Class AsyncSubject<T>

java.lang.Object
io.reactivex.rxjava4.core.Observable<T>
io.reactivex.rxjava4.subjects.Subject<T>
io.reactivex.rxjava4.subjects.AsyncSubject<T>
Type Parameters:
T - the value type
All Implemented Interfaces:
ObservableSource<T>, Observer<T>
public final class AsyncSubject<T> extends Subject<T>
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 Observable, it does not support backpressure.

When this AsyncSubject is terminated via onError(Throwable), the last observed item (if any) is cleared and late 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 Subject.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 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 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);

  • Method Details

    • create

      @CheckReturnValue @NonNull public static <T> @NonNull AsyncSubject<T> create()
      Creates a new AsyncProcessor.
      Type Parameters:
      T - the value type to be received and emitted
      Returns:
      the new AsyncProcessor instance

    • onSubscribe

      public void onSubscribe(Disposable d)
      Description copied from interface: Observer
      Provides the Observer with the means of cancelling (disposing) the connection (channel) with the Observable in both synchronous (from within Observer.onNext(Object)) and asynchronous manner.
      Parameters:
      d - the Disposable instance whose Disposable.dispose() can be called anytime to cancel the connection

    • onNext

      public void onNext(T t)
      Description copied from interface: Observer
      Provides the Observer with a new item to observe.

      The Observable may call this method 0 or more times.

      The Observable will not call this method again after it calls either Observer.onComplete() or Observer.onError(Throwable).

      Parameters:
      t - the item emitted by the Observable

    • onError

      public void onError(Throwable t)
      Description copied from interface: Observer
      Notifies the Observer that the Observable has experienced an error condition.

      If the Observable calls this method, it will not thereafter call Observer.onNext(T) or Observer.onComplete().

      Parameters:
      t - the exception encountered by the Observable

    • onComplete

      public void onComplete()
      Description copied from interface: Observer
      Notifies the Observer that the Observable has finished sending push-based notifications.

      The Observable will not call this method if it calls Observer.onError(Throwable).

    • hasObservers

      @CheckReturnValue public boolean hasObservers()
      Description copied from class: Subject
      Returns true if the subject has any Observers.

      The method is thread-safe.

      Specified by:
      hasObservers in class Subject<T>
      Returns:
      true if the subject has any Observers

    • hasThrowable

      @CheckReturnValue public boolean hasThrowable()
      Description copied from class: Subject
      Returns true if the subject has reached a terminal state through an error event.

      The method is thread-safe.

      Specified by:
      hasThrowable in class Subject<T>
      Returns:
      true if the subject has reached a terminal state through an error event
      See Also:

    • hasComplete

      @CheckReturnValue public boolean hasComplete()
      Description copied from class: Subject
      Returns true if the subject has reached a terminal state through a complete event.

      The method is thread-safe.

      Specified by:
      hasComplete in class Subject<T>
      Returns:
      true if the subject has reached a terminal state through a complete event
      See Also:

    • getThrowable

      @CheckReturnValue public Throwable getThrowable()
      Description copied from class: Subject
      Returns the error that caused the Subject to terminate or null if the Subject hasn't terminated yet.

      The method is thread-safe.

      Specified by:
      getThrowable in class Subject<T>
      Returns:
      the error that caused the Subject to terminate or null if the Subject hasn't terminated yet

    • subscribeActual

      protected void subscribeActual(Observer<? super T> observer)
      Description copied from class: Observable
      Operator implementations (both source and intermediate) should implement this method that performs the necessary business logic and handles the incoming Observers.

      There is no need to call any of the plugin hooks on the current Observable instance or the Observer; all hooks and basic safeguards have been applied by Observable.subscribe(Observer) before this method gets called.

      Specified by:
      subscribeActual in class Observable<T>
      Parameters:
      observer - the incoming Observer, never null

    • hasValue

      @CheckReturnValue public boolean hasValue()
      Returns true if the subject has any value.

      The method is thread-safe.

      Returns:
      true if the subject has any value

    • getValue

      @Nullable @CheckReturnValue public T getValue()
      Returns a single value the Subject currently has or null if no such value exists.

      The method is thread-safe.

      Returns:
      a single value the Subject currently has or null if no such value exists