CODEWITHSUNDEEP
c#reactive-programmingsystem.reactive
Hopeless
Hopeless

Reputation: 4763

Rx code blocks UI thread?

I'm experiencing Reactive Programming (Rx) and one of its interesting feature is subscribing and observing on different threads. But here somehow it blocks the UI thread. Technically I don't have any method returning Task (async method), so here I'm trying to mimic a long process with Thread.Sleep:

IEnumerable<Item> _search(string searchText)
{
        Thread.Sleep(3000);
        //return result by querying ...
        //...
        return someResult;
}

I have a ViewModel class like this:

public class ViewModel {
   public ViewModel(){
       //this SubscribeOn may not be necessary but I just try it here for sure
       SearchTextStream.SubscribeOn(NewThreadScheduler.Default)
                       .ObserveOn(DispatcherScheduler.Current)
                       .Subscribe(searchText => {
                          var items = _search(searchText);
                 }, ex => {
                    //handle error
            });
   }
   public string SearchText
   {
        get
        {
            return _searchText.FirstAsync().Wait();
        }
        set
        {
            _searchText.OnNext(value);
        }
   }
   ISubject<string> _searchText = new BehaviorSubject<string>("");
   public IObservable<string> SearchTextStream
   {
        get
        {
            return _searchText.AsObservable().DistinctUntilChanged();
        }
   }
}

Actually without using Thread.Sleep, I can still see it blocks the UI but not very obvious, so I just use it to make it more obvious. As I said, the scenario here is that I have just a normal method without any task or async. It may be a long-running method. Using with RX, I don't know which should be done to make it behave like async (as when using a Task.Run)?

I'm testing on a WPF application if it matters.

Upvotes: 1

Views: 552

Answers (1)

Enigmativity
Enigmativity

Reputation: 117010

You're calling _search(searchText) on the DispatcherScheduler.Current scheduler - and hence, with the Thread.Sleep you're blocking the UI.

You really should make _search return an observable.

IObservable<IEnumerable<Item>> _search(string searchText)
{
    Thread.Sleep(3000);
    //return result by querying ...
    //...
    return Observable.Return(new [] { new Item() });
}

Now the constructor should look like this:

public ViewModel()
{
    SearchTextStream
        .ObserveOn(System.Reactive.Concurrency.Scheduler.Default)
        .SelectMany(searchText => _search(searchText))
        .ObserveOnDispatcher()
        .Subscribe(items =>
        {
            /* do something with `items` */
        }, ex =>
        {
            //handle error
        });
}

Upvotes: 1

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