Jackson  Watson

Jackson Watson

1625623980

Write your first Line of Code, Right Now - Beyond Code Live 000

What questions do you have about learning to code?
Comment below. I’ll be answering in daily FB Lives.
Original post: https://www.facebook.com/beyondcodebootcamp/posts/269019734684723

#code #line of code

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Write your first Line of Code, Right Now - Beyond Code Live 000
Jackson  Watson

Jackson Watson

1625623980

Write your first Line of Code, Right Now - Beyond Code Live 000

What questions do you have about learning to code?
Comment below. I’ll be answering in daily FB Lives.
Original post: https://www.facebook.com/beyondcodebootcamp/posts/269019734684723

#code #line of code

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 you’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 🎉

(However, be careful, I’m definitely not advocating inheritance as a go-to solution 🙃)

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() // ❌ 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("🎉 \(result)")
    }
}

var consumer: Consumer? = Consumer()

consumer?.consume()

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

// This code prints:
// 🎉 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("🎉 \(arg)", nil)
}

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

chainedServices({ result, _ in
    guard let result = result else { return }
    
    print(result) // Prints: 🎉 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("🎉 \(arg)")
}

let syncFunction = makeSynchrone(myAsyncFunction)

print(syncFunction(42)) // prints 🎉 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 👌

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 🍰

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 Benoît 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 👌

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 Benoît 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) }

🚨 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 🎉

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 2π 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)

🚨 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, it’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

⚠️ 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

📣 NEW 📣 Swift Tips are now available on YouTube 👇

Summary

Tips


Download Details:

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

License: MIT license
#swift 

How to Create a CSS-Only Loader Using One Element

If you have a website, it's helpful to have a loader so users can tell something is happening once they've clicked a link or button.

You can use this loader component in a lot of places, and it should be as simple as possible.

In this post, we will see how to build two types of loaders with only one <div> and a few lines of CSS code. Not only this but we will make them customizable so you can easily create different variations from the same code.

Here's what we'll build:

CSS-only Spinner and Progress Loader

CSS-only Spinner and Progress Loader

How to Create a Spinner Loader

Below is a demo of what we are building:

https://codepen.io/t_afif/pen/PoJyaNy

 <div class="loader"></div>
 <div class="loader" style="--b: 15px;--c: blue;width: 120px;--n: 8"></div>
 <div class="loader" style="--b: 5px;--c: green;width: 80px;--n: 6;--g: 20deg"></div>
 <div class="loader" style="--b: 20px;--c: #000;width: 80px;--n: 15;--g: 7deg"></div> 
 .loader {
   --b: 10px;  /* border thickness */
   --n: 10;    /* number of dashes*/
   --g: 10deg; /* gap between dashes*/
   --c: red;   /* the color */

   width: 100px; /* size */
   aspect-ratio: 1;
   border-radius: 50%;
   padding: 1px;
   background: conic-gradient(#0000,var(--c)) content-box;
   -webkit-mask:
     repeating-conic-gradient(#0000 0deg,
        #000 1deg calc(360deg/var(--n) - var(--g) - 1deg),
        #0000     calc(360deg/var(--n) - var(--g)) calc(360deg/var(--n))),
     radial-gradient(farthest-side,#0000 calc(98% - var(--b)),#000 calc(100% - var(--b)));
           mask:
     repeating-conic-gradient(#0000 0deg,
        #000 1deg calc(360deg/var(--n) - var(--g) - 1deg),
        #0000     calc(360deg/var(--n) - var(--g)) calc(360deg/var(--n))),
     radial-gradient(farthest-side,#0000 calc(98% - var(--b)),#000 calc(100% - var(--b)));
   -webkit-mask-composite: destination-in;
           mask-composite: intersect;
   animation: load 1s infinite steps(var(--n));
 }
 @keyframes load {to{transform: rotate(1turn)}}

We have 4 different loaders using the same code. By only changing a few variables, we can generate a new loader without needing to touch the CSS code.

The variables are defined like below:

  • --b defines the border thickness.
  • --n  defines the number of dashes.
  • --g defines the gap between dashes. Since we're dealing with a circular element, this one is an angle value.
  • --c defines the color.

Here is an illustration to see the different variables.

CSS Variables of the Spinner loader

CSS Variables of the Spinner loader

Let's tackle the CSS code. We will use another figure to illustrate a step-by-step construction of the loader.

Step-by-Step illustration of the Spinner Loader

Step-by-Step illustration of the Spinner Loader

We first start by creating a circle like this:

.loader {
  width: 100px; /* size */
  aspect-ratio: 1;
  border-radius: 50%;
}

Nothing complex so far. Note the use of aspect-ratio which allows us to only modify one value (the width) in order to control the size.

Then we add a conic gradient coloration from transparent to the defined color (the variable --c):

.loader {
  width:100px; /* size */
  aspect-ratio: 1;
  border-radius: 50%;
  background: conic-gradient(#0000,var(--c));
}

In this step, we introduce the mask property to hide some parts of the circle in a repetitive manner. This will depend on the --n and --d variables. If you look closely at the figure, we will notice the following pattern:

visible part
invisible part
visible part
invisible part
etc

To do this, we use repeating-conic-gradient(#000 0 X, #0000 0 Y). From 0 to X we have an opaque color (visible part) and from X to Y we have a transparent one (invisible part).

