https://github.com/ReactiveX/RxJava: Reactive Extensions for Java
  • David Karnok 
Copyright (c) 2016-present, RxJava Contributors.
Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in
compliance with the License. You may obtain a copy of the License at
http://www.apache.org/licenses/LICENSE-2.0
Unless required by applicable law or agreed to in writing, software distributed under the License is
distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See
the License for the specific language governing permissions and limitations under the License.

/**
 * Copyright (c) 2016-present, RxJava Contributors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in
 * compliance with the License. You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software distributed under the License is
 * distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See
 * the License for the specific language governing permissions and limitations under the License.
 */

package io.reactivex.subjects;

import io.reactivex.annotations.Nullable;
import io.reactivex.annotations.NonNull;
import java.util.Arrays;
import java.util.concurrent.atomic.AtomicReference;

import io.reactivex.Observer;
import io.reactivex.annotations.CheckReturnValue;
import io.reactivex.disposables.Disposable;
import io.reactivex.internal.functions.ObjectHelper;
import io.reactivex.internal.observers.DeferredScalarDisposable;
import io.reactivex.plugins.RxJavaPlugins;


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(), getValue(), getValues() or getValues(Object[]). 

 
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);



Type parameters:
  • <T> – the value type
/**
 * A Subject that emits the very last value followed by a completion event or the received error to Observers.
 * <p>
 * <img width="640" height="239" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/AsyncSubject.png" alt="">
 * <p>
 * This subject does not have a public constructor by design; a new empty instance of this
 * {@code AsyncSubject} can be created via the {@link #create()} method.
 * <p>
 * Since a {@code Subject} is conceptionally derived from the {@code Processor} type in the Reactive Streams specification,
 * {@code null}s are not allowed (<a href="https://github.com/reactive-streams/reactive-streams-jvm#2.13">Rule 2.13</a>)
 * as parameters to {@link #onNext(Object)} and {@link #onError(Throwable)}. Such calls will result in a
 * {@link NullPointerException} being thrown and the subject's state is not changed.
 * <p>
 * Since an {@code AsyncSubject} is an {@link io.reactivex.Observable}, it does not support backpressure.
 * <p>
 * When this {@code AsyncSubject} is terminated via {@link #onError(Throwable)}, the
 * last observed item (if any) is cleared and late {@link io.reactivex.Observer}s only receive
 * the {@code onError} event.
 * <p>
 * The {@code AsyncSubject} caches the latest item internally and it emits this item only when {@code onComplete} is called.
 * Therefore, it is not recommended to use this {@code Subject} with infinite or never-completing sources.
 * <p>
 * Even though {@code AsyncSubject} implements the {@code Observer} interface, calling
 * {@code onSubscribe} is not required (<a href="https://github.com/reactive-streams/reactive-streams-jvm#2.12">Rule 2.12</a>)
 * if the subject is used as a standalone source. However, calling {@code onSubscribe}
 * after the {@code AsyncSubject} reached its terminal state will result in the
 * given {@code Disposable} being disposed immediately.
 * <p>
 * Calling {@link #onNext(Object)}, {@link #onError(Throwable)} and {@link #onComplete()}
 * is required to be serialized (called from the same thread or called non-overlappingly from different threads
 * through external means of serialization). The {@link #toSerialized()} method available to all {@code Subject}s
 * provides such serialization and also protects against reentrance (i.e., when a downstream {@code Observer}
 * consuming this subject also wants to call {@link #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.
 * <p>
 * This {@code AsyncSubject} supports the standard state-peeking methods {@link #hasComplete()}, {@link #hasThrowable()},
 * {@link #getThrowable()} and {@link #hasObservers()} as well as means to read the very last observed value -
 * after this {@code AsyncSubject} has been completed - in a non-blocking and thread-safe
 * manner via {@link #hasValue()}, {@link #getValue()}, {@link #getValues()} or {@link #getValues(Object[])}.
 * <dl>
 *  <dt><b>Scheduler:</b></dt>
 *  <dd>{@code AsyncSubject} does not operate by default on a particular {@link io.reactivex.Scheduler} and
 *  the {@code Observer}s get notified on the thread where the terminating {@code onError} or {@code onComplete}
 *  methods were invoked.</dd>
 *  <dt><b>Error handling:</b></dt>
 *  <dd>When the {@link #onError(Throwable)} is called, the {@code AsyncSubject} enters into a terminal state
 *  and emits the same {@code Throwable} instance to the last set of {@code Observer}s. During this emission,
 *  if one or more {@code Observer}s dispose their respective {@code Disposable}s, the
 *  {@code Throwable} is delivered to the global error handler via
 *  {@link io.reactivex.plugins.RxJavaPlugins#onError(Throwable)} (multiple times if multiple {@code Observer}s
 *  cancel at once).
 *  If there were no {@code Observer}s subscribed to this {@code AsyncSubject} when the {@code onError()}
 *  was called, the global error handler is not invoked.
 *  </dd>
 * </dl>
 * <p>
 * Example usage:
 * <pre><code>
 * AsyncSubject&lt;Object&gt; subject = AsyncSubject.create();
 * 
 * TestObserver&lt;Object&gt; 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&lt;Object&gt; to2 = subject.test();
 *
 * // late Observers receive the last cached item too
 * to2.assertResult(2);
 * </code></pre>
 * @param <T> the value type
 */
public final class AsyncSubject<T> extends Subject<T> {

