirable values

It might happen that your code has to deal with values that come with an expiration date. In a game, it could be a score multiplier that will only last for 30 seconds. Or it could be an authentication token for an API, with a 15 minutes lifespan. In both instances you can rely on the type Expirable to encapsulate the expiration logic.

import Foundation

struct Expirable<T> {
    private var innerValue: T
    private(set) var expirationDate: Date
    
    var value: T? {
        return hasExpired() ? nil : innerValue
    }
    
    init(value: T, expirationDate: Date) {
        self.innerValue = value
        self.expirationDate = expirationDate
    }
    
    init(value: T, duration: Double) {
        self.innerValue = value
        self.expirationDate = Date().addingTimeInterval(duration)
    }
    
    func hasExpired() -> Bool {
        return expirationDate < Date()
    }
}

let expirable = Expirable(value: 42, duration: 3)

sleep(2)
expirable.value // 42
sleep(2)
expirable.value // nil

I share the credit for this tip with Benoรฎt Caron.

Using parallelism to speed-up map()

Almost all Apple devices able to run Swift code are powered by a multi-core CPU, consequently making a good use of parallelism is a great way to improve code performance. map() is a perfect candidate for such an optimization, because it is almost trivial to define a parallel implementation.

import Foundation

extension Array {
    func parallelMap<T>(_ transform: (Element) -> T) -> [T] {
        let res = UnsafeMutablePointer<T>.allocate(capacity: count)
        
        DispatchQueue.concurrentPerform(iterations: count) { i in
            res[i] = transform(self[i])
        }
        
        let finalResult = Array<T>(UnsafeBufferPointer(start: res, count: count))
        res.deallocate(capacity: count)
        
        return finalResult
    }
}

let array = (0..<1_000).map { $0 }

func work(_ n: Int) -> Int {
    return (0..<n).reduce(0, +)
}

array.parallelMap { work($0) }

๐Ÿšจ Make sure to only use parallelMap() when the transform function actually performs some costly computations. Otherwise performances will be systematically slower than using map(), because of the multithreading overhead.

Measuring execution time with minimum boilerplate

During development of a feature that performs some heavy computations, it can be helpful to measure just how much time a chunk of code takes to run. The time() function is a nice tool for this purpose, because of how simple it is to add and then to remove when it is no longer needed.

import Foundation

func time(averagedExecutions: Int = 1, _ code: () -> Void) {
    let start = Date()
    for _ in 0..<averagedExecutions { code() }
    let end = Date()
    
    let duration = end.timeIntervalSince(start) / Double(averagedExecutions)
    
    print("time: \(duration)")
}

time {
    (0...10_000).map { $0 * $0 }
}
// time: 0.183973908424377

Running two pieces of code in parallel

Concurrency is definitely one of those topics were the right encapsulation bears the potential to make your life so much easier. For instance, with this piece of code you can easily launch two computations in parallel, and have the results returned in a tuple.

import Foundation

func parallel<T, U>(_ left: @autoclosure () -> T, _ right: @autoclosure () -> U) -> (T, U) {
    var leftRes: T?
    var rightRes: U?
    
    DispatchQueue.concurrentPerform(iterations: 2, execute: { id in
        if id == 0 {
            leftRes = left()
        } else {
            rightRes = right()
        }
    })
    
    return (leftRes!, rightRes!)
}

let values = (1...100_000).map { $0 }

let results = parallel(values.map { $0 * $0 }, values.reduce(0, +))

Making good use of #file, #line and #function

Swift exposes three special variables #file, #line and #function, that are respectively set to the name of the current file, line and function. Those variables become very useful when writing custom logging functions or test predicates.

import Foundation

func log(_ message: String, _ file: String = #file, _ line: Int = #line, _ function: String = #function) {
    print("[\(file):\(line)] \(function) - \(message)")
}

func foo() {
    log("Hello world!")
}

foo() // [MyPlayground.playground:8] foo() - Hello world!

Comparing Optionals through Conditional Conformance

Swift 4.1 has introduced a new feature called Conditional Conformance, which allows a type to implement a protocol only when its generic type also does.

With this addition it becomes easy to let Optional implement Comparable only when Wrapped also implements Comparable:

import Foundation

extension Optional: Comparable where Wrapped: Comparable {
    public static func < (lhs: Optional, rhs: Optional) -> Bool {
        switch (lhs, rhs) {
        case let (lhs?, rhs?):
            return lhs < rhs
        case (nil, _?):
            return true // anything is greater than nil
        case (_?, nil):
            return false // nil in smaller than anything
        case (nil, nil):
            return true // nil is not smaller than itself
        }
    }
}

let data: [Int?] = [8, 4, 3, nil, 12, 4, 2, nil, -5]
data.sorted() // [nil, nil, Optional(-5), Optional(2), Optional(3), Optional(4), Optional(4), Optional(8), Optional(12)]

Safely subscripting a Collection

Any attempt to access an Array beyond its bounds will result in a crash. While it's possible to write conditions such as if index < array.count { array[index] } in order to prevent such crashes, this approach will rapidly become cumbersome.

