angular - Angular 4 Pipes 中的异步调用

Question

我有一个服务方法，可以从服务器获取时间，我计划在自定义管道中使用它。该管道的目的是将时间戳与当前时间进行比较，并以人类可读的格式返回。

服务

export class TimeService{
  currentServerTime;

  fetchCurrentTimeStamp(){
     //http request using Observable
     return sendGETRequest(.....).map(data => {
        this.currentServerTime = data;

        return this.currentServerTime;
     })
  }
}

管道

export class GetTimeFromNowPipe implements PipeTransform{

   fromNow;
   currentServerTime: number;
   constructor(private timeService: TimeService, private appSettings: AppSettings){}

   transform(value: number){
        var currentTime;
        this.timeService.fetchCurrentTimestamp().subscribe(data => {
            currentTime = data
            if(!value) return "";

            if(value){
               let newDate = moment.unix(value);
               this.fromNow = newDate.from(moment.unix(currentTime), true);
            }
          return this.fromNow;
        });

   }
}

HTML

<ul>
    <li *ngFor = "let model of models">
        <span>{{model.created | getTimeFromNow}}</span>
    </li>
</ul>

我被异步调用困住了。如何仅在从服务器获取数据后才从管道返回值？

Answer 1

缺少return，并且subscribe需要更改map以允许异步管道进行订阅

return this.timeService.fetchCurrentTimestamp().map(data => {

对于异步值，需要使用异步管道

<span>{{model.created | getTimeFromNow | async }}</span>