[Angular] Zones and NgZone

NgZone, Angular uses it to profiling all the async actions such as setTimeout, http request and animation.

For example if you dealing with heavy oprations for hundreds of times, you might want it run outside Angular Zone, so it won't trgger change detection hundreds of times.

import { Component, OnInit, DoCheck, NgZone } from '@angular/core';

@Component({
  selector: 'app-root',
  template: `
    <div>
      Counter: {{ counter }}
    </div>
  `
})
export class AppComponent implements OnInit, DoCheck {
  counter = 0;
  constructor(
    private zone: NgZone
  ) {}
  ngOnInit() {
    this.zone.runOutsideAngular(() => {
      for (let i = 0; i < 100; i++) {
        setTimeout(() => this.counter++);
      }
      this.zone.run(() => {
        setTimeout(() => this.counter = this.counter, 1000);
      });
    });
  }
  ngDoCheck() {
    console.log('Change detection has been run!');
  }
}

To notice that, the operation puts inside 'runOutsideAngular' should be async opreation. Otherwise there is no effect. Now you can think that Angular won't update our 'counter' if we run outside Angular. 

So if we resume (trigger change detection) again, we can do:

      this.zone.run(() => {
        setTimeout(() => this.counter = this.counter, 1000);
      });