EventEmitter's Emit function failing to dispatch data to listeners

Currently, I am in the process of establishing a connection between two components using a service: LoaderComponent and AppComponent with LoaderService. The goal is to display a loader whenever the application fetches data. However, when attempting to utilize an EventEmitter to broadcast changes to the components, they do not seem to receive these updates. Interestingly, the service is able to detect the changes when it subscribes to itself.

Here is the code for LoaderService.ts:

import { EventEmitter, Injectable } from '@angular/core';

@Injectable()

class LoaderService {
  @Output change: EventEmitter<number> = new EventEmitter<number>();
  private state: number = 0;

  constructor() { 
     this.change.subscribe(state => console.log(state)); 
     this.setState(1) 
  }

  setState(state: number) {
     this.state = state;
     this.change.emit(this.state);
  }
}
// The state is displayed within the service, but changes are not detected outside of it. Attempts were made with EventEmitter from events as well.

The expectation is to receive events from the LoaderService by its subscribers.

Answer №1

To ensure that Angular creates the LoaderService in a component, you must actually use it within that component. If the service is not used anywhere, Angular will automatically discard it. Inject the LoaderService in the app component as shown below:

constructor(private _loadService: LoaderService) {}
and then you can log messages to the console.

It is also recommended to utilize either Subject or Behavior Subject from Rxjs instead of Output when working with services.

Answer №2

One important thing to note is that EventEmitters and Outputs should not be placed in a service.

To improve this, I will use subjects instead and ensure the subject is private while exposing a public observable. This practice limits how the subject's state can be modified and is typically considered best practice:

import { Injectable } from '@angular/core';
import {Subject} from 'rxjs/Subject';
import {Observable} from 'rxjs/Observable';

@Injectable()

class LoaderService {
  private change: Subject<number> = new Subject<number>();
  change$: Observable<number> = this.change.asObservable();
  private state: number = 0;

  constructor() { 
     this.change$.subscribe(state => console.log(state)); 
     this.setState(1) 
  }

  setState(state: number) {
     this.state = state;
     this.change.next(this.state);
  }
}

Another issue could arise from how the service is provided. For instance, if you have a template in your app component like:

<loader-component></loader-component>
<loader-component></loader-component>

where two loader components are used side by side, and each loader component has its own providers array like:

providers: [LoaderService]

then these loaders will receive separate copies of the service since they provide and inject their own. As a result, they won't see each other's events.

To resolve this, it's better to provide the service in the app component (not the loader component) so they share the same service instance. If provided in both places, each component gets a different copy.

If provided at the root level (module), every component that injects the service without providing its own will get the same service copy.

The correct location for service provision depends on your app's requirements and the purpose of the specific service.

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