We introduce our variables:

  • We need a gap equal to g between each visible part so the formula between X and Y will be X = Y - g.
  • We need n visible part so the formula of Y should be Y = 360deg/n. A full circle is 360deg so we simply divide it by n

Our code so far is:

.loader {
  width: 100px; /* size */
  aspect-ratio: 1;
  border-radius: 50%;
  background: conic-gradient(#0000,var(--c));
  mask: repeating-conic-gradient(#000 0 calc(360deg/var(--n) - var(--g)) , #0000 0 calc(360deg/var(--n))
}

This next step is the trickiest one, because we need to apply another mask to create a kind of hole in order to get the final shape. To do this we will logically use a radial-gradient() with our variable b:

radial-gradient(farthest-side,#0000 calc(100% - var(--b)),#000 0)

A full circle from where we remove a thickness equal to b.

We add this to the previous mask:

.loader {
  width: 100px; /* size */
  aspect-ratio: 1;
  border-radius: 50%;
  background: conic-gradient(#0000,var(--c));
  mask: 
   radial-gradient(farthest-side,#0000 calc(100% - var(--b)),#000 0),
   repeating-conic-gradient(#000 0 calc(360deg/var(--n) - var(--g)) , #0000 0 calc(360deg/var(--n))
}

We have two mask layers, but the result is not what we want. We get the following:

It may look strange but it's logical. The "final" visible part is nothing but the sum of each visible part of each mask layer. We can change this behavior using mask-composite. I would need a whole article to explain this property so I will simply give the value.

In our case, we need to consider intersect (and destination-out for the prefixed property). Our code will become:

.loader {
  width: 100px; /* size */
  aspect-ratio: 1;
  border-radius: 50%;
  background: conic-gradient(#0000,var(--c));
  mask: 
    radial-gradient(farthest-side,#0000 calc(100% - var(--b)),#000 0),
    repeating-conic-gradient(#000 0 calc(360deg/var(--n) - var(--g)) , #0000 0 calc(360deg/var(--n));
  -webkit-mask-composite: destination-in;
          mask-composite: intersect;
}

We are done with the shape! We are only missing the animation. The latter is an infinite rotation.

The only thing to note is that I am using a steps animation to create the illusion of fixed dashes and moving colors.

Here is an illustration to see the difference

A Linear Animation vs a Steps Animation

The first one is a linear and continuous rotation of the shape (not what we want) and the second one is a discrete animation (the one we want).

Here is the full code including the animation:

 <div class="loader"></div>
 <div class="loader" style="--b: 15px;--c: blue;width: 120px;--n: 8"></div>
 <div class="loader" style="--b: 5px;--c: green;width: 80px;--n: 6;--g: 20deg"></div>
 <div class="loader" style="--b: 20px;--c: #000;width: 80px;--n: 15;--g: 7deg"></div> 
 .loader {
   --b: 10px;  /* border thickness */
   --n: 10;    /* number of dashes*/
   --g: 10deg; /* gap between dashes*/
   --c: red;   /* the color */

   width: 100px; /* size */
   aspect-ratio: 1;
   border-radius: 50%;
   padding: 1px;
   background: conic-gradient(#0000,var(--c)) content-box;
   -webkit-mask:
     repeating-conic-gradient(#0000 0deg,
        #000 1deg calc(360deg/var(--n) - var(--g) - 1deg),
        #0000     calc(360deg/var(--n) - var(--g)) calc(360deg/var(--n))),
     radial-gradient(farthest-side,#0000 calc(98% - var(--b)),#000 calc(100% - var(--b)));
           mask:
     repeating-conic-gradient(#0000 0deg,
        #000 1deg calc(360deg/var(--n) - var(--g) - 1deg),
        #0000     calc(360deg/var(--n) - var(--g)) calc(360deg/var(--n))),
     radial-gradient(farthest-side,#0000 calc(98% - var(--b)),#000 calc(100% - var(--b)));
   -webkit-mask-composite: destination-in;
           mask-composite: intersect;
   animation: load 1s infinite steps(var(--n));
 }
 @keyframes load {to{transform: rotate(1turn)}}

You will notice a few differences with the code I used in the explanation:

  • I am adding padding: 1px and setting the background to content-box
  • There is +/1deg between the colors of the repeating-conic-gradient()
  • There are a few percentages of difference between the color inside the radial-gradient()

Those are some corrections to avoid visual glitches. Gradients are known to produce "strange" results in some cases so we have to adjust some values manually to avoid them.

How to Create a Progress Loader

Like the previous one loader, let's start with an overview:

https://codepen.io/t_afif/pen/bGoNddg

 <div class="loader"></div>
 <div class="loader" style="--s:10px;--n:10;color:red"></div>
 <div class="loader" style="--g:0px;color:darkblue"></div>
 <div class="loader" style="--s:25px;--g:8px;border-radius:50px;color:green"></div>
 .loader {
   --n:5;    /* control the number of stripes */
   --s:30px; /* control the width of stripes */
   --g:5px;  /* control the gap between stripes */

   width:calc(var(--n)*(var(--s) + var(--g)) - var(--g));
   height:30px;
   padding:var(--g);
   margin:5px auto;
   border:1px solid;
   background:
     repeating-linear-gradient(90deg,
       currentColor  0 var(--s),
       #0000 0 calc(var(--s) + var(--g))
     ) left / calc((var(--n) + 1)*(var(--s) + var(--g))) 100% 
     no-repeat content-box;
   animation: load 1.5s steps(calc(var(--n) + 1)) infinite;
 }
 @keyframes load {
   0% {background-size: 0% 100%}
 }

We have the same configuration as the previous loader. CSS variables that control the loader:

  • --n defines the number of dashes/stripes.
  • --s defines the width of each stripe.
  • --g defines the gap between stripes.