    @SuppressWarnings("rawtypes")
    static final AsyncDisposable[] EMPTY = new AsyncDisposable[0];

    @SuppressWarnings("rawtypes")
    static final AsyncDisposable[] TERMINATED = new AsyncDisposable[0];

    final AtomicReference<AsyncDisposable<T>[]> subscribers;

    
Write before updating subscribers, read after reading subscribers as TERMINATED. 
/** Write before updating subscribers, read after reading subscribers as TERMINATED. */
    Throwable error;

    
Write before updating subscribers, read after reading subscribers as TERMINATED. 
/** Write before updating subscribers, read after reading subscribers as TERMINATED. */
    T value;

    
Creates a new AsyncProcessor.

Type parameters:
  • <T> – the value type to be received and emitted
Returns:the new AsyncProcessor instance
/**
     * Creates a new AsyncProcessor.
     * @param <T> the value type to be received and emitted
     * @return the new AsyncProcessor instance
     */
    @CheckReturnValue
    @NonNull
    public static <T> AsyncSubject<T> create() {
        return new AsyncSubject<T>();
    }

    
Constructs an AsyncSubject.

Since:2.0
/**
     * Constructs an AsyncSubject.
     * @since 2.0
     */
    @SuppressWarnings("unchecked")
    AsyncSubject() {
        this.subscribers = new AtomicReference<AsyncDisposable<T>[]>(EMPTY);
    }

    @Override
    public void onSubscribe(Disposable d) {
        if (subscribers.get() == TERMINATED) {
            d.dispose();
        }
    }

    @Override
    public void onNext(T t) {
        ObjectHelper.requireNonNull(t, "onNext called with null. Null values are generally not allowed in 2.x operators and sources.");
        if (subscribers.get() == TERMINATED) {
            return;
        }
        value = t;
    }

    @SuppressWarnings("unchecked")
    @Override
    public void onError(Throwable t) {
        ObjectHelper.requireNonNull(t, "onError called with null. Null values are generally not allowed in 2.x operators and sources.");
        if (subscribers.get() == TERMINATED) {
            RxJavaPlugins.onError(t);
            return;
        }
        value = null;
        error = t;
        for (AsyncDisposable<T> as : subscribers.getAndSet(TERMINATED)) {
            as.onError(t);
        }
    }

