Angular 4 - What is the right way to “wait for ope

问题:

I'm encountering a simple problem which has a hacky solution of setTimeout(...,0).

Looking at this simple code :

@Component({
  selector: 'my-app',
  template: `
    <div>
      <input value='Fill Data' type='button' (click)='fill()'/>
      <span *ngFor="let o of Items" class='mySpan'>Span To Detect<br></span>
    </div>
  `,
})
export class App {
  Items:Array<number> = new Array<number>();

  fill()
  {
   this.Items = [1,2,3,4,5,6,7,8,9,10]
   this.analyzeDom(); //this has to run here
  }

  analyzeDom()
   {
      alert($("div .mySpan").length) // "0"

     //BUT if I set this hacky trick , it works
     // setTimeout(function (){  alert($("div .mySpan").length)},0) // "10"
   }
}

If I click the button , the alert shows "0". I understand why it's happening. It's becuase Angular didn't complete its cycle to actually populate the ngFor.

However - doing this trick with setTimeout(..,0) seems a bit hacky to me and I prefer not to trust on it.

Question:

What is the right way to "wait for operation" in Angular ? (so that I'll see "10") ?

Plnkr

回答1:

1) You can force Angular to update the DOM by calling cdRef.detectChanges

constructor(private cdRef: ChangeDetectorRef) {}

analyzeDom(){
  this.cdRef.detectChanges();
  alert($("div .mySpan").length)

Plunker

As setTimeout is macrotask therefore it is running next digest cycle. It means that after calling setTimeout all components tree will be checked

cdRef.detectChanges doesn't call appRef.tick(). It only executes change detection component itself and its children.

2) or you can wait until Angulat has updated DOM by subscribing to zone.onMicrotaskEmpty

import 'rxjs/add/operator/first';

constructor(private zone: NgZone) {}
...
this.zone.onMicrotaskEmpty.first().subscribe(() => this.analyzeDom());

Note: first operator can cause memory leak. See https://github.com/angular/material2/issues/6905

Subscribing to onMicrotaskEmpty doesn't call change detection cycle

Plunker