IObservable to produce results in infinite loop [duplicate]

Question

This question already has answers here:

How do I implement polling using Observables? (2 answers)

Closed 8 years ago.

This is a code that I've developed so far:

var observable = Observable.Create<string>(async observer =>
{
    var wc = new WebClient { UseDefaultCredentials = true };
    observer.OnNext(await wc.DownloadStringTaskAsync("http://ya.ru"));
});

observable.Subscribe(
    res => Debug.WriteLine("got result: {0}", res), 
    exc => Debug.WriteLine("exception: {0}", exc.Message)
); 

This correctly fetches website's data and triggers my callback once. What I want is to have an infinite loop that acts like this: await result -> call OnNext -> wait n seconds -> repeat an operation.

Creating an Observable.Interval and SelectMany it into my Observable won't quite do because this will be querying a website for a fixed periods of time. I want the next call to be triggered only after the previous succeeds or fails. What's the most elegant and concise way to achieve this?

Answer 1

Without changing too much of your code, you could effectively concat a delay after the yield, and then repeat the whole thing indefinitely.

var observable = Observable.Create<string>(async observer =>
{
    var wc = new WebClient { UseDefaultCredentials = true };
    observer.OnNext(await wc.DownloadStringTaskAsync("http://ya.ru"));
});

observable
    .Concat(Observable.Empty<string>().Delay(TimeSpan.FromSeconds(1)))
    .Repeat()
    .Subscribe(
      res => Debug.WriteLine("got result: {0}", res), 
      exc => Debug.WriteLine("exception: {0}", exc.Message)
    ); 

However, there's better ways of doing this, because in the previous instance you're creating a new WebClient every second. Instead, you could do something like this...

using System.Reactive.Threading.Tasks;

var observable = Observable.Using(() => new WebClient(), (client) =>
    client.DownloadStringTaskAsync("http://ya.ru")
        .ToObservable()
        .Concat(Observable.Empty<string>().Delay(TimeSpan.FromSeconds(1)))
        .Repeat()
        );

And if you wanted to repeat on errors, you could add Retry...

var observable = Observable.Using(() => new WebClient(), (client) =>
    client.DownloadStringTaskAsync("http://ya.ru")
        .ToObservable()
        .Retry(3)
        .Concat(Observable.Empty<string>().Delay(TimeSpan.FromSeconds(1)))
        .Repeat()
        );