io.reactivex.rxjava3.subjects

Class UnicastSubject<T>

java.lang.Object

io.reactivex.rxjava3.core.Observable<T>

io.reactivex.rxjava3.subjects.Subject<T>

io.reactivex.rxjava3.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 unsubscribes.

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 unsubscribes.
• 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 unsubscribes 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 Summary

Modifier and Type	Method and Description
static <T> UnicastSubject<T>	create() Creates an UnicastSubject with an internal buffer capacity hint 16.
static <T> UnicastSubject<T>	create(boolean delayError) Creates an UnicastSubject with an internal buffer capacity hint 16 and given delay error flag.
static <T> UnicastSubject<T>	create(int capacityHint) Creates an UnicastSubject with the given internal buffer capacity hint.
static <T> UnicastSubject<T>	create(int capacityHint, 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.
static <T> UnicastSubject<T>	create(int capacityHint, 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 Subscriber cancels its subscription.
Throwable	getThrowable() Returns the error that caused the Subject to terminate or null if the Subject hasn't terminated yet.
boolean	hasComplete() Returns true if the subject has reached a terminal state through a complete event.
boolean	hasObservers() Returns true if the subject has any Observers.
boolean	hasThrowable() Returns true if the subject has reached a terminal state through an error event.
void	onComplete() Notifies the Observer that the Observable has finished sending push-based notifications.
void	onError(Throwable t) Notifies the Observer that the Observable has experienced an error condition.
void	onNext(T t) Provides the Observer with a new item to observe.
void	onSubscribe(Disposable d) 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.
protected void	subscribeActual(Observer<? super T> observer) Operator implementations (both source and intermediate) should implement this method that performs the necessary business logic and handles the incoming Observers.