A great thing is that this condition can be encapsulated in a custom subscript that will work on any Collection:

import Foundation

extension Collection {
    subscript (safe index: Index) -> Element? {
        return indices.contains(index) ? self[index] : nil
    }
}

let data = [1, 3, 4]

data[safe: 1] // Optional(3)
data[safe: 10] // nil

Easier String slicing using ranges

Subscripting a string with a range can be very cumbersome in Swift 4. Let's face it, no one wants to write lines like someString[index(startIndex, offsetBy: 0)..<index(startIndex, offsetBy: 10)] on a regular basis.

Luckily, with the addition of one clever extension, strings can be sliced as easily as arrays ๐ŸŽ‰

import Foundation

extension String {
    public subscript(value: CountableClosedRange<Int>) -> Substring {
        get {
            return self[index(startIndex, offsetBy: value.lowerBound)...index(startIndex, offsetBy: value.upperBound)]
        }
    }
    
    public subscript(value: CountableRange<Int>) -> Substring {
        get {
            return self[index(startIndex, offsetBy: value.lowerBound)..<index(startIndex, offsetBy: value.upperBound)]
        }
    }
    
    public subscript(value: PartialRangeUpTo<Int>) -> Substring {
        get {
            return self[..<index(startIndex, offsetBy: value.upperBound)]
        }
    }
    
    public subscript(value: PartialRangeThrough<Int>) -> Substring {
        get {
            return self[...index(startIndex, offsetBy: value.upperBound)]
        }
    }
    
    public subscript(value: PartialRangeFrom<Int>) -> Substring {
        get {
            return self[index(startIndex, offsetBy: value.lowerBound)...]
        }
    }
}

let data = "This is a string!"

data[..<4]  // "This"
data[5..<9] // "is a"
data[10...] // "string!"

Concise syntax for sorting using a KeyPath

By using a KeyPath along with a generic type, a very clean and concise syntax for sorting data can be implemented:

import Foundation

extension Sequence {
    func sorted<T: Comparable>(by attribute: KeyPath<Element, T>) -> [Element] {
        return sorted(by: { $0[keyPath: attribute] < $1[keyPath: attribute] })
    }
}

let data = ["Some", "words", "of", "different", "lengths"]

data.sorted(by: \.count) // ["of", "Some", "words", "lengths", "different"]

If you like this syntax, make sure to checkout KeyPathKit!

Manufacturing cache-efficient versions of pure functions

By capturing a local variable in a returned closure, it is possible to manufacture cache-efficient versions of pure functions. Be careful though, this trick only works with non-recursive function!

import Foundation

func cached<In: Hashable, Out>(_ f: @escaping (In) -> Out) -> (In) -> Out {
    var cache = [In: Out]()
    
    return { (input: In) -> Out in
        if let cachedValue = cache[input] {
            return cachedValue
        } else {
            let result = f(input)
            cache[input] = result
            return result
        }
    }
}

let cachedCos = cached { (x: Double) in cos(x) }

cachedCos(.pi * 2) // value of cos for 2ฯ€ is now cached

Simplifying complex conditions with pattern matching

When distinguishing between complex boolean conditions, using a switch statement along with pattern matching can be more readable than the classic series of if {} else if {}.

import Foundation

let expr1: Bool
let expr2: Bool
let expr3: Bool

if expr1 && !expr3 {
    functionA()
} else if !expr2 && expr3 {
    functionB()
} else if expr1 && !expr2 && expr3 {
    functionC()
}

switch (expr1, expr2, expr3) {
    
case (true, _, false):
    functionA()
case (_, false, true):
    functionB()
case (true, false, true):
    functionC()
default:
    break
}

Easily generating arrays of data

Using map() on a range makes it easy to generate an array of data.

import Foundation

func randomInt() -> Int { return Int(arc4random()) }

let randomArray = (1...10).map { _ in randomInt() }

Using @autoclosure for cleaner call sites

Using @autoclosure enables the compiler to automatically wrap an argument within a closure, thus allowing for a very clean syntax at call sites.

import UIKit

extension UIView {
    class func animate(withDuration duration: TimeInterval, _ animations: @escaping @autoclosure () -> Void) {
        UIView.animate(withDuration: duration, animations: animations)
    }
}

let view = UIView()

UIView.animate(withDuration: 0.3, view.backgroundColor = .orange)

Observing new and old value with RxSwift

When working with RxSwift, it's very easy to observe both the current and previous value of an observable sequence by simply introducing a shift using skip().

import RxSwift

let values = Observable.of(4, 8, 15, 16, 23, 42)

let newAndOld = Observable.zip(values, values.skip(1)) { (previous: $0, current: $1) }
    .subscribe(onNext: { pair in
        print("current: \(pair.current) - previous: \(pair.previous)")
    })

//current: 8 - previous: 4
//current: 15 - previous: 8
//current: 16 - previous: 15
//current: 23 - previous: 16
//current: 42 - previous: 23

Implicit initialization from literal values

Using protocols such as ExpressibleByStringLiteral it is possible to provide an init that will be automatically when a literal value is provided, allowing for nice and short syntax. This can be very helpful when writing mock or test data.

import Foundation

extension URL: ExpressibleByStringLiteral {
    public init(stringLiteral value: String) {
        self.init(string: value)!
    }
}

let url: URL = "http://www.google.fr"

NSURLConnection.canHandle(URLRequest(url: "http://www.google.fr"))

Achieving systematic validation of data

Through some clever use of Swift private visibility it is possible to define a container that holds any untrusted value (such as a user input) from which the only way to retrieve the value is by making it successfully pass a validation test.

