Understanding Angular's @Component Decorator and Service Providers

When working with Angular, you may encounter a scenario where you provide a custom service in two components’ providers section of the @Component decorator. You may wonder how many instances of the service will be created in this situation. The answer is 2. Yes, two separate instances of the service will be created.

In Angular, services use a hierarchical injector system, and every component can potentially have its own injector. When a service is listed in the providers array of a component, a new instance of the service is created for that component and any of its child components if they request the service. If you provide the service in two components, then two separate service instances are created.

Here's a simple example. Let's imagine we have a LoggingService which we provide in two components, ComponentA and ComponentB:

@Component({
  selector: 'app-component-a',
  templateUrl: './component-a.component.html',
  providers: [LoggingService]
})
export class ComponentA {}

@Component({
  selector: 'app-component-b',
  templateUrl: './component-b.component.html',
  providers: [LoggingService]
})
export class ComponentB {}

In this setup, both ComponentA and ComponentB will get their own instances of LoggingService. This means that if LoggingService had state (like a count of log messages), changes made in ComponentA would not be reflected in ComponentB.

However, it's important to note that best practice in Angular is to minimize the number of instances where a service is provided more than once. If many components need to use a service, it's much more efficient to provide it once in a higher-level component or the application's root injector (using the AppModule's providers array or the @Injectable decorator's providedIn property).

Remember that understanding this behavior of Angular's dependency injection system and the @Component decorator's providers array is crucial when designing your Angular applications. It allows you to control exactly how and when services are instantiated, which is vital for efficient resource management and correct application logic.