Methods inherited from class io.reactivex.rxjava3.subjects.Subject

toSerialized

Methods inherited from class io.reactivex.rxjava3.core.Observable

all, amb, ambArray, ambWith, any, blockingFirst, blockingFirst, blockingForEach, blockingIterable, blockingIterable, blockingLast, blockingLast, blockingLatest, blockingMostRecent, blockingNext, blockingSingle, blockingSingle, blockingSubscribe, blockingSubscribe, blockingSubscribe, blockingSubscribe, blockingSubscribe, buffer, buffer, buffer, buffer, buffer, buffer, buffer, buffer, buffer, buffer, buffer, buffer, buffer, buffer, buffer, buffer, buffer, bufferSize, cache, cacheWithInitialCapacity, cast, collect, collectInto, combineLatest, combineLatest, combineLatest, combineLatest, combineLatest, combineLatest, combineLatest, combineLatest, combineLatest, combineLatest, combineLatestArray, combineLatestArray, combineLatestDelayError, combineLatestDelayError, combineLatestDelayError, combineLatestDelayError, compose, concat, concat, concat, concat, concat, concat, concatArray, concatArrayDelayError, concatArrayEager, concatArrayEager, concatArrayEagerDelayError, concatArrayEagerDelayError, concatDelayError, concatDelayError, concatDelayError, concatEager, concatEager, concatEager, concatEager, concatMap, concatMap, concatMap, concatMapCompletable, concatMapCompletable, concatMapCompletableDelayError, concatMapCompletableDelayError, concatMapCompletableDelayError, concatMapDelayError, concatMapDelayError, concatMapDelayError, concatMapEager, concatMapEager, concatMapEagerDelayError, concatMapEagerDelayError, concatMapIterable, concatMapIterable, concatMapMaybe, concatMapMaybe, concatMapMaybeDelayError, concatMapMaybeDelayError, concatMapMaybeDelayError, concatMapSingle, concatMapSingle, concatMapSingleDelayError, concatMapSingleDelayError, concatMapSingleDelayError, concatWith, concatWith, concatWith, concatWith, contains, count, create, debounce, debounce, debounce, defaultIfEmpty, defer, delay, delay, delay, delay, delay, delay, delaySubscription, delaySubscription, delaySubscription, dematerialize, distinct, distinct, distinct, distinctUntilChanged, distinctUntilChanged, distinctUntilChanged, doAfterNext, doAfterTerminate, doFinally, doOnComplete, doOnDispose, doOnEach, doOnEach, doOnError, doOnLifecycle, doOnNext, doOnSubscribe, doOnTerminate, elementAt, elementAt, elementAtOrError, empty, error, error, filter, first, firstElement, firstOrError, flatMap, flatMap, flatMap, flatMap, flatMap, flatMap, flatMap, flatMap, flatMap, flatMap, flatMap, flatMap, flatMapCompletable, flatMapCompletable, flatMapIterable, flatMapIterable, flatMapMaybe, flatMapMaybe, flatMapSingle, flatMapSingle, forEach, forEachWhile, forEachWhile, forEachWhile, fromArray, fromCallable, fromFuture, fromFuture, fromFuture, fromFuture, fromIterable, fromPublisher, fromSupplier, generate, generate, generate, generate, generate, groupBy, groupBy, groupBy, groupBy, groupBy, groupJoin, hide, ignoreElements, interval, interval, interval, interval, intervalRange, intervalRange, isEmpty, join, just, just, just, just, just, just, just, just, just, just, last, lastElement, lastOrError, lift, map, materialize, merge, merge, merge, merge, merge, merge, merge, merge, mergeArray, mergeArray, mergeArrayDelayError, mergeArrayDelayError, mergeDelayError, mergeDelayError, mergeDelayError, mergeDelayError, mergeDelayError, mergeDelayError, mergeDelayError, mergeDelayError, mergeWith, mergeWith, mergeWith, mergeWith, never, observeOn, observeOn, observeOn, ofType, onErrorResumeNext, onErrorResumeWith, onErrorReturn, onErrorReturnItem, onTerminateDetach, publish, publish, range, rangeLong, reduce, reduce, reduceWith, repeat, repeat, repeatUntil, repeatWhen, replay, replay, replay, replay, replay, replay, replay, replay, replay, replay, replay, replay, replay, replay, replay, replay, replay, replay, retry, retry, retry, retry, retry, retryUntil, retryWhen, safeSubscribe, sample, sample, sample, sample, sample, sample, scan, scan, scanWith, sequenceEqual, sequenceEqual, sequenceEqual, sequenceEqual, serialize, share, single, singleElement, singleOrError, skip, skip, skip, skipLast, skipLast, skipLast, skipLast, skipLast, skipLast, skipUntil, skipWhile, sorted, sorted, startWith, startWithArray, startWithItem, startWithIterable, subscribe, subscribe, subscribe, subscribe, subscribe, subscribeOn, subscribeWith, switchIfEmpty, switchMap, switchMap, switchMapCompletable, switchMapCompletableDelayError, switchMapDelayError, switchMapDelayError, switchMapMaybe, switchMapMaybeDelayError, switchMapSingle, switchMapSingleDelayError, switchOnNext, switchOnNext, switchOnNextDelayError, switchOnNextDelayError, take, take, take, takeLast, takeLast, takeLast, takeLast, takeLast, takeLast, takeLast, takeLast, takeLast, takeUntil, takeUntil, takeWhile, test, test, throttleFirst, throttleFirst, throttleLast, throttleLast, throttleLatest, throttleLatest, throttleLatest, throttleLatest, throttleWithTimeout, throttleWithTimeout, timeInterval, timeInterval, timeInterval, timeInterval, timeout, timeout, timeout, timeout, timeout, timeout, timeout, timeout, timer, timer, timestamp, timestamp, timestamp, timestamp, to, toFlowable, toFuture, toList, toList, toList, toMap, toMap, toMap, toMultimap, toMultimap, toMultimap, toMultimap, toSortedList, toSortedList, toSortedList, toSortedList, unsafeCreate, unsubscribeOn, using, using, window, window, window, window, window, window, window, window, window, window, window, window, window, window, window, window, window, withLatestFrom, withLatestFrom, withLatestFrom, withLatestFrom, withLatestFrom, withLatestFrom, wrap, zip, zip, zip, zip, zip, zip, zip, zip, zip, zip, zip, zip, zipArray, zipWith, zipWith, zipWith, zipWith

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Method Detail

create

@CheckReturnValue
 @NonNull
public static <T> 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> 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

create

@CheckReturnValue
 @NonNull
public static <T> UnicastSubject<T> create(int capacityHint,
                                                                       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.

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

create

@CheckReturnValue
 @NonNull
public static <T> UnicastSubject<T> create(int capacityHint,
                                                                       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 Subscriber cancels its subscription.

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

Since:

2.2

create

@CheckReturnValue
 @NonNull
public static <T> 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(java.lang.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(java.lang.Throwable).

hasObservers

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
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

hasThrowable

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:

Subject.getThrowable(), Subject.hasComplete()

hasComplete

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()