Using Abstract Classes As Dependency-Injection Tokens

sing Abstract Classes As Dependency-Injection Tokens With “providedIn” Semantics In Angular 9.1.9

By Ben Nadel on May 29, 2020

Tags: Javascript / DHTML

Earlier this week, I looked at saving temporary form-data to the [SessionStorage](https://www.bennadel.com/blog/3834-saving-temporary-form-data-to-the-sessionstorage-api-in-angular-9-1-9.htm) API in Angular 9 so that a user wouldn’t lose their form-data if they accidentally refreshed the page. In that exploration, I had a Storage class that used the SessionStorage API internally in a hard-coded fashion. But, I wanted to take a look at how I could make that behavior more dynamic using Dependency-Injection (DI) tokens. A few years ago, I wrote about using abstract classes as DI tokens in Angular 4; however, with the new providedIn @Injectable decorator, I didn’t know how to do this. That is, until I came across a post on tree-shakeable providers by Manfred Steyer. What follows in my attempt to take Manfred’s insights and translate them over to my temporary storage exploration in Angular 9.1.9.

Run this demo in my JavaScript Demos project on GitHub.

View this code in my JavaScript Demos project on GitHub.

A couple of versions ago, Angular introduced a new @Injectable() decorator semantic that would allow us to “provide” Services within our Angular application without having to explicitly define those services in the @NgModule decorators:

@Injectable({
		providedIn: "root"
	})
	export class MyServiceClass { 
		// ....
	}

I couldn’t really tell you what this providedIn:"root" concept is doing mechanically; I can only tell you that it magically adds the given Service to the Dependency-Injection (DI) container. This allows other classes to then inject this Service using the traditional type-based annotations:

import { MyServiceClass } from "./my-service";

	@Injectable({
		providedIn: "root"
	})
	export class SomeConsumerClass { 
		constructor( myService: MyServiceClass ) {
			// ....
		}
	}

Now, getting back to the idea of using an abstract class as a dependency-injection token, I wasn’t sure how to turn the MyServiceClass reference into an abstract class that concrete implementations could implement. I figured that I could always go and add the default, concrete implementation to the @NgModule decorator; however, that would defeat the ease-of-use that the providedIn syntax is bringing to the table.

It wasn’t until I read Manfred’s blog post that I even realized that the @Injectable() decorator has more than just the providedIn key. In fact, it allows for all the same “use” options that we can have in the @NgModule providers:

• useClass
• useValue
• useFactory
• useExisting

ASIDE: The _documentation for the _[_@Injectable()_](https://angular.io/api/core/Injectable)_ decorator_ lists _providedIn_ as the only option, which is definitely part of my confusion on the matter.

With this new insight, we can expose an abstract class as a dependency-injection token and then use the useClass option to tell it which concrete implementation to use as the default provider.

Circling back to my temporary storage demo, I can now create a TemporaryStorageService class that is abstract, provides a default, concrete implementation, and acts as a dependency-injection token in the Angular application:

// Import the core angular services.
	import { forwardRef } from "@angular/core";
	import { Injectable } from "@angular/core";

	// ----------------------------------------------------------------------------------- //
	// ----------------------------------------------------------------------------------- //

	// By using an ABSTRACT CLASS as the dependency-injection (DI) token, it allows us to
	// use the class as both the token and as an INTERFACE that the concrete classes have to
	// implement. And, by including the "useClass" property in our decorator, it allows us
	// to define the DEFAULT IMPLEMENTATION to be used with this injection token.
	@Injectable({
		providedIn: "root",
		useClass: forwardRef( () => SessionStorageService ) // Default implementation.
	})
	export abstract class TemporaryStorageService {
		public abstract get<T>( key: string ) : Promise<T | null>;
		public abstract remove( key: string ) : void;
		public abstract set( key: string, value: any ) : void;
	}

Here, we’re using the abstract class, TemporaryStorageService, as both the DI token and the Interface for the concrete implementations. We’re then using the useClass option to tell the Angular Injector to provide the SessionStorageService class as the default implementation for said DI token.

NOTE: I’m using the _forwardRef()_ function here because the _SessionStorageService_ class is defined lower-down in the code-file. I could have swapped the order of the definitions and used the class reference directly; but, I wanted to the code-file to read top-to-bottom.

Now, from my other Services and Components, I can inject the TemporaryStorageService token and remain blissfully ignorant of whichever implementation is actually being provided. To see this in action, I’ve created an App component that injects the TemporaryStorageService token and receives one of the following three concrete implementations:

• SessionStorageService
• LocalStorageService
• InMemoryStorageService

#javascript / dhtml #angular

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What Is The Use Of WordPress?

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1. Multiple Websites Under A Single Installation
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2. WordPress Social Network
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6. Security Features
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By choosing hosting providers like WP Engine, you can improve the security of the website. It helps in threat detection, manage patching and updates, and internal security audits for the customers, and so much more.

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