Justin Schwartzenberger Justin Schwartzenberger - 6 months ago 16
AngularJS Question

Is there a way to check for @Output wire up from within a component in Angular 2?

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 2 beta 2 and TypeScript)

import {Component, Output, EventEmitter} from 'angular2/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

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).

Comments