Queue like Subject in RxJava

Question

I\'m looking for a Subject (or something similar) that can:

1. Could receive items and hold them in a queue or buffer if there are no subscribers
2. Once we

Answer 1

I had similar problem, my requirements were:

• Should support replay of values when no Observer subscribed
• Should allow only one Observer subscribed at a time
• Should allow another Observer to subscribe when first Observer is disposed

I've implemented it as RxRelay but the implementation for Subject would be similar:

public final class CacheRelay extends Relay {

    private final ConcurrentLinkedQueue queue = new ConcurrentLinkedQueue<>();
    private final PublishRelay relay = PublishRelay.create();

    private CacheRelay() {
    }

    public static  CacheRelay create() {
        return new CacheRelay<>();
    }

    @Override
    public void accept(T value) {
        if (relay.hasObservers()) {
            relay.accept(value);
        } else {
            queue.add(value);
        }
    }

    @Override
    public boolean hasObservers() {
        return relay.hasObservers();
    }

    @Override
    protected void subscribeActual(Observer observer) {
        if (hasObservers()) {
            EmptyDisposable.error(new IllegalStateException("Only a single observer at a time allowed."), observer);
        } else {
            for (T element; (element = queue.poll()) != null; ) {
                observer.onNext(element);
            }
            relay.subscribeActual(observer);
        }
    }
}

Look at this Gist for more