import Foundation

struct Untrusted<T> {
    private(set) var value: T
}

protocol Validator {
    associatedtype T
    static func validation(value: T) -> Bool
}

extension Validator {
    static func validate(untrusted: Untrusted<T>) -> T? {
        if self.validation(value: untrusted.value) {
            return untrusted.value
        } else {
            return nil
        }
    }
}

struct FrenchPhoneNumberValidator: Validator {
    static func validation(value: String) -> Bool {
       return (value.count) == 10 && CharacterSet(charactersIn: value).isSubset(of: CharacterSet.decimalDigits)
    }
}

let validInput = Untrusted(value: "0122334455")
let invalidInput = Untrusted(value: "0123")

FrenchPhoneNumberValidator.validate(untrusted: validInput) // returns "0122334455"
FrenchPhoneNumberValidator.validate(untrusted: invalidInput) // returns nil

Implementing the builder pattern with keypaths

With the addition of keypaths in Swift 4, it is now possible to easily implement the builder pattern, that allows the developer to clearly separate the code that initializes a value from the code that uses it, without the burden of defining a factory method.

import UIKit

protocol With {}

extension With where Self: AnyObject {
    @discardableResult
    func with<T>(_ property: ReferenceWritableKeyPath<Self, T>, setTo value: T) -> Self {
        self[keyPath: property] = value
        return self
    }
}

extension UIView: With {}

let view = UIView()

let label = UILabel()
    .with(\.textColor, setTo: .red)
    .with(\.text, setTo: "Foo")
    .with(\.textAlignment, setTo: .right)
    .with(\.layer.cornerRadius, setTo: 5)

view.addSubview(label)

๐Ÿšจ The Swift compiler does not perform OS availability checks on properties referenced by keypaths. Any attempt to use a KeyPath for an unavailable property will result in a runtime crash.

I share the credit for this tip with Marion Curtil.

Storing functions rather than values

When a type stores values for the sole purpose of parametrizing its functions, itโ€™s then possible to not store the values but directly the function, with no discernable difference at the call site.

import Foundation

struct MaxValidator {
    let max: Int
    let strictComparison: Bool
    
    func isValid(_ value: Int) -> Bool {
        return self.strictComparison ? value < self.max : value <= self.max
    }
}

struct MaxValidator2 {
    var isValid: (_ value: Int) -> Bool
    
    init(max: Int, strictComparison: Bool) {
        self.isValid = strictComparison ? { $0 < max } : { $0 <= max }
    }
}

MaxValidator(max: 5, strictComparison: true).isValid(5) // false
MaxValidator2(max: 5, strictComparison: false).isValid(5) // true

Defining operators on function types

Functions are first-class citizen types in Swift, so it is perfectly legal to define operators for them.

import Foundation

let firstRange = { (0...3).contains($0) }
let secondRange = { (5...6).contains($0) }

func ||(_ lhs: @escaping (Int) -> Bool, _ rhs: @escaping (Int) -> Bool) -> (Int) -> Bool {
    return { value in
        return lhs(value) || rhs(value)
    }
}

(firstRange || secondRange)(2) // true
(firstRange || secondRange)(4) // false
(firstRange || secondRange)(6) // true

Typealiases for functions

Typealiases are great to express function signatures in a more comprehensive manner, which then enables us to easily define functions that operate on them, resulting in a nice way to write and use some powerful API.

import Foundation

typealias RangeSet = (Int) -> Bool

func union(_ left: @escaping RangeSet, _ right: @escaping RangeSet) -> RangeSet {
    return { left($0) || right($0) }
}

let firstRange = { (0...3).contains($0) }
let secondRange = { (5...6).contains($0) }

let unionRange = union(firstRange, secondRange)

unionRange(2) // true
unionRange(4) // false

Encapsulating state within a function

By returning a closure that captures a local variable, it's possible to encapsulate a mutable state within a function.

import Foundation

func counterFactory() -> () -> Int {
    var counter = 0
    
    return {
        counter += 1
        return counter
    }
}

let counter = counterFactory()

counter() // returns 1
counter() // returns 2

Generating all cases for an Enum

โš ๏ธ Since Swift 4.2, allCases can now be synthesized at compile-time by simply conforming to the protocol CaseIterable. The implementation below should no longer be used in production code.

Through some clever leveraging of how enums are stored in memory, it is possible to generate an array that contains all the possible cases of an enum. This can prove particularly useful when writing unit tests that consume random data.

import Foundation

enum MyEnum { case first; case second; case third; case fourth }

protocol EnumCollection: Hashable {
    static var allCases: [Self] { get }
}

extension EnumCollection {
    public static var allCases: [Self] {
        var i = 0
        return Array(AnyIterator {
            let next = withUnsafePointer(to: &i) {
                $0.withMemoryRebound(to: Self.self, capacity: 1) { $0.pointee }
            }
            if next.hashValue != i { return nil }
            i += 1
            return next
        })
    }
}

extension MyEnum: EnumCollection { }

MyEnum.allCases // [.first, .second, .third, .fourth]

Using map on optional values

The if-let syntax is a great way to deal with optional values in a safe manner, but at times it can prove to be just a little bit to cumbersome. In such cases, using the Optional.map() function is a nice way to achieve a shorter code while retaining safeness and readability.