Illustration of the CSS Variables

Illustration of the CSS Variables

From the above figure we can see that the width of the element will depend on the 3 variables. The CSS will be as follows:

.loader {
  width: calc(var(--n)*(var(--s) + var(--g)) - var(--g));
  height: 30px; /* use any value you want here */
  padding: var(--g);
  border: 1px solid;
}

We use padding to set the gap on each side. Then the width will be equal to the number of stripes multiplied by their width and the gap. We remove one gap because for N stripes we have N-1 gaps.

To create the stripes we will use the below gradient.

repeating-linear-gradient(90deg,
  currentColor 0 var(--s),
  #0000        0 calc(var(--s) + var(--g))
 )

From 0 to s is the defined color and from s to s + g a transparent color (the gap).

I am using currentColor which is the value of the color property. Note that I didn't define any color inside border so it will also use to the value of color. If we want to change the color of the loader, we only need to set the color property.

Our code so far:

.loader {
  width: calc(var(--n)*(var(--s) + var(--g)) - var(--g));
  height: 30px;
  padding: var(--g);
  border: 1px solid;
  background:
    repeating-linear-gradient(90deg,
      currentColor  0 var(--s),
      #0000 0 calc(var(--s) + var(--g))
    ) left / 100% 100% content-box no-repeat;
}

I am using content-box to make sure the gradient doesn't cover the padding area. Then I define a size equal to 100% 100% and a left position.

It's time for the animation. For this loader, we will animate the background-size from 0% 100% to 100% 100% which means the width of our gradient from 0%  to 100%

Like the previous loader, we will rely on steps() to have a discrete animation instead of a continuous one.

A Linear Animation vs a Steps Animation

The second one is what we want to create, and we can achieve it by adding the following code:

.loader {
  animation: load 1.5s steps(var(--n)) infinite;
}
@keyframes load {
  0% {background-size: 0% 100%}
}

If you look closely at the last figure, you will notice that the animation is not complete. We are missing one stripe at the end, even if we have used N. This is not a bug but how steps() is supposed to work.

To overcome this, we need to add an extra step. We increase the background-size of our gradient to contain N+1 stripes and use steps(N+1). This will get us to the final code:

.loader {
  width: calc(var(--n)*(var(--s) + var(--g)) - var(--g));
  height: 30px;
  padding: var(--g);
  margin: 5px auto;
  border: 1px solid;
  background:
    repeating-linear-gradient(90deg,
      currentColor  0 var(--s),
      #0000 0 calc(var(--s) + var(--g))
    ) left / calc((var(--n) + 1)*(var(--s) + var(--g))) 100% 
    content-box no-repeat;
  animation: load 1.5s steps(calc(var(--n) + 1)) infinite;
}
@keyframes load {
  0% {background-size: 0% 100%}
}

Note that the width of the gradient is equal to N+1 multiplied by the width of one stripe and a gap (instead of being 100% )

Conclusion

I hope you enjoyed this tutorial. If you are interested, I have made more than 500 CSS-only single div loaders. I also wrote another tutorial to explain how to create the Dots loader using only background properties.

Find below useful links to get more detail about some properties I have used that I didn't explain thoroughly due to their complexity:

Thank you for reading!

Link: https://www.freecodecamp.org/news/how-to-create-a-css-only-loader/

#css 

1つの要素を使用してCSSのみのローダーを作成する

Webサイトがある場合は、ローダーを使用すると、ユーザーがリンクまたはボタンをクリックすると何かが起こっていることを知ることができるので便利です。

このローダーコンポーネントは多くの場所で使用でき、可能な限りシンプルにする必要があります。

<div>この投稿では、1行と数行のCSSコードで2種類のローダーを構築する方法を説明します。これだけでなく、同じコードからさまざまなバリエーションを簡単に作成できるようにカスタマイズできるようにします。

これが私たちが構築するものです:

CSSのみのスピナーとプログレスローダー

CSSのみのスピナーとプログレスローダー

スピナーローダーを作成する方法

以下は、私たちが構築しているもののデモです。

https://codepen.io/t_afif/pen/PoJyaNy

 <div class="loader"></div>
 <div class="loader" style="--b: 15px;--c: blue;width: 120px;--n: 8"></div>
 <div class="loader" style="--b: 5px;--c: green;width: 80px;--n: 6;--g: 20deg"></div>
 <div class="loader" style="--b: 20px;--c: #000;width: 80px;--n: 15;--g: 7deg"></div> 
 .loader {
   --b: 10px;  /* border thickness */
   --n: 10;    /* number of dashes*/
   --g: 10deg; /* gap between dashes*/
   --c: red;   /* the color */

   width: 100px; /* size */
   aspect-ratio: 1;
   border-radius: 50%;
   padding: 1px;
   background: conic-gradient(#0000,var(--c)) content-box;
   -webkit-mask:
     repeating-conic-gradient(#0000 0deg,
        #000 1deg calc(360deg/var(--n) - var(--g) - 1deg),
        #0000     calc(360deg/var(--n) - var(--g)) calc(360deg/var(--n))),
     radial-gradient(farthest-side,#0000 calc(98% - var(--b)),#000 calc(100% - var(--b)));
           mask:
     repeating-conic-gradient(#0000 0deg,
        #000 1deg calc(360deg/var(--n) - var(--g) - 1deg),
        #0000     calc(360deg/var(--n) - var(--g)) calc(360deg/var(--n))),
     radial-gradient(farthest-side,#0000 calc(98% - var(--b)),#000 calc(100% - var(--b)));
   -webkit-mask-composite: destination-in;
           mask-composite: intersect;
   animation: load 1s infinite steps(var(--n));
 }
 @keyframes load {to{transform: rotate(1turn)}}

