Dependency Injection in Angular

Dependency Injection (DI) is a design pattern and fundamental principle in Angular. It is a technique in a component or service receives its dependencies from an external source, rather than creating those dependencies internally. This pattern brings significant benefits in terms of code reuse, unit testing, and component decoupling.

In the Angular context, dependency injection is widely used to provide service instances to components and other services. Here are the main concepts related to dependency injection in Angular:

1. Provide

A "provider" is an entity that can create or provide an instance of a service. In Angular, providers are generally configured in the application module using the "providers" property or in the service itself using the "@Injectable" decorator.

2. Injector Hierarchy

Angular has an injector hierarchy, which means each component and module has its own injector. When a service is injected into a component, Angular looks for a provider in the component injector. If it doesn't find it, it looks in the module injector. This continues up to the application's root injector.

3. Dependency Injection in Constructors

Dependency injection is often performed in the constructor of a class. When declaring a dependency in the constructor of a component or service, Angular automatically provides an instance of the dependency when the class is instantiated.

Example:

import { Component } from '@angular/core';
import { MyServiceService } from './my-service.service';

@Component({
   selector: 'app-my-component',
   template: `...`,
   providers: [MyServiceService] // can be configured in the module or component
})
export class MyComponentComponent {
   constructor(private myService: MyServiceService) {
     // this.myService is automatically injected
   }
}

4. Singleton by Default

Angular provides a single service instance for the entire application by default. This means that if a service is injected into multiple components, they all share the same service instance.

5. Dependency Injection into Services

Services may depend on other services. Dependency injection allows services to be easily composed and reused.

Benefits of Dependency Injection

Decoupling

Dependency injection reduces coupling between components and services, facilitating code maintenance and evolution.

Reusability

Services can be reused across multiple components and modules, promoting code reusability.

Testability

It makes writing unit tests easier as you can easily replace real dependencies with mocks or alternative implementations.

Configurability

Allows you to easily configure different implementations for a specific interface, providing significant flexibility.

In summary, dependency injection is one of the key principles of Angular that promotes modular, decoupled, and testable design in applications.