What you think about DefereedObject instead of callbacks?

[lexer]

AsyncResult<ResultClass> doSmth(String param1);

Usage:

AsyncResult<ResultClass> result = apiClient.getSmth("foo");

result.onSuccess(new ResultCallback<ResultClass>(ResultClass result) { ... } );
result.onError(new ErrorCallback<ErroClass>(ErrorClass err) { ... } );
result.onFinally(new Runnable() { ... } );

Also we can do sync call:

ResultClass res = result.doSync();

It is very close to Guava ListenableFutures https://code.google.com/p/guava-libraries/wiki/ListenableFutureExplained. The only difference that ListenableFuture cannot execute itself and you have to execute them with another object.

I also like that with Guava ListenableFutures async operations can be merged to be executed in parallel or one by one.

[JakeWharton]

Interesting. I like the concept because it aligns closely with what I wanted for #119.

[lexer]

@JakeWharton There is also very good alternative to Guava Listenable Futures.

It's called Reactive Extensions for Java:

http://www.infoq.com/presentations/netflix-functional-rx
https://github.com/Netflix/RxJava

In this rest methods will return Observable

public intefrace Observable<T> {
    public Subscription subscribe(Observer<T> observer);
}

public interface Observer<T> {
     public void onCompleted();
     public void onError(Exception e);
     public void onNext(T args);
}

The benefit that Rx provides bunch of methods to merge observables to single observer to perform complex sequence of parallel or step by step execution.

[felipeleusin]

Anything up with this? Do you guys need help implementing it? This is a dearly missed feature for me

[codefromthecrypt]

there's a related issue here. if helpful, we could consolidate #262

in any case, we could support returning Subscription from a Callback, which would be of incremental value as you could have a lever to say cancel.

[JakeWharton]

Discussion moved to #297.