同じコードを使用する4つの異なるローダーがあります。いくつかの変数を変更するだけで、CSSコードに触れることなく新しいローダーを生成できます。

変数は次のように定義されます。

  • --b 境界線の太さを定義します。
  • --n  ãƒ€ãƒƒã‚·ãƒ¥ã®æ•°ã‚’定義します。
  • --gダッシュ間のギャップを定義します。円形の要素を扱っているので、これは角度の値です。
  • --c 色を定義します。

これは、さまざまな変数を確認するための図です。

スピナーローダーのCSS変数

スピナーローダーのCSS変数

CSSコードに取り組みましょう。別の図を使用して、ローダーの段階的な構成を説明します。

スピナーローダーのステップバイステップの図

スピナーローダーのステップバイステップの図

まず、次のような円を作成します。

.loader {
  width: 100px; /* size */
  aspect-ratio: 1;
  border-radius: 50%;
}

これまでのところ複雑なことはありません。これを使用すると、サイズを制御するためにaspect-ratio1つの値()のみを変更できることに注意してください。width

次に、透明から定義された色(変数--c)に円錐曲線の色を追加します。

.loader {
  width:100px; /* size */
  aspect-ratio: 1;
  border-radius: 50%;
  background: conic-gradient(#0000,var(--c));
}

このステップでmaskは、円の一部を繰り返し非表示にするプロパティを紹介します。--nこれはと--d変数に依存します。図をよく見ると、次のパターンに気付くでしょう。

visible part
invisible part
visible part
invisible part
etc

これを行うには、を使用しますrepeating-conic-gradient(#000 0 X, #0000 0 Y)。から0までXは不透明な色(可視部分)があり、からXまでYは透明な色(不可視部分)があります。

変数を紹介します。

  • との間の式がになるgように、各可視部分の間に等しいギャップが必要です。XYX = Y - g
  • n目に見える部分が必要なので、の式YはY = 360deg/nです。完全な円は360deg、単純にで割ったものです。n

これまでのコードは次のとおりです。

.loader {
  width: 100px; /* size */
  aspect-ratio: 1;
  border-radius: 50%;
  background: conic-gradient(#0000,var(--c));
  mask: repeating-conic-gradient(#000 0 calc(360deg/var(--n) - var(--g)) , #0000 0 calc(360deg/var(--n))
}

この次のステップは最も難しいステップです。最終的な形状を取得するために、別のマスクを適用して一種の穴を作成する必要があるためです。これを行うにはradial-gradient()、変数で論理的にaを使用しますb。

radial-gradient(farthest-side,#0000 calc(100% - var(--b)),#000 0)

に等しい厚さを削除するところから完全な円b。

これを前のマスクに追加します。

.loader {
  width: 100px; /* size */
  aspect-ratio: 1;
  border-radius: 50%;
  background: conic-gradient(#0000,var(--c));
  mask: 
   radial-gradient(farthest-side,#0000 calc(100% - var(--b)),#000 0),
   repeating-conic-gradient(#000 0 calc(360deg/var(--n) - var(--g)) , #0000 0 calc(360deg/var(--n))
}

2つのマスクレイヤーがありますが、結果は私たちが望むものではありません。次のようになります。

奇妙に見えるかもしれませんが、それは論理的です。「最終的な」可視部分は、各マスクレイヤーの各可視部分の合計に他なりません。この動作は、を使用して変更できますmask-composite。このプロパティを説明するために記事全体が必要になるので、単純に値を示します。

intersect私たちの場合、 (そしてdestination-out接頭辞付きのプロパティについて)考慮する必要があります。コードは次のようになります。

.loader {
  width: 100px; /* size */
  aspect-ratio: 1;
  border-radius: 50%;
  background: conic-gradient(#0000,var(--c));
  mask: 
    radial-gradient(farthest-side,#0000 calc(100% - var(--b)),#000 0),
    repeating-conic-gradient(#000 0 calc(360deg/var(--n) - var(--g)) , #0000 0 calc(360deg/var(--n));
  -webkit-mask-composite: destination-in;
          mask-composite: intersect;
}

