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

Since an AsyncProcessor is a Reactive Streams Processor type, 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 processor's state is not changed.

AsyncProcessor is a io.reactivex.rxjava3.core.Flowable as well as a FlowableProcessor and supports backpressure from the downstream but its Subscriber-side consumes items in an unbounded manner.

When this AsyncProcessor is terminated via onError(Throwable), the last observed item (if any) is cleared and late Subscribers only receive the onError event.

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

Even though AsyncProcessor implements the Subscriber interface, calling onSubscribe is not required (Rule 2.12) if the processor is used as a standalone source. However, calling onSubscribe after the AsyncProcessor reached its terminal state will result in the given Subscription being canceled 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 FlowableProcessors provides such serialization and also protects against reentrance (i.e., when a downstream Subscriber consuming this processor also wants to call onNext(Object) on this processor 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 Subscribers.

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

Backpressure:

The AsyncProcessor honors the backpressure of the downstream Subscribers and won't emit its single value to a particular Subscriber until that Subscriber has requested an item. When the AsyncProcessor is subscribed to a io.reactivex.rxjava3.core.Flowable, the processor consumes this Flowable in an unbounded manner (requesting Long#MAX_VALUE) as only the very last upstream item is retained by it.

Scheduler:

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

Error handling:

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

Example usage:

AsyncProcessor<Object> processor = AsyncProcessor.create();

TestSubscriber<Object> ts1 = processor.test();

ts1.assertEmpty();

processor.onNext(1);

// AsyncProcessor only emits when onComplete was called.
ts1.assertEmpty();

processor.onNext(2);
processor.onComplete();

// onComplete triggers the emission of the last cached item and the onComplete event.
ts1.assertResult(2);

TestSubscriber<Object> ts2 = processor.test();

// late Subscribers receive the last cached item too
ts2.assertResult(2);