What do the various ISubject implementations do and when would they be used?

Question

I have a fairly good idea of what the Subject class does and when to use it, but I\'ve just been looking through the language reference on msdn and see there are various other I

Answer 1

In light of the latest version (v1.0.2856.0) and to keep this question up to date, there has been a new set of subject classes:

FastSubject, FastBehaviorSubject, FastAsyncSubject and FastReplaySubject

As per the release notes they

are much faster than regular subjects but:

• don’t decouple producer and consumer by an IScheduler (effectively limiting them to ImmediateScheduler);
• don’t protect against stack overflow;
• don’t synchronize input messages.

Fast subjects are used by Publish and Prune operators if no scheduler is specified.

Answer 2

Paul's answer pretty much nails it. There's a few things worth adding, though:

AsyncSubject works as Paul says, but only after the source completes. Before that, it works like Subject (where "live" values are received by subscribers)

AsyncSubject has changed since I last ran tests against it. It no longer acts as a live subject before completion, but waits for completion before it emits a value. And, as Sergey mentions, it returns the last value, not the first (though I should have caught that as that's always been the case)

AsyncSubject is used by Prune, FromAsyncPattern, ToAsync and probably a few others

BehaviorSubject is used by overloads of Publish that accept an initial value

ReplaySubject is used by Replay

NOTE: All operator references above refer to the publishing set of operators as they were before they were replaced with generalised publish operators in rev 2838 (Christmas '10) as it has been mentioned that the original operators will be re-added

Answer 3

In regards to AsyncSubject

This code:

        var s = new AsyncSubject<int>();
        s.OnNext(1);
        s.Subscribe(Console.WriteLine);
        s.OnNext(2);
        s.OnNext(3);
        s.OnCompleted();

prints a single value 3. And it prints same if subscription is moved to after completion. So it plays back not the first, but the last item, plays it after completion (until complete, it does not produce values), and it does not work like Subject before completion. See this Prune discussion for more info (AsyncSubject is basically the same as Prune)

Answer 4

These subjects all share a common property - they take some (or all) of what gets posted to them via OnNext and record it and play it back to you - i.e. they take a Hot Observable and make it Cold. This means, that if you Subscribe to any of these more than once (i.e. Subscribe => Unsubscribe => Subscribe again), you'll see at least one of the same value again.

ReplaySubject: Every time you subscribe to the Subject, you get the entire history of what has been posted replayed back to you, as fast as possible (or a subset, like the last n items)

AsyncSubject: Always plays back the last item posted and completes, but only after the source has completed. This Subject is awesome for async functions, since you can write them without worrying about race conditions: even if someone Subscribes after the async method completes, they get the result.

BehaviorSubject: Kind of like ReplaySubject but with a buffer of one, so you always get the last thing that was posted. You also can provide an initial value. Always provides one item instantly on Subscribe.