Package io.reactivex.rxjava3.subjects

Class 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);

    • Constructor Detail

      • AsyncSubject

        AsyncSubject()
        Constructs an AsyncSubject.
        Since:
        2.0

    • Method Detail

      • onError

        public void onError​(java.lang.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

      • 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

      • 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:
        Subject.hasThrowable()

      • getThrowable

        @CheckReturnValue
public java.lang.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

      • add

        boolean add​(AsyncSubject.AsyncDisposable<T> ps)
        Tries to add the given subscriber to the subscribers array atomically or returns false if the subject has terminated.
        Parameters:
        ps - the subscriber to add
        Returns:
        true if successful, false if the subject has terminated

      • remove

        void remove​(AsyncSubject.AsyncDisposable<T> ps)
        Atomically removes the given subscriber if it is subscribed to the subject.
        Parameters:
        ps - the subject to remove

      • 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