Class UnicastSubject<T>

java.lang.Object
io.reactivex.rxjava4.core.Observable<T>
io.reactivex.rxjava4.subjects.Subject<T>
io.reactivex.rxjava4.subjects.UnicastSubject<T>
Type Parameters:
T - the value type received and emitted by this Subject subclass
All Implemented Interfaces:
ObservableSource<T>, Observer<T>
public final class UnicastSubject<T> extends Subject<T>
A Subject that queues up events until a single Observer subscribes to it, replays those events to it until the Observer catches up and then switches to relaying events live to this single Observer until this UnicastSubject terminates or the Observer disposes.

Note that UnicastSubject holds an unbounded internal buffer.

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

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

If more than one Observer attempts to subscribe to this UnicastSubject, they will receive an IllegalStateException indicating the single-use-only nature of this UnicastSubject, even if the UnicastSubject already terminated with an error.

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 a UnicastSubject is an Observable, it does not support backpressure.

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

Even though UnicastSubject 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 UnicastSubject 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).

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

Scheduler:
UnicastSubject does not operate by default on a particular Scheduler and the single Observer gets notified on the thread the respective onXXX methods were invoked.
Error handling:
When the onError(Throwable) is called, the UnicastSubject enters into a terminal state and emits the same Throwable instance to the current single Observer. During this emission, if the single Observers disposes its respective Disposable, the Throwable is delivered to the global error handler via RxJavaPlugins.onError(Throwable). If there were no Observers subscribed to this UnicastSubject when the onError() was called, the global error handler is not invoked.

Example usage: 

UnicastSubject<Integer> subject = UnicastSubject.create();

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

// fresh UnicastSubjects are empty
to1.assertEmpty();

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

// A UnicastSubject only allows one Observer during its lifetime
to2.assertFailure(IllegalStateException.class);

subject.onNext(1);
to1.assertValue(1);

subject.onNext(2);
to1.assertValues(1, 2);

subject.onComplete();
to1.assertResult(1, 2);

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

UnicastSubject<Integer> subject2 = UnicastSubject.create();

// a UnicastSubject caches events until its single Observer subscribes
subject2.onNext(1);
subject2.onNext(2);
subject2.onComplete();

TestObserver<Integer> to3 = subject2.test();

// the cached events are emitted in order
to3.assertResult(1, 2);
Since:
2.0

  • Method Details

    • create

      @CheckReturnValue @NonNull public static <T> @NonNull UnicastSubject<T> create()
      Creates an UnicastSubject with an internal buffer capacity hint 16.
      Type Parameters:
      T - the value type
      Returns:
      an UnicastSubject instance

    • create

      @CheckReturnValue @NonNull public static <T> @NonNull UnicastSubject<T> create(int capacityHint)
      Creates an UnicastSubject with the given internal buffer capacity hint.
      Type Parameters:
      T - the value type
      Parameters:
      capacityHint - the hint to size the internal unbounded buffer
      Returns:
      an UnicastSubject instance
      Throws:
      IllegalArgumentException - if capacityHint is non-positive

    • create

      @CheckReturnValue @NonNull public static <T> @NonNull UnicastSubject<T> create(int capacityHint, @NonNull @NonNull Runnable onTerminate)
      Creates an UnicastSubject with the given internal buffer capacity hint and a callback for the case when the single Subscriber cancels its subscription or the subject is terminated.

      The callback, if not null, is called exactly once and non-overlapped with any active replay.

      Type Parameters:
      T - the value type
      Parameters:
      capacityHint - the hint to size the internal unbounded buffer
      onTerminate - the callback to run when the Subject is terminated or cancelled, null not allowed
      Returns:
      an UnicastSubject instance
      Throws:
      NullPointerException - if onTerminate is null
      IllegalArgumentException - if capacityHint is non-positive

    • create

      @CheckReturnValue @NonNull public static <T> @NonNull UnicastSubject<T> create(int capacityHint, @NonNull @NonNull Runnable onTerminate, boolean delayError)
      Creates an UnicastSubject with the given internal buffer capacity hint, delay error flag and a callback for the case when the single Observer disposes its Disposable or the subject is terminated.

      The callback, if not null, is called exactly once and non-overlapped with any active replay.

      History: 2.0.8 - experimental

      Type Parameters:
      T - the value type
      Parameters:
      capacityHint - the hint to size the internal unbounded buffer
      onTerminate - the callback to run when the Subject is terminated or cancelled, null not allowed
      delayError - deliver pending onNext events before onError
      Returns:
      an UnicastSubject instance
      Throws:
      NullPointerException - if onTerminate is null
      IllegalArgumentException - if capacityHint is non-positive
      Since:
      2.2

    • create

      @CheckReturnValue @NonNull public static <T> @NonNull UnicastSubject<T> create(boolean delayError)
      Creates an UnicastSubject with an internal buffer capacity hint 16 and given delay error flag.

      The callback, if not null, is called exactly once and non-overlapped with any active replay.

      History: 2.0.8 - experimental

      Type Parameters:
      T - the value type
      Parameters:
      delayError - deliver pending onNext events before onError
      Returns:
      an UnicastSubject instance
      Since:
      2.2

    • 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

    • 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

    • getThrowable

      @Nullable @CheckReturnValue public @Nullable 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

    • 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: