Angular 2 runOutsideAngular still change the UI

From my understanding of runOutsideAngular(), if I need to run something that won't trigger the Angular change detection, I need to use this function. My code is not working, however; when I click the button, the UI is changing and the number is 2.

@Component({selector: 'my-cmp', 
template: `<h1>{{num}}</h1>
           <button (click)="onClick()">Change number</button>`})

class MyComponent implements OnChanges {

  num = 1; 
  constructor(private _ngZone: NgZone ) {

  }

  onClick() {
    this._ngZone.runOutsideAngular(() => {
      this.num = 2;
    }}));
  }
}

If anything is causing change detection, and a bound event like (click)="onClick()" does cause change detection, then Angular will detect the change.

runOutsideAngular doesn't mean Angular won't see the change, it only means that the code run this way doesn't cause change detection, but because the click event already does, it's meaningless in your example.

[In short] you need to change one line in your current code

onClick() {
    this._ngZone.runOutsideAngular(() => {
        setTimeout(()=>this.num = 2,0); // instead of this.num = 2;
    }}));
  }

now if you click the on the <button>, this.num will become 2, but you won't see any change in the UI (a temporary inconsistency between view and model)

[Explanation] without runOutsideAngular(), async functions like addEventListener() or setTimeout() behaves differently (monkey patched). their callbacks will try to update UI with Angular after running user's code. For example, you can treat (click)="onClick()" as:

addEventListener("click",function modifiedCallback(){
    onClick();
    updateUIifModelChanges(); //call to Angular
})

In order to not triggered UI update we need to satisfy the following two conditions:

1. not modify model in function onClick (so, modify inside setTimeout())
2. when the model is indeed modified, do not invoke updateUIifModelChanges (call setTimeout() inside runOutsideAngular)

[More] of cause, the explanation I gave is a very very...simplified version of what happens. setTimeout() has the same function signature whether it's running inside runOutsideAngular() or not. The reason that it behaves differently is because it's running in a different Zone

If you want to prevent change detection then you can

1) subscribe on ngZone.onMicrotaskEmpty like this:

import { NgZone, ChangeDetectorRef } from '@angular/core';
import 'rxjs/add/operator/first';

...
export class MyComponent {
  constructor(private ngZone: NgZone, private cdRef: ChangeDetectorRef) {}

  onClick() {
    // to do something

    this.cdRef.detach();
    this.ngZone.onMicrotaskEmpty.first().subscribe(() => {
      // reattach changeDetector after application.tick()
      this.cdRef.reattach();
    });
  }
}

This handler will run after Application.tick

See also Plunker Example

2) use custom directive like this:

@Directive({
  selector: '[outSideEventHandler]'
})
class OutSideEventHandlerDirective {
  private handler: Function;

  @Input() event: string = 'click'; // pass desired event
  @Output('outSideEventHandler') emitter = new EventEmitter();

  constructor(private ngZone: NgZone, private elRef: ElementRef) {}

  ngOnInit() {
    this.ngZone.runOutsideAngular(() => {
      this.handler = $event => this.emitter.emit($event);
      this.elRef.nativeElement.addEventListener(this.event, this.handler);
    });
  }

  ngOnDestory() {
    this.elRef.nativeElement.removeEventListener(this.event, this.handler);
  }
}

and then in template you can write:

<button (outSideEventHandler)="onClick()">Click outside zone</button>

or

<button event="mousedown" (outSideEventHandler)="onClick()">Click outside zone</button>

Plunker

3) write custom DOM event handler as described in this article.

• https://medium.com/@TheLarkInn/creating-custom-dom-events-in-angular2-f326d348dc8b#.bx4uggfdy

Other solutions see here:

• Angular 2 how to keep event from triggering digest loop/detection cycle?

Using ngZone.run is a bit better than the setTimeout solutions since it uses angular specific functionality. Run is meant to be used within ngZone.runOutsideAngular functions.

From the docs:

Running functions via run allows you to reenter Angular zone from a task that was executed outside of the Angular zone (typically started via {@link #runOutsideAngular}).

This is actually a very practical example of say a button that increments a number by one but only triggers change detection when the number is even.

    @Component({selector: 'my-cmp', 
    template: `<h1>{{num}}</h1>
               <button (click)="onClick()">Change number</button>`})

    class MyComponent implements OnChanges {

      num = 1; 
      constructor(private _ngZone: NgZone ) {

      }

      onClick() {
        this._ngZone.runOutsideAngular(() => {
          if(this.num % 2 === 0){
              // modifying the state here wont trigger change.
              this.num++;
          } else{
            this._ngZone.run(() => {
                this.num++;
            })
          }
        
        }}));
      }
    }