在管道中实现倒数计时器，并在计时器结束时使用附加服务更新组件

Question

我正在使用我的 Angular 应用程序在屏幕上显示 date/time 倒计时。这是从特定日期和时间开始的倒计时。

运行它每一秒都在用 Google Chrome 杀死我的 CPU。可能是 moment() 库有一些效果，但还不确定。我可能会马上尝试一下，看看是否有帮助。

是否有更好（更有效）的方法来做到这一点？

这是我的代码

calculateTimeRemaining(endDate: Date, timeZone: string, eventId: number) {
  setInterval(() => {
    const element = document.getElementById("event-" + eventId);
    if (element) {
      const currentDateUtc = moment().utc();
      const endDateUtc = moment(endDate).utc(true);
      endDateUtc.tz(timeZone);
      const dif = moment.duration(endDateUtc.diff(currentDateUtc));
      const hours = dif.hours();
      const minutes = dif.minutes();
      const seconds = dif.seconds();
      if (hours < 1) {
        element.innerHTML = minutes + ' m ' + seconds + ' s';
      } else {
        element.innerHTML = hours + ' h ' + minutes + ' m ' + seconds + ' s';
      }
    }
  }, 1000);
}

<span id="event-{{event.id}}" class="float-right yb-color">{{calculateTimeRemaining(event.datePendingUtc, event.yogabandTimeZoneId, event.id)}}</span>

编辑（解决方案） - 我已经在管道中实现了倒数计时器（按照推荐）来解决这个问题。我将在下面包含管道的完整代码以及我如何调用它。我还有一项服务，用于在计时器启动时向我的组件发送通知，以便我可以执行其他操作。

import { Pipe, PipeTransform } from '@angular/core';
import { Observable, of, timer } from 'rxjs';
import { map, takeWhile } from 'rxjs/operators';
import { EventService } from 'src/app/core/services/event.service';

@Pipe({
  name: 'timeRemaining'
})
export class TimeRemainingPipe implements PipeTransform {
  eventId: number;
  expired = false;

  constructor(private eventService: EventService) {}

  /**
   * @param futureDate    should be in a valid Date Time format
   *                      e.g. YYYY-MM-DDTHH:mm:ss.msz
   *                      e.g. 2021-10-06T17:27:10.740z
   */
   public transform(futureDateUtc: string, eventId: number): Observable<string> {
    
    this.eventId = eventId;
    /**
     * Initial check to see if time remaining is in the future
     * If not, don't bother creating an observable
     */

    if (!futureDateUtc || this.getMsDiff(futureDateUtc) < 0) {
      console.info('Pipe - Time Expired Event: ' + eventId);
      return of('EXPIRED');
    }
    
    return timer(0, 1000).pipe(
        takeWhile(() => !this.expired),
        map(() => {
            return this.msToTime(this.getMsDiff(futureDateUtc));
        })
    );
  }

  /**
   * Gets the millisecond difference between a future date and now
   * @private
   * @param   futureDateUtc: string
   * @returns number  milliseconds remaining
   */
   // Z converts to local time
   private getMsDiff = (futureDate: string): number => (+(new Date(futureDate + 'Z')) - Date.now());


   /**
    * Converts milliseconds to the
    *
    * @private
    * @param msRemaining
    * @returns null    when no time is remaining
    *          string  in the format `HH:mm:ss`
    */
   private msToTime(msRemaining: number): string | null {
       if (msRemaining < 0) {
           console.info('Pipe - No Time Remaining:', msRemaining);
           this.expired = true;
           this.eventService.expired(this.eventId);
           return 'EXPIRED';
       }

       let seconds: string | number = Math.floor((msRemaining / 1000) % 60),
           minutes: string | number = Math.floor((msRemaining / (1000 * 60)) % 60),
           hours: string | number = Math.floor((msRemaining / (1000 * 60 * 60)) % 24);

       /**
        * Add the relevant `0` prefix if any of the numbers are less than 10
        * i.e. 5 -> 05
        */
       seconds = (seconds < 10) ? '0' + seconds : seconds;
       minutes = (minutes < 10) ? '0' + minutes : minutes;
       hours = (hours < 10) ? '0' + hours : hours;

       return `${hours}:${minutes}:${seconds}`;
   }

}

<span class="float-right yb-color">{{(event.dateUtc | timeRemaining: event.id | async}}</span>

这是服务

export class EventService {
  private subject = new Subject <any> ();

  expired(eventId: number) {
    this.subject.next(eventId);
  }

  expiredEvents(): Observable <any> {
    return this.subject.asObservable();
  }
}

// how it's called in the component

this.eventService.expiredEvents().subscribe(eventId => {
  console.info('Service - Expired Event:', eventId);
  // do something now w/ expired event
});

Answer 1

为了性能，不要在 Angular 模板中调用任何函数，因为它会在每次更改检测中运行。

这篇文章很好地描述了这个问题https://medium.com/showpad-engineering/why-you-should-never-use-function-calls-in-angular-template-expressions-e1a50f9c0496

我们可以使用纯管道更改您的实现

time-remaining.pipe.ts

@Pipe({
  name: 'timeRemaining',
})
export class TimeRemainingPipe {
  transform(event: any): any {
    // put implementation here
    // return time remaining
  }
}

我相信管道，你可以删除 const element = document.getElementById("event-" + eventId); 部分。

component.html

<span id="event-{{event.id}}" class="float-right yb-color">{{ event | timeRemaining }}</span>

参考： https://angular.io/guide/pipes

在管道中实现倒数计时器，并在计时器结束时使用附加服务更新组件

Implementing a countdown timer in a pipe with additional service to update component when timer finishes

javascript

angular

performance

momentjs

setinterval