完成后 n 秒重复请求 (Angular2 - http.get)

Question

我玩了一下angular2，过了一会儿就卡住了。

使用 http.get 可以很好地处理单个请求，但我想每 4 秒轮询一次实时数据，经过一段时间的修修补补并阅读了很多 reactivex 的东西，我最终得到了：

Observable.timer(0,4000)
  .flatMap(
    () => this._http.get(this._url)
       .share()
       .map(this.extractData)
       .catch(this.handleError)
  )
  .share();

在 http.get-observable 发出请求结果后，是否有一种简单方法来启动（4 秒）间隔？（或者我最终会进入 observable-hell 吗？）

我想要的时间表：

Time(s): 0 - - - - - 1 - - - - - 2 - - - - - 3 - - - - - 4 - - - - - 5 - - - - - 6
Action:  Request - - Response - - - - - - - - - - - - - - - - - - - -Request-... 
Wait:                | wait for 4 seconds -------------------------> |

Answer 1

如果方便的话可以试试间隔。调用 subscribe 会给你 Subscription ，让你可以在一段时间后取消轮询。

let observer = Observable.interval(1000 * 4);
let subscription = observer.subsscribe(x => {
    this._http.get(this._url)
     .share()
     .map(this.extractData)
     .catch(this.handleError)
});

....
// if you don't require to poll anymore..
subscription.unsubscribe();

Answer 2

我自己设法做到了，唯一的缺点是 http.get 不能更容易地重复。

pollData(): Observable<any> {

  //Creating a subject
  var pollSubject = new Subject<any>();

  //Define the Function which subscribes our pollSubject to a new http.get observable (see _pollLiveData() below)
  var subscribeToNewRequestObservable = () => {
    this._pollLiveData()
      .subscribe(
      (res) => { pollSubject.next(res) }
      );
  };

  //Subscribe our "subscription-function" to custom subject (observable) with 4000ms of delay added
  pollSubject.delay(4000).subscribe(subscribeToNewRequestObservable);

  //Call the "subscription-function" to execute the first request
  subscribeToNewRequestObservable();

  //Return observable of our subject
  return pollSubject.asObservable();

}

private _pollLiveData() {

  var url = 'http://localhost:4711/poll/';

  return this._http.get(url)
    .map(
    (res) => { return res.json(); }
    );
};

以下是您不能使用更直接订阅的原因：

var subscribeToNewRequestObservable = () => {
    this._pollLiveData()
      .subscribe(pollSubject);
  };

完成 http.get-observable 也会完成您的主题并防止它发出更多项目。

这仍然是一个 cold observable，因此除非您订阅它，否则不会发出任何请求。

this._pollService.pollData().subscribe(
  (res) => { this.count = res.count; }
);

Answer 3

更新到 RxJS 6

import { timer } from 'rxjs';
import { concatMap, map, expand, catchError } from 'rxjs/operators';

pollData$ = this._http.get(this._url)
  .pipe(
    map(this.extractData),
    catchError(this.handleError)
  );

pollData$.pipe(
  expand(_ => timer(4000).pipe(concatMap(_ => pollData$)))
).subscribe();

我正在使用 RxJS 5，但不确定 RxJS 4 等效运算符是什么。无论如何，这是我的 RxJS 5 解决方案，希望对您有所帮助：

var pollData = this._http.get(this._url)
            .map(this.extractData)
            .catch(this.handleError);
pollData.expand(
  () => Observable.timer(4000).concatMap(() => pollData)
).subscribe();

扩展运算符将发射数据并递归地开始一个新的 Observable 每次发射

Answer 4

Can Nguyen 对的小修改，以防您希望轮询延迟取决于之前的请求完成状态。

var pollData = () => request()   // make request
    .do(handler, errorHandler)   // handle response data or error
    .ignoreElements()            // ignore request progress notifications
    .materialize();              // wrap error/complete notif-ns into Notification

pollData()                            // get our Observable<Notification>...
  .expand(                            // ...and recursively map...
    (n) => Rx.Observable              // ...each Notification object...
      .timer(n.error ? 1000 : 5000)   // ...(with delay depending on previous completion status)...
      .concatMap(() => pollData()))   // ...to new Observable<Notification>
  .subscribe();

Plunk.

或者：

var pollData = () => request()             // make request
    .last()                                // take last progress value
    .catch(() => Rx.Observable.of(null));  // replace error with null-value

pollData()
  .expand(
    (data) => Rx.Observable
      .timer(data ? 5000 : 1000)           // delay depends on a value
      .concatMap(() => pollData()))
  .subscribe((d) => {console.log(d);});    // can subscribe to the value stream at the end

Plunk.

完成后 n 秒重复请求 (Angular2 - http.get)

Repeat request (Angular2 - http.get) n seconds after finished

javascript

observable

rxjs

reactivex

angular