形が出来上がりました!アニメーションが欠けているだけです。後者は無限回転です。

注意すべき唯一のことは、stepsアニメーションを使用して、固定されたダッシュと動く色の錯覚を作成しているということです。

これが違いを見るためのイラストです

線形アニメーションとステップアニメーション

最初のものは形状の線形で連続的な回転であり(私たちが望むものではありません)、2番目のものは離散アニメーション(私たちが望むもの)です。

アニメーションを含む完全なコードは次のとおりです。

 <div class="loader"></div>
 <div class="loader" style="--b: 15px;--c: blue;width: 120px;--n: 8"></div>
 <div class="loader" style="--b: 5px;--c: green;width: 80px;--n: 6;--g: 20deg"></div>
 <div class="loader" style="--b: 20px;--c: #000;width: 80px;--n: 15;--g: 7deg"></div> 
 .loader {
   --b: 10px;  /* border thickness */
   --n: 10;    /* number of dashes*/
   --g: 10deg; /* gap between dashes*/
   --c: red;   /* the color */

   width: 100px; /* size */
   aspect-ratio: 1;
   border-radius: 50%;
   padding: 1px;
   background: conic-gradient(#0000,var(--c)) content-box;
   -webkit-mask:
     repeating-conic-gradient(#0000 0deg,
        #000 1deg calc(360deg/var(--n) - var(--g) - 1deg),
        #0000     calc(360deg/var(--n) - var(--g)) calc(360deg/var(--n))),
     radial-gradient(farthest-side,#0000 calc(98% - var(--b)),#000 calc(100% - var(--b)));
           mask:
     repeating-conic-gradient(#0000 0deg,
        #000 1deg calc(360deg/var(--n) - var(--g) - 1deg),
        #0000     calc(360deg/var(--n) - var(--g)) calc(360deg/var(--n))),
     radial-gradient(farthest-side,#0000 calc(98% - var(--b)),#000 calc(100% - var(--b)));
   -webkit-mask-composite: destination-in;
           mask-composite: intersect;
   animation: load 1s infinite steps(var(--n));
 }
 @keyframes load {to{transform: rotate(1turn)}}

説明で使用したコードとの違いに気付くでしょう。

  • padding: 1px背景を追加して設定していますcontent-box
  • +/1degの色の間にありますrepeating-conic-gradient()
  • 内側の色には数パーセントの違いがあります radial-gradient()

これらは、視覚的な不具合を回避するためのいくつかの修正です。グラデーションは「奇妙な」結果を生成することが知られているため、それらを回避するためにいくつかの値を手動で調整する必要があります。

プログレスローダーを作成する方法

前のローダーと同様に、概要から始めましょう。

https://codepen.io/t_afif/pen/bGoNddg

 <div class="loader"></div>
 <div class="loader" style="--s:10px;--n:10;color:red"></div>
 <div class="loader" style="--g:0px;color:darkblue"></div>
 <div class="loader" style="--s:25px;--g:8px;border-radius:50px;color:green"></div>
 .loader {
   --n:5;    /* control the number of stripes */
   --s:30px; /* control the width of stripes */
   --g:5px;  /* control the gap between stripes */

   width:calc(var(--n)*(var(--s) + var(--g)) - var(--g));
   height:30px;
   padding:var(--g);
   margin:5px auto;
   border:1px solid;
   background:
     repeating-linear-gradient(90deg,
       currentColor  0 var(--s),
       #0000 0 calc(var(--s) + var(--g))
     ) left / calc((var(--n) + 1)*(var(--s) + var(--g))) 100% 
     no-repeat content-box;
   animation: load 1.5s steps(calc(var(--n) + 1)) infinite;
 }
 @keyframes load {
   0% {background-size: 0% 100%}
 }

以前のローダーと同じ構成になっています。ローダーを制御するCSS変数:

  • --n ダッシュ/ストライプの数を定義します。
  • --s 各ストライプの幅を定義します。
  • --g ストライプ間のギャップを定義します。

CSS変数の図

CSS変数の図

上の図から、要素の幅が3つの変数に依存することがわかります。CSSは次のようになります。

.loader {
  width: calc(var(--n)*(var(--s) + var(--g)) - var(--g));
  height: 30px; /* use any value you want here */
  padding: var(--g);
  border: 1px solid;
}

padding両側にギャップを設定するために使用します。その場合、幅はストライプの数に幅とギャップを掛けたものに等しくなります。Nストライプにはギャップがあるため、1つのギャップを削除しN-1ます。

ストライプを作成するには、以下のグラデーションを使用します。

repeating-linear-gradient(90deg,
  currentColor 0 var(--s),
  #0000        0 calc(var(--s) + var(--g))
 )

From 0tosは定義された色であり、from stos + gは透明色(ギャップ)です。

currentColorプロパティの値であるwhichを使用していcolorます。内部に色を定義しなかったためborder、の値にも使用されることに注意してくださいcolor。ローダーの色を変更したい場合は、colorプロパティを設定するだけです。

これまでのコード:

.loader {
  width: calc(var(--n)*(var(--s) + var(--g)) - var(--g));
  height: 30px;
  padding: var(--g);
  border: 1px solid;
  background:
    repeating-linear-gradient(90deg,
      currentColor  0 var(--s),
      #0000 0 calc(var(--s) + var(--g))
    ) left / 100% 100% content-box no-repeat;
}

content-boxグラデーションがパディング領域をカバーしないようにするために使用しています。100% 100%次に、左の位置に等しいサイズを定義します。

アニメーションの時間です。このローダーでは、background-sizefrom 0% 100%toをアニメーション化します。これは、fromから  toへ100% 100%のグラデーションの幅を意味します。0%100%

steps()以前のローダーと同様に、連続的なアニメーションではなく、個別のアニメーションを使用することに依存します。

線形アニメーションとステップアニメーション

2つ目は作成したいもので、次のコードを追加することで実現できます。

.loader {
  animation: load 1.5s steps(var(--n)) infinite;
}
@keyframes load {
  0% {background-size: 0% 100%}
}

