Angular Dependency Injection

Angular Dependency Injection

23 Feb 2024
Beginner
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12 min read

Angular Dependency Injection: An Overview

Angular, a popular open-source web application framework developed by Google, incorporates a powerful and flexible concept known as Dependency Injection (DI). It is employed to create the fundamental ideas of Angular. In this Angular tutorial, we'll explore the fundamentals of Angular Dependency Injection, its benefits, and how to leverage it to enhance the structure of your Angular applications. For more in-depth knowledge, consider our Angular Certification Online Training.

Read More: Top 50 Angular Interview Questions & Answers

What Is Dependency Injection?

Dependency Injection is a design pattern that promotes the separation of concerns by decoupling components and their dependencies. In Angular, dependencies are typically services, but they also can be values, such as strings or functions. DI is used to inject instances of services, components, and other objects into classes that depend on them, promoting modularity, reusability, and testability within the application.

Angular's dependency injection concept.

There are three types of classes in Dependency Injection:

types of classes in Dependency Injection

  1. Client Class - This is the dependent class, which depends on the service class.
  2. Service Class - Class that provides the service to the client class.
  3. Injector Class - Injects the service class object into the client class.

Key Concepts of Angular Dependency Injection

Injector

The Angular Injector creates and manages instances of components, directives, services, and other Angular constructs. It maintains a hierarchy of injectors, with the root injector at the top. When a component requests a dependency, the injector looks for the required service in its hierarchy and provides it.

Providers

Providers are responsible for telling the injector how to create and configure instances of a particular dependency. They are defined in the metadata of Angular components using the provider's property. Providers can be specified at various levels, such as at the component level, module level, or even in the application's root module.

Injectable Decorator

To make a class injectable, the @Injectable() decorator is used. This decorator informs Angular that the class may have dependencies that need to be injected. It is typically applied to services, allowing Angular to recognize and manage their dependencies.

Types of Dependency Injection in Angular

In Angular, there are three types of Dependency Injection (DI):

  1. Constructor injection
  2. Setter injection
  3. Interface injection

Let's explore each type in detail:

1. Constructor injection

Constructor injection is the most common type of DI in Angular. Dependencies are injected through the constructor of a class, typically a component or a service. The Angular framework automatically resolves and provides the required dependencies when creating an instance of the class.

import { Injectable } from '@angular/core';

@Injectable({
  providedIn: 'root',
})
export class MyService {
  constructor(private dependencyService: DependencyService) {
    // ...
  }
}

In the example above, DependencyService is injected into the MyService class through its constructor.

2. Setter injection

Setter injection involves injecting dependencies through setter methods. Instead of receiving dependencies through the constructor, the class provides setter methods that can be called to set the dependencies after the instance is created.

import { Injectable } from '@angular/core';

@Injectable({
  providedIn: 'root',
})
export class MyService {
  private dependencyService: DependencyService;

  setDependencyService(dependencyService: DependencyService): void {
    this.dependencyService = dependencyService;
  }
}

Setter injection can be useful in scenarios where it's not feasible or desirable to inject dependencies through the constructor.

3. Interface injection

Interface-based injection involves creating an interface for a service and injecting the interface rather than the concrete implementation. This allows for greater flexibility and easier substitution of implementations, especially during testing.

import { Injectable } from '@angular/core';

interface DependencyInterface {
  // Define methods and properties
}

@Injectable({
  providedIn: 'root',
})
export class DependencyService implements DependencyInterface {
  // Implementation of the interface
}

@Injectable({
  providedIn: 'root',
})
export class MyService {
  constructor(private dependency: DependencyInterface) {
    // ...
  }
}

In this example, MyService depends on the DependencyInterface, which is implemented by the DependencyService. During testing, a mock implementation of the interface can be provided to isolate the unit being tested.

How To Implement Angular Dependency Injection?

Implementing Angular Dependency Injection involves the following steps to set up and use services within your components.

