有很多方法可以实现，一个基本的例子就是使用take操作符

There are many ways to achieve this, a basic example is to use the take operator

import { Observable, timer } from 'rxjs';
import { take, map } from 'rxjs/operators';

@Component({
   selector: 'my-app',
   template: `<h2>{{counter$ | async}}</h2>`
})
export class App {
   counter$: Observable<number>;
   count = 60;

   constructor() {
     this.counter$ = timer(0,1000).pipe(
       take(this.count),
       map(() => --this.count)
     );
   }
}

更好的方法是创建一个反指令！

import { Directive, Input, Output, EventEmitter, OnChanges, OnDestroy } from '@angular/core';

import { Subject, Observable, Subscription, timer } from 'rxjs';
import { switchMap, take, tap } from 'rxjs/operators';

@Directive({
  selector: '[counter]'
})
export class CounterDirective implements OnChanges, OnDestroy {

  private _counterSource$ = new Subject<any>();
  private _subscription = Subscription.EMPTY;

  @Input() counter: number;
  @Input() interval: number;
  @Output() value = new EventEmitter<number>();

  constructor() {

    this._subscription = this._counterSource$.pipe(
      switchMap(({ interval, count }) =>
        timer(0, interval).pipe(
          take(count),
          tap(() => this.value.emit(--count))
        )
      )
    ).subscribe();
  }

  ngOnChanges() {
    this._counterSource$.next({ count: this.counter, interval: this.interval });
  }

  ngOnDestroy() {
    this._subscription.unsubscribe();
  }

}

用法:

<ng-container [counter]="60" [interval]="1000" (value)="count = $event">
  <span> {{ count }} </span>
</ng-container>

这是一个实时stackblitz