import UIKit

let date: Date? = Date() // or could be nil, doesn't matter
let formatter = DateFormatter()
let label = UILabel()

if let safeDate = date {
    label.text = formatter.string(from: safeDate)
}

label.text = date.map { return formatter.string(from: $0) }

label.text = date.map(formatter.string(from:)) // even shorter, tough less readable

๐Ÿ“ฃ NEW ๐Ÿ“ฃ Swift Tips are now available on YouTube ๐Ÿ‘‡

Summary

Tips


Download Details:

Author: vincent-pradeilles
Source code: https://github.com/vincent-pradeilles/swift-tips

License: MIT license
#swift 

Nat  Grady

Nat Grady

1658879220

A Command Line tool To Generate PDF From URL, HTML Or Markdown File

electron-pdf   

A command line tool to generate PDF from URL, HTML or Markdown files with electron.

Versioning

Starting with version 4.0.x the master branch will always have the latest electron version.

Semantic Versioning is used, and corresponds to electron versions in the following way:

  • electron-pdf 15.0.x (master) => electron=15.1.1, node=16.5.0, chrome=94.0.4606.61
  • electron-pdf 10.0.x => electron=10.1.3, node=12.16.3, chrome=85.0.4183.121
  • electron-pdf 7.0.x => electron 7.x (Chromium 78, Node 12.8.1)
  • electron-pdf 4.0.x => electron 4.x (Chromium 69, Node 10.11.0)
  • electron-pdf 1.3.x => electron 1.6.x (Chromium 56, Node 7.4)
  • electron-pdf 1.2.x => electron 1.4.x (Chromium 53, Node 6.5)

Note: The Chromium versions employed by electron have impacts based on the functionality you may be exporting.
Choose the version you need based on Chromium.

Install

npm install electron-pdf

Note: If you're installing electron-pdf as root using the system level npm (vs a user-level install like with NVM) then you may need to run the following command instead:

sudo npm install electron-pdf -g --unsafe-perm

Please see the npm docs for more information.

For gnu/linux installations without a graphical environment:

$ sudo apt-get install xvfb # or equivalent
$ export DISPLAY=':99.0'
$ Xvfb :99 -screen 0 1024x768x24 > /dev/null 2>&1 &
$ electron-pdf ...

There is also an example docker machine here.

Node Usage

Electron PDF can be used inside of an application, or more commonly as the engine for a pdf rendering service. For instance, to handle http requests using Express. The following snippets show you how you can get started.

The application must run in an Electron process

In package.json

"start": "DEBUG=electronpdf:* electron index.js",
"watch": "DEBUG=electronpdf:* nodemon --exec electron index.js"

You can use the same instance

var ElectronPDF = require('electron-pdf')
var express = require('express')
var bodyParser = require('body-parser')
var app = express()
app.use(bodyParser.json())

var exporter = new ElectronPDF()
exporter.on('charged', () => {
    //Only start the express server once the exporter is ready
    app.listen(port, hostname, function() {
        console.log(`Export Server running at http://${hostname}:${port}`);
    })
})
exporter.start()

And handle multiple export job instances

app.post('/pdfexport', function(req,res){
    // derive job arguments from request here
    // 
    const jobOptions = {
      /**
        r.results[] will contain the following based on inMemory
          false: the fully qualified path to a PDF file on disk
          true: The Buffer Object as returned by Electron
        
        Note: the default is false, this can not be set using the CLI
       */
      inMemory: false 
    }
    const options = {
          pageSize : "A4"
    }
    exporter.createJob(source, target, options, jobOptions).then( job => {
    job.on('job-complete', (r) => {
            console.log('pdf files:', r.results)
            // Process the PDF file(s) here
        })
        job.render()
    })    
})

Using an in memory Buffer

If you set the inMemory setting to true, you must also set closeWindow=false or you will get a segmentation fault anytime the window is closed before the buffer is sent on the response. You then need to invoke job.destroy to close the window.

Sample Code:

const jobOptions = { inMemory: true, closeWindow: false }
exporter.createJob(source, target, options, jobOptions).then( job => {
    job.on('job-complete', (r) => {
      //Send the Buffer here
      process.nextTick(() => {job.destroy()})
    })
})

Events

The API is designed to emit noteworthy events rather than use callbacks. Full documentation of all events is a work in progress.

Environment Variables

  • ELECTRONPDF_RENDERER_MAX_MEMORY : The --max-old-space-size option for each Electron renderer process (browser window); default: 75% of total system memory up to 8GB
  • ELECTRONPDF_WINDOW_CLEANUP_INTERVAL : Interval for which to check for hung windows, in milliseconds; default: 30 seconds
  • ELECTRONPDF_WINDOW_LIFE_THRESHOLD : How long a window can remain open before it is terminated, in milliseconds; default: 5 minutes
  • ELECTRONPDF_PNG_CAPTURE_DELAY : Amount of millis to wait before invoking WebContents.capturePage for PNG exports; default: 100ms

Command Line Usage

For Ad-hoc conversions, Electron PDF comes with support for a CLI.

To generate a PDF from a HTML file

$ electron-pdf index.html ~/Desktop/index.pdf

To generate a PDF from a Markdown file

$ electron-pdf index.md ~/Desktop/index.pdf

To generate a PDF from a Markdown file with custom CSS (defaults to Github markdown style)

$ electron-pdf index.html ~/Desktop/index.pdf -c my-awesome-css.css

To generate a PDF from a URL

$ electron-pdf https://fraserxu.me ~/Desktop/fraserxu.pdf

Rendering Options

Electron PDF gives you complete control of how the BrowserWindow should be configured, and when the window contents should be captured.