最後の図をよく見ると、アニメーションが完全ではないことがわかります。を使用したとしても、最後に1つのストライプがありませんN。これはバグではありませんが、どのように機能するsteps()はずです。

これを克服するには、追加のステップを追加する必要があります。background-sizeグラデーションを増やしてN+1ストライプを含め、を使用しますsteps(N+1)。これにより、最終的なコードが表示されます。

.loader {
  width: calc(var(--n)*(var(--s) + var(--g)) - var(--g));
  height: 30px;
  padding: var(--g);
  margin: 5px auto;
  border: 1px solid;
  background:
    repeating-linear-gradient(90deg,
      currentColor  0 var(--s),
      #0000 0 calc(var(--s) + var(--g))
    ) left / calc((var(--n) + 1)*(var(--s) + var(--g))) 100% 
    content-box no-repeat;
  animation: load 1.5s steps(calc(var(--n) + 1)) infinite;
}
@keyframes load {
  0% {background-size: 0% 100%}
}

グラデーションの幅は、N+1(ではなく100%) 1つのストライプとギャップの幅を掛けたものに等しいことに注意してください。

結論

このチュートリアルを楽しんでいただけたでしょうか。興味があれば、私は500以上のCSSのみのシングルdivローダーを作成しました。また、バックグラウンドプロパティのみを使用してドットローダーを作成する方法を説明する別のチュートリアルを作成しました。

以下の便利なリンクを見つけて、複雑さのために完全には説明しなかった、私が使用したいくつかのプロパティの詳細を確認してください。

読んでくれてありがとう!

リンク:https ://www.freecodecamp.org/news/how-to-create-a-css-only-loader/

#css 

Cómo Crear Un Cargador De Solo CSS Usando Un Elemento

Si tiene un sitio web, es útil tener un cargador para que los usuarios puedan saber que algo está sucediendo una vez que hayan hecho clic en un enlace o botón.

Puede usar este componente del cargador en muchos lugares y debería ser lo más simple posible.

En esta publicación, veremos cómo construir dos tipos de cargadores con solo una <div>y unas pocas líneas de código CSS. No solo esto, sino que los haremos personalizables para que pueda crear fácilmente diferentes variaciones del mismo código.

Esto es lo que construiremos:

Spinner y Progress Loader solo para CSS

Spinner y Progress Loader solo para CSS

Cómo crear un cargador giratorio

A continuación se muestra una demostración de lo que estamos construyendo:

https://codepen.io/t_afif/pen/PoJyaNy

 <div class="loader"></div>
 <div class="loader" style="--b: 15px;--c: blue;width: 120px;--n: 8"></div>
 <div class="loader" style="--b: 5px;--c: green;width: 80px;--n: 6;--g: 20deg"></div>
 <div class="loader" style="--b: 20px;--c: #000;width: 80px;--n: 15;--g: 7deg"></div> 
 .loader {
   --b: 10px;  /* border thickness */
   --n: 10;    /* number of dashes*/
   --g: 10deg; /* gap between dashes*/
   --c: red;   /* the color */

   width: 100px; /* size */
   aspect-ratio: 1;
   border-radius: 50%;
   padding: 1px;
   background: conic-gradient(#0000,var(--c)) content-box;
   -webkit-mask:
     repeating-conic-gradient(#0000 0deg,
        #000 1deg calc(360deg/var(--n) - var(--g) - 1deg),
        #0000     calc(360deg/var(--n) - var(--g)) calc(360deg/var(--n))),
     radial-gradient(farthest-side,#0000 calc(98% - var(--b)),#000 calc(100% - var(--b)));
           mask:
     repeating-conic-gradient(#0000 0deg,
        #000 1deg calc(360deg/var(--n) - var(--g) - 1deg),
        #0000     calc(360deg/var(--n) - var(--g)) calc(360deg/var(--n))),
     radial-gradient(farthest-side,#0000 calc(98% - var(--b)),#000 calc(100% - var(--b)));
   -webkit-mask-composite: destination-in;
           mask-composite: intersect;
   animation: load 1s infinite steps(var(--n));
 }
 @keyframes load {to{transform: rotate(1turn)}}

Tenemos 4 cargadores diferentes usando el mismo código. Con solo cambiar algunas variables, podemos generar un nuevo cargador sin necesidad de tocar el código CSS.

Las variables se definen como sigue:

  • --b define el grosor del borde.
  • --n  define el número de guiones.
  • --gdefine el espacio entre guiones. Como estamos tratando con un elemento circular, este es un valor de ángulo.
  • --c define el color.

Aquí hay una ilustración para ver las diferentes variables.

Variables CSS del cargador Spinner

Variables CSS del cargador Spinner

Abordemos el código CSS. Usaremos otra figura para ilustrar una construcción paso a paso del cargador.

Ilustración paso a paso del cargador giratorio

Ilustración paso a paso del cargador giratorio

Primero comenzamos creando un círculo como este:

.loader {
  width: 100px; /* size */
  aspect-ratio: 1;
  border-radius: 50%;
}

Nada complejo hasta ahora. Tenga en cuenta que su uso aspect-rationos permite modificar solo un valor (el width) para controlar el tamaño.

