Introduction

Dependency Injection (DI) is a design pattern that allows us to decouple the dependencies of an object from its implementation. This pattern is extensively used in Angular to create scalable and maintainable applications.

What is Dependency Injection in Angular?

In Angular, Dependency Injection is the process of providing a service or value to a class or function that needs it, without the receiving class or function having to know where the service comes from or how to create it. Angular provides the `@Injectable()` decorator to create services and manages the creation and lifecycle of these services through the `Injector`.

Why Use Dependency Injection in Angular?

Dependency Injection in Angular provides several benefits:

– **Loosely Coupled Code:** By decoupling the dependencies, the code becomes more modular and easier to maintain.
– **Testability:** Dependencies can be easily mocked during testing, making it easier to write unit tests for components.
– **Code Reusability:** Services can be shared among multiple components, reducing code duplication.

Basic Dependency Injection in Angular

To create a service, we use the `@Injectable()` decorator and provide a constructor with its dependencies. Here’s an example of a simple service:

“`typescript
import { Injectable } from ‘@angular/core’;

@Injectable({
providedIn: ‘root’
})
export class ExampleService {
constructor() {}

// Service methods go here
}
“`

To inject the service into a component, we use the `constructor` and the `Inject` decorator:

“`typescript
import { Component, Inject } from ‘@angular/core’;
import { ExampleService } from ‘./example.service’;

@Component({
selector: ‘app-example’,
templateUrl: ‘./example.component.html’,
styleUrls: [‘./example.component.scss’]
})
export class ExampleComponent {
constructor(private exampleService: ExampleService) {}
}
“`

Providing Services at Different Levels

The `providedIn` property in the `@Injectable()` decorator can be used to specify where the service should be provided. The possible values are:

– `root`: The service is provided at the application root and can be injected into any component.
– `module`: The service is provided at the module level and can only be injected into components within that module.
– `class`: The service is provided only for the specific class or component where it is defined.

Conclusion

Dependency Injection is a powerful pattern that helps in creating maintainable and scalable Angular applications. By understanding and effectively using this pattern, developers can simplify their codebase, improve testability, and promote code reusability.