To specify browser options

The BrowserWindow supports many options which you may define by passing a JSON Object to the --browserConfig option.

Some common use cases may include:

  • height and width - electron-pdf calculates the browser height and width based off of the dimensions of PDF page size multiplied by the HTML standard of 96 pixels/inch. So only set these values if you need to override this behavior
  • show - to display the browser window during generation
$ electron-pdf https://fraserxu.me ~/Desktop/fraserxu.pdf --browserConfig '{"show":true}'

To generate a PDF after the an async task in the HTML

electron-pdf ./index.html ~/Desktop/README.pdf -e

In your application, at the point which the view is ready for rendering

document.body.dispatchEvent(new Event('view-ready'))

Warning: It is possible that your application will be ready and emit the event before the main electron process has had a chance execute the javascript in the renderer process which listens for this event.

If you are finding that the event is not effective and your page waits until the full timeout has occurred, then you should use setInterval to emit the event until it is acknowledged like so:

  var eventEmitInterval = setInterval(function () {
    document.body.dispatchEvent(new Event('view-ready'))
  }, 25)

  document.body.addEventListener('view-ready-acknowledged', function(){
    clearInterval(eventEmitInterval)
  })

When the main process first receives your ready event it will emit a single acknowlegement on document.body with whatever event name you are using suffixed with -acknowledged. So the default would be view-ready-acknowledged

Observing your own event

If the page you are rending is under your control, and you wish to modify the behavior of the rendering process you can use a CustomEvent and an observer that will be triggered after the view is ready but before it is captured.

your-page.html

document.body.dispatchEvent(new CustomEvent('view-ready', { detail: {layout: landscape} }))

your-exporter.js

You are required to provide a function that accepts the detail object from the CustomEvent and returns a Promise. You may optionally fulfill the promise with and object that will amend/override any of the contextual attributes assigned to resource (url) currently being exported.

As an example, suppose you wanted to change the orientation of the PDF, and capture the output as PNG instead of a PDF.

job.observeReadyEvent( (detail) => {
    return new Promise( (resolve,reject) => {
      const context = {}
      if( detail && detail.landscape ){
        job.changeArgValue('landscape', true)
        context.type = 'png'
      }
      resolve(context)
    })
})

Note: Future versions of the library will only allow you to provide context overrides, and not allow you to change job level attributes.

All Available Options

Electron PDF exposes the printToPDF settings (i.e. pageSize, orientation, margins, etc.) available from the Electron API. See the following options for usage.


  A command line tool to generate PDF from URL, HTML or Markdown files

  Options
    --help                     Show this help
    --version                  Current version of package
    
    -i | --input               String - The path to the HTML file or url
    -o | --output              String - The path of the output PDF
    
    -b | --printBackground     Boolean - Whether to print CSS backgrounds.
    
    --acceptLanguage           String - A valid value for the 'Accept-Language' http request header
    
    --browserConfig            String - A valid JSON String that will be parsed into the options passed to electron.BrowserWindow
    
    -c | --css                 String - The path to custom CSS (can be specified more than once)
    
    -d | --disableCache        Boolean - Disable HTTP caching
                                 false - default
    
    -e | --waitForJSEvent      String - The name of the event to wait before PDF creation
                                 'view-ready' - default
    
    -l | --landscape           Boolean - true for landscape, false for portrait (don't pass a string on the CLI, just the `-l` flag)
                                 false - default
    
    -m | --marginsType         Integer - Specify the type of margins to use
                                 0 - default margins
                                 1 - no margins (electron-pdf default setting)
                                 2 - minimum margins
    
    --noprint                  Boolean - Do not run printToPDF, useful if the page downloads a file that needs captured instead of a PDF.  
                                         The Electron `win.webContents.session.on('will-download')` event will be implemented 
                                         and the file saved to the location provided in `--output`.
                                         Currently only supports a single import url.
                                         The page is responsible for initiating the download itself.
    
    -p | --pageSize            String - Can be A3, A4, A5, Legal, Letter, Tabloid or an Object containing height and width in microns
                                 "A4" - default
    
    -r | --requestHeaders      String - A valid JSON String that will be parsed into an Object where each key/value pair is: <headerName>: <headerValue>
                                 Example: '{"Authorization": "Bearer token", "X-Custom-Header": "Hello World"}'  
    
    -s | --printSelectionOnly  Boolean - Whether to print selection only
                                 false - default
                                 
    -t | --trustRemoteContent  Boolean - Whether to trust remote content loaded in the Electron webview.  False by default.
    --type                     String - The type of export, will dictate the output file type.  'png': PNG image, anything else: PDF File
    
    -w | --outputWait          Integer โ€“ Time to wait (in MS) between page load and PDF creation.  
                                         If used in conjunction with -e this will override the default timeout of 10 seconds    
    --ignoreCertificateErrors  Boolean - If true, all certificate errors thrown by Electron will be ignored.  This can be used to accept self-signed and untrusted certificates.  You should be aware of the security implications of setting this flag.
                             false - default

Find more information on Electron Security here.

