A Subject that emits the very last value followed by a completion event or the received error to Observers.

This subject does not have a public constructor by design; a new empty instance of this AsyncSubject can be created via the create() method.

Since a Subject is conceptionally derived from the Processor type in the Reactive Streams specification, nulls are not allowed (Rule 2.13) as parameters to onNext(Object) and onError(Throwable). Such calls will result in a NullPointerException being thrown and the subject's state is not changed.

Since an AsyncSubject is an io.reactivex.rxjava3.core.Observable, it does not support backpressure.

When this AsyncSubject is terminated via onError(Throwable), the last observed item (if any) is cleared and late io.reactivex.rxjava3.core.Observers only receive the onError event.

The AsyncSubject caches the latest item internally and it emits this item only when onComplete is called. Therefore, it is not recommended to use this Subject with infinite or never-completing sources.

Even though AsyncSubject implements the Observer interface, calling onSubscribe is not required (Rule 2.12) if the subject is used as a standalone source. However, calling onSubscribe after the AsyncSubject reached its terminal state will result in the given Disposable being disposed immediately.

Calling onNext(Object), onError(Throwable) and onComplete() is required to be serialized (called from the same thread or called non-overlappingly from different threads through external means of serialization). The toSerialized() method available to all Subjects provides such serialization and also protects against reentrance (i.e., when a downstream Observer consuming this subject also wants to call onNext(Object) on this subject recursively). The implementation of onXXX methods are technically thread-safe but non-serialized calls to them may lead to undefined state in the currently subscribed Observers.

This AsyncSubject supports the standard state-peeking methods hasComplete(), hasThrowable(), getThrowable() and hasObservers() as well as means to read the very last observed value - after this AsyncSubject has been completed - in a non-blocking and thread-safe manner via hasValue() or getValue().

Scheduler:

AsyncSubject does not operate by default on a particular io.reactivex.rxjava3.core.Scheduler and the Observers get notified on the thread where the terminating onError or onComplete methods were invoked.

Error handling:

When the onError(Throwable) is called, the AsyncSubject enters into a terminal state and emits the same Throwable instance to the last set of Observers. During this emission, if one or more Observers dispose their respective Disposables, the Throwable is delivered to the global error handler via io.reactivex.rxjava3.plugins.RxJavaPlugins#onError(Throwable) (multiple times if multiple Observers cancel at once). If there were no Observers subscribed to this AsyncSubject when the onError() was called, the global error handler is not invoked.

Example usage:

AsyncSubject<Object> subject = AsyncSubject.create();

TestObserver<Object> to1 = subject.test();

to1.assertEmpty();

subject.onNext(1);

// AsyncSubject only emits when onComplete was called.
to1.assertEmpty();

subject.onNext(2);
subject.onComplete();

// onComplete triggers the emission of the last cached item and the onComplete event.
to1.assertResult(2);

TestObserver<Object> to2 = subject.test();

// late Observers receive the last cached item too
to2.assertResult(2);