Injecting Services in Angular

Injecting services in Angular allows components, directives, and other services to use functionality defined in shared services. This is a core part of Angular's dependency injection (DI) system.

Overview of Service Injection

Service injection in Angular is done through the constructor of the class where the service is needed. Angular's DI framework takes care of providing the correct instances of services where required.

Injecting a Service into a Component

To inject a service into a component, you add the service to the component's constructor. Angular's DI framework automatically provides an instance of the service:

// my-service.service.ts
import { Injectable } from '@angular/core';

@Injectable({
  providedIn: 'root'
})
export class MyService {
  constructor() { }

  getMessage(): string {
    return 'Hello from MyService!';
  }
}

// my-component.component.ts
import { Component, OnInit } from '@angular/core';
import { MyService } from '../my-service.service';

@Component({
  selector: 'app-my-component',
  template: '
{{ message }}
',
})
export class MyComponent implements OnInit {
  message: string;

  constructor(private myService: MyService) {}

  ngOnInit() {
    this.message = this.myService.getMessage();
  }
}

In this example, MyService is injected into MyComponent via the constructor. The ngOnInit lifecycle hook is used to call a method on the service and set a component property.

Injecting a Service into Another Service

You can also inject services into other services in a similar way:

// another-service.service.ts
import { Injectable } from '@angular/core';
import { MyService } from './my-service.service';

@Injectable({
  providedIn: 'root'
})
export class AnotherService {
  constructor(private myService: MyService) {}

  getCombinedMessage(): string {
    return this.myService.getMessage() + ' and AnotherService!';
  }
}

Here, MyService is injected into AnotherService, allowing AnotherService to use the functionality provided by MyService.

Providing and Injecting Services in Modules

Services can also be provided and injected within modules:

// 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';
import { AnotherService } from './another-service.service';

@NgModule({
  declarations: [AppComponent],
  imports: [BrowserModule],
  providers: [MyService, AnotherService],
  bootstrap: [AppComponent]
})
export class AppModule { }

In this example, both MyService and AnotherService are provided in the AppModule, making them available for injection throughout the application.

Key Points

Conclusion

Injecting services in Angular is a fundamental aspect of the framework's DI system. Properly understanding and utilizing service injection allows for more modular, reusable, and maintainable code.