Debugging

Sentry

If you have a Sentry account and setup a new app to get a new DSN, you can set a SENTRY_DSN environment variable which will activate sentry logs. See lib/sentry.js for implementation details.

This will allow you to easily see/monitor errors that are occuring inside of the Chromium renderer (browser window). It also automatically integrates with Electron's Crash Reporter

CLI Usage

You can see some additional logging (if you're getting errors or unexpected output) by setting DEBUG=electron* For example: DEBUG=electron* electron-pdf <input> <output> -l

  Usage
    $ electron-pdf <input> <output>
    $ electron-pdf <input> <output> -l

  Examples
    $ electron-pdf http://fraserxu.me ~/Desktop/fraserxu.pdf
    $ electron-pdf ./index.html ~/Desktop/index.pdf
    $ electron-pdf ./README.md ~/Desktop/README.pdf -l
    $ electron-pdf ./README.md ~/Desktop/README.pdf -l -c my-awesome-css.css

Inspired by electron-mocha

Other Formats

Want to use the same options, but export to PNG or snapshot the rendered HTML? Just set the output filename to end in .png or .html instead!

  Examples
    $ electron-pdf http://fraserxu.me ~/Desktop/fraserxu.pdf
    $ electron-pdf http://fraserxu.me ~/Desktop/fraserxu.html
    $ electron-pdf http://fraserxu.me ~/Desktop/fraserxu.png

Extensions

If you need powerpoint support, pdf-powerpoint picks up where Electron PDF leaves off by converting each page in the PDF to a PNG and placing them on individual slides.

Author: Fraserxu
Source Code: https://github.com/fraserxu/electron-pdf 
License: MIT license

#electron #pdf #url #html #markdown 

Cayla  Erdman

Cayla Erdman

1594369800

Introduction to Structured Query Language SQL pdf

SQL stands for Structured Query Language. SQL is a scripting language expected to store, control, and inquiry information put away in social databases. The main manifestation of SQL showed up in 1974, when a gathering in IBM built up the principal model of a social database. The primary business social database was discharged by Relational Software later turning out to be Oracle.

Models for SQL exist. In any case, the SQL that can be utilized on every last one of the major RDBMS today is in various flavors. This is because of two reasons:

1. The SQL order standard is genuinely intricate, and it isnโ€™t handy to actualize the whole standard.

2. Every database seller needs an approach to separate its item from others.

Right now, contrasts are noted where fitting.

#programming books #beginning sql pdf #commands sql #download free sql full book pdf #introduction to sql pdf #introduction to sql ppt #introduction to sql #practical sql pdf #sql commands pdf with examples free download #sql commands #sql free bool download #sql guide #sql language #sql pdf #sql ppt #sql programming language #sql tutorial for beginners #sql tutorial pdf #sql #structured query language pdf #structured query language ppt #structured query language

Yvonne  Hickle

Yvonne Hickle

1662610320

Mockit: A Tasty Mocking Framework for Unit Tests in Swift 5.0

Mockit

Introduction

Mockit is a Tasty mocking framework for unit tests in Swift 5.0. It's at an early stage of development, but its current features are almost completely usable.

Mockit is a mocking framework that tastes brilliant. It lets you write beautiful tests with a clean & simple API. Tests written using Mockit are very readable and they produce clean verification errors. It's inspired by the famous mocking framework for Java - Mockito.

Documentation

Mockit is yet to be documented fully but it comes with a sample project that lets you try all its features and become familiar with the API. You can find it in Mockit.xcworkspace.

There's an example test file called ExampleTests.swift. Look there for some tests that can be run. This tests a class Example against a mocked collaborator ExampleCollaborator.

File an issue if you have any question.

To run the example project, clone the repo, and run pod install from the Example directory first.

Limitations

  • There's some boiler-plate code needed to create mocks. See MockExampleCollaborator for an example in the Basic Usage section below. However, a plugin development is on its way for both Xcode and AppCode to minimize writing this boiler-plate code every time a mock needs to be created.

Features

Stubbing. Mockit lets you stub a method and then perform any of 3 actions (thenReturn, thenDo, thenAnswer) individually or in any order via chaining;

Mocking. You can create a subclass extending the Mock protocol to mock required methods;

Call Verification. You can verify method calls using one of 8 supported modes (Once, AtLeastOnce, AtMostOnce, Times, AtLeastTimes, AtMostTimes, Never and Only);

Arguments of specific call. Mockit allows you to obtain the arguments of a specific method call to use custom assertions on them;

Helpful messages. If method verification fails or something goes wrong, Mockit provides readable messages that describes the issue;

Default Type matchers. Out of the box, Mockit can match the following types:

  • String / String?
  • Bool / Bool?
  • Int / Int?
  • Double / Double?
  • Float / Float?
  • Array / Array? of the above primitive types
  • Dictionary / Dictionary? of the above primitive types

Given that Swift does not have reflection, Mockit cannot magically match your custom types, so you need to subclass TypeMatcher protocol to write your one custom type matcher. For an example, see the Basic Usage section below.

