Is there a way to check for @Output wire up from within a component in Angular?

Question

In the ngOnInit method of a component the @Input values will have been bound so you can check those properties on the component, but there doesn't seem to be a way to check @Output event bindings. I want to be able to know if the @Output was wired up on the component or not.

(using Angular and TypeScript)

import {Component, Output, EventEmitter} from '@angular/core';  @Component({     selector: 'sample',     template: `<p>a sample</p>` }) export class SampleComponent {     @Output() cancel = new EventEmitter();          ngOnInit() {         // would like to check and see if cancel was used         // on the element <sample (cancel)="doSomething()"></sample>          // or not <sample></sample>     } } 

Answer 1

Same approach as user1448982 but without using the ObservableWrapper that is meant to be platform code that is not exposed for use via the api.

(Using Angular 2 RC1 and TypeScript)
Note: This only started working from 2.0.0-beta.16 and greater

import {Component, Output, EventEmitter} from '@angular/core';  @Component({     selector: 'sample',     template: `<p>a sample</p>` }) export class SampleComponent {     @Output() cancel = new EventEmitter();     private isCancelUsed = false;      ngOnInit() {         this.isCancelUsed = this.cancel.observers.length > 0;     } } 

Plunker

The ObservableWrapper.hasSubscribers method does this line internally, so you can just do the same thing here.

When using TypeScript you will get a transpile time error if Angular ever ends up changing the EventEmitter from a Subject (which is part Observable, thus the .observers property).

Update (03 / 2022)

The observers attribute is deprecated since rxjs v7. Instead of checking the length of the observers array, you can now use a boolean that indicates if the subject is in use.

// Old approach this.isCancelUsed = this.cancel.observers.length > 0;  // New approach this.isCancelUsed = this.cancel.observed;