如何使用Reactive Extensions（Rx.Net）等待一个值或直到经过一段固定的时间

Question

I want to wait (block) a thread until either a time has elapsed or another stream pumps a value, I thought the following might achieve this but it throws an exception because the first stream is empty,

 // class level subject manipulated by another thread...
 _updates = new Subject<Unit>();
 ...
 // wait for up to 5 seconds before carrying on...    
 var result = Observable.Timer(DateTime.Now.AddSeconds(5))
    .TakeUntil(_updates)
    .Wait();

How can I achieve the ability to block for up to 5 seconds or until the other stream pumps a value?

Answer 1

You can use Observable.Timeout like this:

 var result = _updates.Take(1).Timeout(DateTime.Now.AddSeconds(5)).Wait();

I use Take(1) because timeout expects sequence to complete, not just produce next value. On timeout it will throw System.TimeoutException .

If you don't want exception - you can use Catch to provide some value instead:

var result = _updates.Take(1).Timeout(DateTime.Now.AddSeconds(5))
    .Catch(Observable.Return(default(Unit))).Wait();
// should catch specific exception, not all

If your Unit is indeed that rx unit mentioned by @Shlomo - you can change it like this:

var result = _updates.Select(c => (Unit?) c).Take(1)
    .Timeout(DateTime.Now.AddSeconds(5)).Catch(Observable.Return((Unit?) null)).Wait();

or just catch that exception as usual.

Answer 2

Here's another alternative if you don't want to deal with exceptions:

var _updates = new Subject<Unit>();

var result = Observable.Merge(
    _updates.Materialize(),
    Observable.Empty<Unit>()
       .Delay(TimeSpan.FromSeconds(5))
       .Materialize()
   )
   .Take(1)
   .Wait();

switch (result.Kind)
{
    case NotificationKind.OnCompleted:
        //time's up, or someone called _updates.OnCompleted().
        break;
    case NotificationKind.OnNext:
        var message = result.Value;
        //Message received. Handle message
        break;
    case NotificationKind.OnError:
        var exception = result.Exception;
        //Exception thrown. Handle exception
        break;
}

Answer 3

Can you create two tasks and then do a wait any? That would likely be the "easiest" way to achieve that.

var waitTask = new Task(() => Thread.Sleep(5000));
var messageTask = new Task(() => <listen for input and return>);

await Task.WhenAny(waitTask, messageTask);

<the rest of your code>

Edit: Title changed to specify React, answer probably wouldn't be applicable anymore.