Basic Usage

The examples below assume we are mocking this class:

class ExampleCollaborator {

    func voidFunction() {
    }

    func function(int: Int, _ string: String) -> String {
      return ""
    }

    func stringDictFunction(dict: [String: String]) -> String {
      return ""
    }

}

In your test code, you'll need to create a MockExampleCollaborator, which extends ExampleCollaborator and adopts Mock. The mock creates a CallHandler, and forwards all calls to it:

class MockExampleCollaborator: ExampleCollaborator, Mock {

    let callHandler: CallHandler

    init(testCase: XCTestCase) {
      callHandler = CallHandlerImpl(withTestCase: testCase)
    }

    func instanceType() -> MockExampleCollaborator {
      return self
    }

    override func voidFunction() {
      callHandler.accept(nil, ofFunction: #function, atFile: #file, inLine: #line, withArgs: nil)
    }

    override func function(int: Int, _ string: String) -> String {
      return callHandler.accept("", ofFunction: #function, atFile: #file, inLine: #line, withArgs: int, string) as! String
    }

    override func stringDictFunction(dict: Dictionary<String, String>) -> String {
      return callHandler.accept("", ofFunction: #function, atFile: #file, inLine: #line, withArgs: dict) as! String
    }

}

To write a custom type matcher:

public class CustomMatcher: TypeMatcher {

    public func match(argument arg: Any, withArgument withArg: Any) -> Bool {
      switch (arg, withArg) {
        case ( _ as CustomType, _ as CustomType):
          // custom matching code here
          return true
        default:
          return false
      }
    }

}

It is good practice to put the mock objects and custom type matchers in a separate group in the test part of your project.

Currently-supported syntax

// stub a call on a method with parameters, then return value
mockCollaborator.when().call(withReturnValue: mockCollaborator.function(42, "frood")).thenReturn("hoopy")
// stub a call on a method with dictionary parameter, then answer value
mockCollaborator.when().call(withReturnValue: mockCollaborator.stringDictFunction(["Hello": "Pong"])).thenAnswer {
      (args: [Any?]) -> String in

      // custom code here
    }
// stub a call on a void method , then do action
mockCollaborator.when().call(withReturnValue: mockCollaborator.voidFunction()).thenDo {
      (args: [Any?]) -> Void in

      // if the call is received, this closure will be executed
      print("===== thenDo closure called =====")
    }
// stub a call on a method , then return values on multiple calls
mockCollaborator.when().call(withReturnValue: mockCollaborator.stringDictFunction(["Hello": "Pong"])).thenReturn("ping", "hoopy")
// stub a call on a method , then chain multiple actions for corresponding calls
mockCollaborator.when().call(withReturnValue: mockCollaborator.stringDictFunction(["Hello": "Pong"])).thenReturn("ping", "hoopy").thenAnswer {
      (args: [Any?]) -> String in

      // custom code here
    }
// call a method and then get arguments of a specific call which can be asserted later
systemUnderTest.doSomethingWithParamters(42, "frood")
systemUnderTest.doSomethingWithParamters(18, "hoopy")

let argumentsOfFirstCall = mockCollaborator.getArgs(callOrder: 1).of(mockCollaborator.function(AnyValue.int, AnyValue.string))
let argumentsOfSecondCall = mockCollaborator.getArgs(callOrder: 2).of(mockCollaborator.function(AnyValue.int, AnyValue.string))

With nice mocks, Mockit supports the "verify expectations after mocking" style using 8 supported verification modes

// call method on the system under test
systemUnderTest.expectMethodOneAndThree()

// Then
mockCollaborator.verify(verificationMode: Once()).methodOne()
mockCollaborator.verify(verificationMode: Never()).methodTwo()
mockCollaborator.verify(verificationMode: Once()).methodThree()


// call method on the system under test
sut.expectMethodOneTwice()

// Then
mockCollaborator.verify(verificationMode: Times(times: 2)).methodOne()


// call method on the system under test
sut.expectOnlyMethodThree()

// Then
mockCollaborator.verify(verificationMode: Only()).methodThree()


// call method on system under test
sut.expectAllThreeMethods()

// Then
mockCollaborator.verify(verificationMode: Once()).methodOne()
mockCollaborator.verify(verificationMode: AtLeastOnce()).methodTwo()
mockCollaborator.verify(verificationMode: AtMostOnce()).methodThree()


// call method on the system under test
sut.expectMethodTwoAndThree()

// Then
mockCollaborator.verify(verificationMode: AtLeastTimes(times: Times(times: 1))).methodTwo()
mockCollaborator.verify(verificationMode: Never()).methodOne()
mockCollaborator.verify(verificationMode: AtMostTimes(times: Times(times: 3))).methodThree()

Requirements

  • Xcode 9+
  • XCTest

Installation

Mockit is built with Swift 5.0.

CocoaPods

Mockit is available through CocoaPods. To install it, simply add the following line to your Podfile:

pod 'Mockit', '1.5.0'

Manually

  1. Download and drop /Mockit folder in your project.
  2. Congratulations!

Feedback

Issues and pull-requests are most welcome - especially about improving the API further.

Author

Syed Sabir Salman-Al-Musawi, sabirvirtuoso@gmail.com

I'd also like to thank Sharafat Ibn Mollah Mosharraf for his big support during the API design and development phase.

License

Mockit is available under the MIT license. See the LICENSE file for more info.

The PNG at the top of this README.md is from www.mockit.co.uk/about.html


Download Details:

Author: sabirvirtuoso
Source code: https://github.com/sabirvirtuoso/Mockit

License: MIT license
#swift 

 iOS App Dev

iOS App Dev

1653910080

Xcprofiler: CLI to Profile Compilation Time Of Swift Project

xcprofiler

Command line utility to profile compilation time of Swift project.

