RxJs 6 中的可观察轮询

Question

Just wanted to migrate some of my legacy code to a newer version of RxJs. The code was used for polling:

Observable
  .interval(5000)
  .startWith(0)
  .switchMap(() => this.apiService.getData())
  .subscribe(data => /* ... */)

This worked fine in RxJs 5.2.0 .

In RxJs 6.0.0 this doesn't work for multiple reasons ( Observable only method is create , interval seems to be static, no method chaining, ...).

How to do this in latest RxJs?

Answer 1

I did not run it:

interval(5000).pipe(
    switchMap(() => this.apiService.getData())
).subscribe(data => /* ... */)

Interval is now a function that returns an Observable and transformations are chained inside pipe function.

Answer 2

Referencing LeeCampbell's RxCookbook about reactive programming. Using interval is actually not practical. Because if you don't get a response from this.apiService.getData() in the interval value another value will be emitted by the interval and a new request is performed.

What you need to do is use a timer that emits a single value, use a switchMap to perform your request, and a repeat to poll again after success. Also, you can use a retry before the repeat if you want to retry after errors.

This is the implementation in C# using Rx.Net ( Ref ) :

public static IObservable<T> Poll<T>(this IObservable<T> request, TimeSpan period, IScheduler scheduler)
{
    return Observable.Timer(period, scheduler)
      .SelectMany(_ => request)
      .Retry()    //Loop on errors
      .Repeat();  //Loop on success
}

Which can be translated to RxJS 6 as following:

timer(5000).pipe(
    switchMap(tick => this.apiService.getData()),
    retry(3), // retry 3 times before bubbling an error
    repeat()
);

Further enhancements can be found in the LeeCampbell's RxCookbook .