A FlowableProcessor variant that queues up events until a single Subscriber subscribes to it, replays those events to it until the Subscriber catches up and then switches to relaying events live to this single Subscriber until this UnicastProcessor terminates or the Subscriber cancels its subscription.

This processor does not have a public constructor by design; a new empty instance of this UnicastProcessor can be created via the following create methods that allow specifying the retention policy for items:

• create() - creates an empty, unbounded UnicastProcessor that caches all items and the terminal event it receives.
• create(int) - creates an empty, unbounded UnicastProcessor with a hint about how many total items one expects to retain.
• create(boolean) - creates an empty, unbounded UnicastProcessor that optionally delays an error it receives and replays it after the regular items have been emitted.
• create(int, Runnable) - creates an empty, unbounded UnicastProcessor with a hint about how many total items one expects to retain and a callback that will be called exactly once when the UnicastProcessor gets terminated or the single Subscriber cancels.
• create(int, Runnable, boolean) - creates an empty, unbounded UnicastProcessor with a hint about how many total items one expects to retain and a callback that will be called exactly once when the UnicastProcessor gets terminated or the single Subscriber cancels and optionally delays an error it receives and replays it after the regular items have been emitted.

If more than one Subscriber attempts to subscribe to this Processor, they will receive an IllegalStateException if this UnicastProcessor hasn't terminated yet, or the Subscribers receive the terminal event (error or completion) if this Processor has terminated.

The UnicastProcessor buffers notifications and replays them to the single Subscriber as requested, for which it holds upstream items an unbounded internal buffer until they can be emitted.

Since a UnicastProcessor is a Reactive Streams Processor, 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.

Since a UnicastProcessor is a io.reactivex.rxjava3.core.Flowable as well as a FlowableProcessor, it honors the downstream backpressure but consumes an upstream source in an unbounded manner (requesting Long#MAX_VALUE).

When this UnicastProcessor is terminated via onError(Throwable) the current or late single Subscriber may receive the Throwable before any available items could be emitted. To make sure an onError event is delivered to the Subscriber after the normal items, create a UnicastProcessor with the create(boolean) or create(int, Runnable, boolean) factory methods.

Even though UnicastProcessor 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 UnicastProcessor 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).

This UnicastProcessor supports the standard state-peeking methods hasComplete(), hasThrowable(), getThrowable() and hasSubscribers().

Backpressure:

UnicastProcessor honors the downstream backpressure but consumes an upstream source (if any) in an unbounded manner (requesting Long#MAX_VALUE).

Scheduler:

UnicastProcessor does not operate by default on a particular io.reactivex.rxjava3.core.Scheduler and the single Subscriber gets notified on the thread the respective onXXX methods were invoked.

Error handling:

When the onError(Throwable) is called, the UnicastProcessor enters into a terminal state and emits the same Throwable instance to the current single Subscriber. During this emission, if the single Subscribers cancels its respective Subscriptions, the Throwable is delivered to the global error handler via io.reactivex.rxjava3.plugins.RxJavaPlugins#onError(Throwable). If there were no Subscribers subscribed to this UnicastProcessor when the onError() was called, the global error handler is not invoked.

Example usage:

UnicastProcessor<Integer> processor = UnicastProcessor.create();

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

// fresh UnicastProcessors are empty
ts1.assertEmpty();

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

// A UnicastProcessor only allows one Subscriber during its lifetime
ts2.assertFailure(IllegalStateException.class);

processor.onNext(1);
ts1.assertValue(1);

processor.onNext(2);
ts1.assertValues(1, 2);

processor.onComplete();
ts1.assertResult(1, 2);

// ----------------------------------------------------

UnicastProcessor<Integer> processor2 = UnicastProcessor.create();

// a UnicastProcessor caches events until its single Subscriber subscribes
processor2.onNext(1);
processor2.onNext(2);
processor2.onComplete();

TestSubscriber<Integer> ts3 = processor2.test();

// the cached events are emitted in order
ts3.assertResult(1, 2);