Luego agregamos una coloración de degradado cónico de transparente al color definido (la variable --c):

.loader {
  width:100px; /* size */
  aspect-ratio: 1;
  border-radius: 50%;
  background: conic-gradient(#0000,var(--c));
}

En este paso, introducimos la maskpropiedad para ocultar algunas partes del círculo de forma repetitiva. Esto dependerá de las variables --ny . --dSi observa detenidamente la figura, notaremos el siguiente patrón:

visible part
invisible part
visible part
invisible part
etc

Para hacer esto, usamos repeating-conic-gradient(#000 0 X, #0000 0 Y). De 0a Xtenemos un color opaco (parte visible) y de Xa Ytenemos uno transparente (parte invisible).

Introducimos nuestras variables:

  • Necesitamos un espacio igual a gentre cada parte visible por lo que la fórmula entre Xy Yserá X = Y - g.
  • Necesitamos nla parte visible, por lo que la fórmula de Ydebería ser Y = 360deg/n. Un círculo completo es 360degasí que simplemente lo dividimos porn

Nuestro código hasta ahora es:

.loader {
  width: 100px; /* size */
  aspect-ratio: 1;
  border-radius: 50%;
  background: conic-gradient(#0000,var(--c));
  mask: repeating-conic-gradient(#000 0 calc(360deg/var(--n) - var(--g)) , #0000 0 calc(360deg/var(--n))
}

El siguiente paso es el más complicado, porque necesitamos aplicar otra máscara para crear una especie de agujero para obtener la forma final. Para ello usaremos lógicamente a radial-gradient()con nuestra variable b:

radial-gradient(farthest-side,#0000 calc(100% - var(--b)),#000 0)

Un círculo completo del que quitamos un espesor igual a b.

Añadimos esto a la máscara anterior:

.loader {
  width: 100px; /* size */
  aspect-ratio: 1;
  border-radius: 50%;
  background: conic-gradient(#0000,var(--c));
  mask: 
   radial-gradient(farthest-side,#0000 calc(100% - var(--b)),#000 0),
   repeating-conic-gradient(#000 0 calc(360deg/var(--n) - var(--g)) , #0000 0 calc(360deg/var(--n))
}

Tenemos dos capas de máscara, pero el resultado no es el que queremos. Obtenemos lo siguiente:

Puede parecer extraño pero es lógico. La parte visible "final" no es más que la suma de cada parte visible de cada capa de máscara. Podemos cambiar este comportamiento usando mask-composite. Necesitaría un artículo completo para explicar esta propiedad, así que simplemente daré el valor.

En nuestro caso, debemos considerar intersect(y destination-outpara la propiedad prefijada). Nuestro código se convertirá en:

.loader {
  width: 100px; /* size */
  aspect-ratio: 1;
  border-radius: 50%;
  background: conic-gradient(#0000,var(--c));
  mask: 
    radial-gradient(farthest-side,#0000 calc(100% - var(--b)),#000 0),
    repeating-conic-gradient(#000 0 calc(360deg/var(--n) - var(--g)) , #0000 0 calc(360deg/var(--n));
  -webkit-mask-composite: destination-in;
          mask-composite: intersect;
}

¡Hemos terminado con la forma! Solo nos falta la animación. Esta última es una rotación infinita.

Lo único a tener en cuenta es que estoy usando una stepsanimación para crear la ilusión de guiones fijos y colores en movimiento.

Aquí hay una ilustración para ver la diferencia.

Una animación lineal frente a una animación de pasos

La primera es una rotación lineal y continua de la forma (no la que queremos) y la segunda es una animación discreta (la que queremos).

Aquí está el código completo, incluida la animación:

 <div class="loader"></div>
 <div class="loader" style="--b: 15px;--c: blue;width: 120px;--n: 8"></div>
 <div class="loader" style="--b: 5px;--c: green;width: 80px;--n: 6;--g: 20deg"></div>
 <div class="loader" style="--b: 20px;--c: #000;width: 80px;--n: 15;--g: 7deg"></div> 
 .loader {
   --b: 10px;  /* border thickness */
   --n: 10;    /* number of dashes*/
   --g: 10deg; /* gap between dashes*/
   --c: red;   /* the color */

   width: 100px; /* size */
   aspect-ratio: 1;
   border-radius: 50%;
   padding: 1px;
   background: conic-gradient(#0000,var(--c)) content-box;
   -webkit-mask:
     repeating-conic-gradient(#0000 0deg,
        #000 1deg calc(360deg/var(--n) - var(--g) - 1deg),
        #0000     calc(360deg/var(--n) - var(--g)) calc(360deg/var(--n))),
     radial-gradient(farthest-side,#0000 calc(98% - var(--b)),#000 calc(100% - var(--b)));
           mask:
     repeating-conic-gradient(#0000 0deg,
        #000 1deg calc(360deg/var(--n) - var(--g) - 1deg),
        #0000     calc(360deg/var(--n) - var(--g)) calc(360deg/var(--n))),
     radial-gradient(farthest-side,#0000 calc(98% - var(--b)),#000 calc(100% - var(--b)));
   -webkit-mask-composite: destination-in;
           mask-composite: intersect;
   animation: load 1s infinite steps(var(--n));
 }
 @keyframes load {to{transform: rotate(1turn)}}

Notarás algunas diferencias con el código que usé en la explicación:

  • Estoy agregando padding: 1pxy configurando el fondo paracontent-box
  • Hay +/1degentre los colores de larepeating-conic-gradient()
  • Hay algunos porcentajes de diferencia entre el color dentro del radial-gradient()

Esas son algunas correcciones para evitar fallas visuales. Se sabe que los degradados producen resultados "extraños" en algunos casos, por lo que debemos ajustar algunos valores manualmente para evitarlos.

