Understanding Service Instances and Dependency Injection in Angular: Why Root-Level Services Aren’t Always Singletons
When working with Angular services, one might expect that services provided at the root level, using the `providedIn: ‘root’` option, should behave as singletons. In most cases, this is true, but there are certain scenarios where the behavior might seem counterintuitive. One such situation arises when a service is injected multiple times using the providers array, leading to the creation of a new instance of that service. This article will explore why the value of state.name$ inside the `AppChild` component remains `’Initial value’` instead of reflecting `’Hello world’` and why injecting a root-level service sometimes creates multiple instances.
Scenario Setup
Let’s break down the provided code example:
- StateService:
@Injectable({
providedIn: 'root',
})
export class StateService {
nameSubject = new BehaviorSubject('Initial value');
name$ = this.nameSubject.asObservable();
}This service manages a `BehaviorSubject`, which starts with an initial value of `’Initial value’`. It exposes the observable `name$` to which other components can subscribe.
2. AppChild Component:
@Component({
selector: 'app-child',
standalone: true,
providers: [StateService], // Injects a new instance of StateService
template: `<p> State name is - {{state.name$ | async}}</p>`,
changeDetection: ChangeDetectionStrategy.OnPush,
})
export class AppChild {
state = inject(StateService);
}This component uses a local `StateService` instance (defined in the `providers` array), which is distinct from the one provided at the root level.
3. App Component:
@Component({
selector: 'app-root',
standalone: true,
imports: [AppChild],
template: `<app-child></app-child>`,
})
export class App {
state = inject(StateService);
ngOnInit() {
// Changes the root-level StateService instance's value
this.state.nameSubject.next('Hello world');
}
}This component modifies the `StateService`’s value, but it is important to note that this refers to the root-level service instance.
Why `AppChild` Shows `’Initial value’` Instead of `’Hello world’`
At first glance, it might seem counterintuitive that the `state.name$` value inside the `AppChild` component displays `’Initial value’`, even though the `App` component changes the `StateService`’s `nameSubject` to `’Hello world’`. Here’s why this happens:
1. Local vs. Global Service Instances:
Even though `StateService` is provided at the root level (via `providedIn: ‘root’`), the `AppChild` component creates a new instance of `StateService` because it lists the service in its `providers` array. This means `AppChild` uses a completely different instance of the service, with its own `BehaviorSubject` that still holds the `’Initial value’`.
2. Separate State Management:
The root-level `StateService` instance (used by the `App` component) and the local `StateService` instance (used by the `AppChild` component) are not connected. As a result, when `App` changes the state to `’Hello world’`, it is updating the root-level instance’s `nameSubject`, but the `AppChild` component is still subscribed to its own local instance of the service, which continues to have `’Initial value’`.
3. The Key to Understanding:
- The `StateService` inside `App` is the root-level service instance, and the change to `’Hello world’` affects only that instance.
- The `StateService` inside `AppChild` is a separate instance because of the `providers: [StateService]` declaration, which overrides the root-level service with a new instance.
Why This Behavior Might Be Counterintuitive
Angular’s dependency injection system is designed to create singletons for services provided at the root level. When you specify `providedIn: ‘root’`, you expect the service to be shared across the entire application. However, when you explicitly provide the service again using the `providers` array in a component, Angular treats that as a request for a **new instance** of the service, even though it is provided in the root.
This behavior can be confusing because developers often assume that root-level services are singletons across the app. While this is generally true, the presence of a `providers` array in a component can override this, leading to multiple instances of the same service in different parts of the application.
Why Injecting a Root-Level Service Can Create Multiple Instances
The key takeaway is that root-level services are singletons unless explicitly overridden. If a component or module includes the same service in its `providers` array, Angular will treat it as a request for a new instance of that service.
- Root-Level Service Behavior:
If you inject a service provided in the root using `providedIn: ‘root’`, it behaves as a singleton throughout the application unless you provide it again locally in a component or module.
- Local Service Behavior:
When you declare a service in the `providers` array of a component, Angular creates a new instance of that service for that specific component and its children. This local instance is entirely separate from the root-level instance.
This allows for a lot of flexibility but can also lead to confusion, as it’s easy to assume that root-level services should always be singleton and shared across the entire app.
Conclusion
To sum up, the `state.name$` value inside `AppChild` shows `’Initial value’` instead of `’Hello world’` because `AppChild` is using a new instance of `StateService` due to the `providers` array declaration. Even though `StateService` is provided in the root, each component can request a new instance by adding the service to its own `providers` array.
This can be counterintuitive for developers who expect root-level services to always behave as singletons, but understanding that Angular creates new instances when explicitly requested through the `providers` array helps clarify this behavior. To avoid such issues, it’s essential to be mindful of where and how services are provided to ensure proper state sharing across your application.