1. Create a Service

First, create a service that will provide functionality or data to other components. You can use Angular CLI to generate a service:
ng generate service my-service
This will create a my-service.service.ts file. Open the file and define your service:
import { Injectable } from '@angular/core';

@Injectable({
  providedIn: 'root', // Provides the service at the root level
})
export class MyService {
  // Implement your service logic here
}

2. Inject the Service into a Component

Now, you can inject the service into a component that needs to use its functionality. Open the component file (e.g., app.component.ts) and inject the service through the constructor:
import { Component } from '@angular/core';
import { MyService } from './my-service.service';

@Component({
  selector: 'app-root',
  templateUrl: './app.component.html',
  styleUrls: ['./app.component.css'],
})
export class AppComponent {
  constructor(private myService: MyService) {
    // Use myService in the component
  }
}

3. Register the Service in a Module

Angular needs to know about the service and how to create an instance of it. Register the service in the providers array of an Angular module. If you want the service to be available throughout the entire application, use the root module (app.module.ts):
import { NgModule } from '@angular/core';
import { BrowserModule } from '@angular/platform-browser';
import { AppComponent } from './app.component';
import { MyService } from './my-service.service';

@NgModule({
  declarations: [AppComponent],
  imports: [BrowserModule],
  providers: [MyService], // Register the service here
  bootstrap: [AppComponent],
})
export class AppModule {}

4. Use the Service in the Component

Now that the service is injected into the component, you can use its methods and properties within the component:
import { Component } from '@angular/core';
import { MyService } from './my-service.service';

@Component({
  selector: 'app-root',
  templateUrl: './app.component.html',
  styleUrls: ['./app.component.css'],
})
export class AppComponent {
  constructor(private myService: MyService) {
    // Use myService in the component
    const data = this.myService.getData();
    console.log(data);
  }
}
In this example, assume that MyService has a method called getData().

5. Optional: Use Dependency Injection in Templates

You can also use dependency injection directly in templates by accessing injected services using the *ngIf directive. For example:
<div *ngIf="myService.someCondition()">Content to show based on service condition</div>

Benefits of Angular Dependency Injection

  • Modularity: DI promotes modularity by encapsulating the functionality of each component and service. This makes it easier to develop, test, and maintain individual parts of the application independently.
  • Testability: With DI, it becomes straightforward to replace real services with mock services during testing. This facilitates unit testing, ensuring that each component can be tested in isolation.
  • Maintainability: Dependency Injection helps in creating loosely coupled components, making it easier to modify and extend the application without affecting other parts. This enhances the overall maintainability of the codebase.
  • Reusability: Components and services can be reused across different parts of the application or even in multiple projects, thanks to the decoupling facilitated by Dependency Injection.

Read More:

Summary

Angular Dependency Injection is a powerful mechanism that contributes significantly to the framework's architecture. By promoting modularity, testability, maintainability, and reusability, Dependency Injection plays a vital role in building scalable and robust Angular applications. Implementing this design pattern into your development behaviors and understanding its fundamental ideas will surely result in simpler and easily maintained code.

FAQs

Q1. What is Angular Dependency Injection (DI)?

Angular Dependency Injection (DI) is a design pattern and mechanism in the Angular framework that manages the creation and injection of dependencies into components, services, and other objects. DI promotes modularity, testability, and maintainability by decoupling components from their dependencies.

Q2. Why is Dependency Injection important in Angular?

Dependency Injection is crucial in Angular for several reasons. It promotes modular and maintainable code by decoupling components, enables easy testing through the substitution of dependencies, and enhances code reusability.

Q3. What are the benefits of using Dependency Injection in Angular?

The benefits of using Dependency Injection in Angular include modularity, testability, maintainability, and reusability. It simplifies code organization, allows for easier unit testing, makes code more maintainable by reducing coupling, and enables the reuse of components and services across different parts of the application.

Q4. What are the different types of Dependency Injection in Angular?

ngular supports three types of Dependency Injection: constructor injection, setter injection, and interface-based injection. 

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Shailendra Chauhan (Microsoft MVP, Founder & CEO at Scholarhat by DotNetTricks)

Shailendra Chauhan is the Founder and CEO at ScholarHat by DotNetTricks which is a brand when it comes to e-Learning. He provides training and consultation over an array of technologies like Cloud, .NET, Angular, React, Node, Microservices, Containers and Mobile Apps development. He has been awarded Microsoft MVP 8th time in a row (2016-2023). He has changed many lives with his writings and unique training programs. He has a number of most sought-after books to his name which has helped job aspirants in cracking tough interviews with ease.
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