Cómo crear un cargador de progreso

Al igual que el cargador anterior, comencemos con una descripción general:

https://codepen.io/t_afif/pen/bGoNddg

 <div class="loader"></div>
 <div class="loader" style="--s:10px;--n:10;color:red"></div>
 <div class="loader" style="--g:0px;color:darkblue"></div>
 <div class="loader" style="--s:25px;--g:8px;border-radius:50px;color:green"></div>
 .loader {
   --n:5;    /* control the number of stripes */
   --s:30px; /* control the width of stripes */
   --g:5px;  /* control the gap between stripes */

   width:calc(var(--n)*(var(--s) + var(--g)) - var(--g));
   height:30px;
   padding:var(--g);
   margin:5px auto;
   border:1px solid;
   background:
     repeating-linear-gradient(90deg,
       currentColor  0 var(--s),
       #0000 0 calc(var(--s) + var(--g))
     ) left / calc((var(--n) + 1)*(var(--s) + var(--g))) 100% 
     no-repeat content-box;
   animation: load 1.5s steps(calc(var(--n) + 1)) infinite;
 }
 @keyframes load {
   0% {background-size: 0% 100%}
 }

Tenemos la misma configuración que el cargador anterior. Variables CSS que controlan el cargador:

  • --n define el número de guiones/rayas.
  • --s define el ancho de cada franja.
  • --g define el espacio entre las rayas.

Ilustración de las variables CSS

Ilustración de las variables CSS

De la figura anterior podemos ver que el ancho del elemento dependerá de las 3 variables. El CSS será el siguiente:

.loader {
  width: calc(var(--n)*(var(--s) + var(--g)) - var(--g));
  height: 30px; /* use any value you want here */
  padding: var(--g);
  border: 1px solid;
}

Usamos paddingpara establecer el espacio en cada lado. Entonces el ancho será igual al número de rayas multiplicado por su ancho y el espacio. Eliminamos un espacio porque para Nlas rayas tenemos N-1espacios.

Para crear las rayas usaremos el siguiente degradado.

repeating-linear-gradient(90deg,
  currentColor 0 var(--s),
  #0000        0 calc(var(--s) + var(--g))
 )

De 0a ses el color definido y de sa s + gun color transparente (la brecha).

Estoy usando currentColorcuál es el valor de la colorpropiedad. Tenga en cuenta que no definí ningún color dentro border, por lo que también se usará para el valor de color. Si queremos cambiar el color del cargador, solo necesitamos establecer la colorpropiedad.

Nuestro código hasta ahora:

.loader {
  width: calc(var(--n)*(var(--s) + var(--g)) - var(--g));
  height: 30px;
  padding: var(--g);
  border: 1px solid;
  background:
    repeating-linear-gradient(90deg,
      currentColor  0 var(--s),
      #0000 0 calc(var(--s) + var(--g))
    ) left / 100% 100% content-box no-repeat;
}

Estoy usando content-boxpara asegurarme de que el degradado no cubra el área de relleno. Luego defino un tamaño igual a 100% 100%y una posición izquierda.

Es hora de la animación. Para este cargador, animaremos el background-sizede 0% 100%a 100% 100%lo que significa el ancho de nuestro degradado de 0%  a100%

Al igual que el cargador anterior, confiaremos en steps()tener una animación discreta en lugar de una continua.

Una animación lineal frente a una animación de pasos

El segundo es el que queremos crear, y lo podemos lograr agregando el siguiente código:

.loader {
  animation: load 1.5s steps(var(--n)) infinite;
}
@keyframes load {
  0% {background-size: 0% 100%}
}

Si observa detenidamente la última figura, notará que la animación no está completa. Nos falta una raya al final, incluso si hemos usado N. Esto no es un error, sino cómo steps()se supone que funciona.

Para superar esto, necesitamos agregar un paso adicional. Aumentamos el background-sizede nuestro degradado para contener N+1rayas y usar steps(N+1). Esto nos llevará al código final:

.loader {
  width: calc(var(--n)*(var(--s) + var(--g)) - var(--g));
  height: 30px;
  padding: var(--g);
  margin: 5px auto;
  border: 1px solid;
  background:
    repeating-linear-gradient(90deg,
      currentColor  0 var(--s),
      #0000 0 calc(var(--s) + var(--g))
    ) left / calc((var(--n) + 1)*(var(--s) + var(--g))) 100% 
    content-box no-repeat;
  animation: load 1.5s steps(calc(var(--n) + 1)) infinite;
}
@keyframes load {
  0% {background-size: 0% 100%}
}

Tenga en cuenta que el ancho del degradado es igual a N+1multiplicado por el ancho de una franja y un espacio (en lugar de ser 100%)

Conclusión

Espero que disfrutes este tutorial. Si está interesado, he creado más de 500 cargadores div únicos solo para CSS . También escribí otro tutorial para explicar cómo crear el cargador de puntos usando solo propiedades de fondo .

Encuentre a continuación enlaces útiles para obtener más detalles sobre algunas propiedades que he usado y que no expliqué a fondo debido a su complejidad:

¡Gracias por leer!

Enlace: https://www.freecodecamp.org/news/how-to-create-a-css-only-loader/

#css