This tool is developed in working time for Cookpad.

Installation

gem install xcprofiler

xcprofiler is tested on latest Ruby 2.3/2.4.

Usage

Add -Xfrontend -debug-time-function-bodies build flags in Build Settings -> Other Swift Flags section of your Xcode project.

Build your project

Execute xcprofiler

$ xcprofiler [PRODUCT_NAME or ACTIVITY_LOG_PATH] [options]

xcprofiler searches the latest build log on your DerivedData directory.

You can also specify the .xcactivitylog.

$ xcprofiler MyApp
$ xcprofiler ~/Library/Developer/Xcode/DerivedData/MyApp-xxxxxxxxxxx/Logs/Build/0761C73D-3B6C-449A-BE89-6D11DAB748FE.xcactivitylog

Sample output is here

+----------------------+------+---------------------------------------------------------------------------------------------------------------------------------------------------------------+----------+
| File                 | Line | Method name                                                                                                                                                   | Time(ms) |
+----------------------+------+---------------------------------------------------------------------------------------------------------------------------------------------------------------+----------+
| ResultProtocol.swift | 132  | public func ==<T : ResultProtocol where T.Value : Equatable, T.Error : Equatable>(left: T, right: T) -> Bool                                                  | 14.2     |
| Result.swift         | 66   | get {}                                                                                                                                                        | 13.1     |
| Result.swift         | 78   | public static func error(_ message: String? = default, function: String = #function, file: String = #file, line: Int = #line) -> NSError                      | 6.3      |
| Result.swift         | 69   | get {}                                                                                                                                                        | 2.2      |
| Result.swift         | 132  | public func `try`<T>(_ function: String = #function, file: String = #file, line: Int = #line, try: (NSErrorPointer) -> T?) -> Result<T, NSError>              | 1.7      |
| Result.swift         | 95   | get {}                                                                                                                                                        | 1.4      |
| Result.swift         | 21   | public init(_ value: T?, failWith: @autoclosure () -> Error)                                                                                                  | 0.9      |
| Result.swift         | 142  | public func `try`(_ function: String = #function, file: String = #file, line: Int = #line, try: (NSErrorPointer) -> Bool) -> Result<(), NSError>              | 0.9      |
| ResultProtocol.swift | 172  | @available(*, unavailable, renamed: "recover(with:)") public func recoverWith(_ result: @autoclosure () -> Self) -> Self                                      | 0.7      |
| Result.swift         | 72   | get {}                                                                                                                                                        | 0.6      |
| Result.swift         | 75   | get {}                                                                                                                                                        | 0.6      |
| ResultProtocol.swift | 72   | public func recover(_ value: @autoclosure () -> Value) -> Value                                                                                               | 0.5      |
| ResultProtocol.swift | 111  | public func &&&<L : ResultProtocol, R : ResultProtocol where L.Error == R.Error>(left: L, right: @autoclosure () -> R) -> Result<(L.Value, R.Value), L.Error> | 0.5      |
| ResultProtocol.swift | 144  | public func !=<T : ResultProtocol where T.Value : Equatable, T.Error : Equatable>(left: T, right: T) -> Bool                                                  | 0.5      |
| ResultProtocol.swift | 92   | public func tryMap<U>(_ transform: (Value) throws -> U) -> Result<U, Error>                                                                                   | 0.4      |
| Result.swift         | 175  | @available(*, unavailable, renamed: "success") public static func Success(_: T) -> Result<T, Error>                                                           | 0.3      |
| ResultProtocol.swift | 55   | public func mapError<Error2>(_ transform: (Error) -> Error2) -> Result<Value, Error2>                                                                         | 0.3      |
| ResultProtocol.swift | 77   | public func recover(with result: @autoclosure () -> Self) -> Self                                                                                             | 0.3      |
| ResultProtocol.swift | 93   | (closure)                                                                                                                                                     | 0.3      |
| Result.swift         | 31   | public init(attempt f: () throws -> T)                                                                                                                        | 0.2      |
+----------------------+------+---------------------------------------------------------------------------------------------------------------------------------------------------------------+----------+

Available Options

optionshorthanddescription
--limit-lLimit for display
--threshold Threshold of time to display (ms)
--show-invalids Show invalid location results
--order-oSort order (default,time,file)
--derived-data-path Root path of DerivedData directory
--truncate-at-tTruncate the method name with specified length
--no-unique Show the duplicated results

Use custom reporters

You can use reporters to output tracking logs.

require 'xcprofiler'

profiler = Xcprofiler::Profiler.by_product_name('MyApp')
profiler.reporters = [
  Xcprofiler::StandardOutputReporter.new(limit: 20, order: :time),
  Xcprofiler::JSONReporter.new(output_path: 'result.json'),
  Xcprofiler::BlockReporter.new do |executions|
    do_something(executions)
  end,
]
profiler.report!

You can also implement your own reporters.

See implementation of built-in reporters for detail.

danger-xcprofiler

You can integrate xcprofiler to danger.

https://github.com/giginet/danger-xcprofiler

Demo

Download Details:
Author: giginet
Source Code: https://github.com/giginet/xcprofiler
License: MIT license

#swift  #ios  #mobileapp