    @SuppressWarnings("unchecked")
    @Override
    public void onComplete() {
        if (subscribers.get() == TERMINATED) {
            return;
        }
        T v = value;
        AsyncDisposable<T>[] array = subscribers.getAndSet(TERMINATED);
        if (v == null) {
            for (AsyncDisposable<T> as : array) {
                as.onComplete();
            }
        } else {
            for (AsyncDisposable<T> as : array) {
                as.complete(v);
            }
        }
    }

    @Override
    public boolean hasObservers() {
        return subscribers.get().length != 0;
    }

    @Override
    public boolean hasThrowable() {
        return subscribers.get() == TERMINATED && error != null;
    }

    @Override
    public boolean hasComplete() {
        return subscribers.get() == TERMINATED && error == null;
    }

    @Override
    public Throwable getThrowable() {
        return subscribers.get() == TERMINATED ? error : null;
    }

    @Override
    protected void subscribeActual(Observer<? super T> observer) {
        AsyncDisposable<T> as = new AsyncDisposable<T>(observer, this);
        observer.onSubscribe(as);
        if (add(as)) {
            if (as.isDisposed()) {
                remove(as);
            }
        } else {
            Throwable ex = error;
            if (ex != null) {
                observer.onError(ex);
            } else {
                T v = value;
                if (v != null) {
                    as.complete(v);
                } else {
                    as.onComplete();
                }
            }
        }
    }

    
Tries to add the given subscriber to the subscribers array atomically
or returns false if the subject has terminated.

Params:
  • ps – the subscriber to add
Returns:true if successful, false if the subject has terminated
/**
     * Tries to add the given subscriber to the subscribers array atomically
     * or returns false if the subject has terminated.
     * @param ps the subscriber to add
     * @return true if successful, false if the subject has terminated
     */
    boolean add(AsyncDisposable<T> ps) {
        for (;;) {
            AsyncDisposable<T>[] a = subscribers.get();
            if (a == TERMINATED) {
                return false;
            }

            int n = a.length;
            @SuppressWarnings("unchecked")
            AsyncDisposable<T>[] b = new AsyncDisposable[n + 1];
            System.arraycopy(a, 0, b, 0, n);
            b[n] = ps;

            if (subscribers.compareAndSet(a, b)) {
                return true;
            }
        }
    }

    
Atomically removes the given subscriber if it is subscribed to the subject.

Params:
  • ps – the subject to remove
/**
     * Atomically removes the given subscriber if it is subscribed to the subject.
     * @param ps the subject to remove
     */
    @SuppressWarnings("unchecked")
    void remove(AsyncDisposable<T> ps) {
        for (;;) {
            AsyncDisposable<T>[] a = subscribers.get();
            int n = a.length;
            if (n == 0) {
                return;
            }

            int j = -1;
            for (int i = 0; i < n; i++) {
                if (a[i] == ps) {
                    j = i;
                    break;
                }
            }

            if (j < 0) {
                return;
            }

            AsyncDisposable<T>[] b;

            if (n == 1) {
                b = EMPTY;
            } else {
                b = new AsyncDisposable[n - 1];
                System.arraycopy(a, 0, b, 0, j);
                System.arraycopy(a, j + 1, b, j, n - j - 1);
            }
            if (subscribers.compareAndSet(a, b)) {
                return;
            }
        }
    }

    
Returns true if the subject has any value.

The method is thread-safe.

Returns:true if the subject has any value
/**
     * Returns true if the subject has any value.
     * <p>The method is thread-safe.
     * @return true if the subject has any value
     */
    public boolean hasValue() {
        return subscribers.get() == TERMINATED && value != null;
    }

    
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
/**
     * Returns a single value the Subject currently has or null if no such value exists.
     * <p>The method is thread-safe.
     * @return a single value the Subject currently has or null if no such value exists
     */
    @Nullable
    public T getValue() {
        return subscribers.get() == TERMINATED ? value : null;
    }

    
Returns an Object array containing snapshot all values of the Subject.

The method is thread-safe.

Returns:the array containing the snapshot of all values of the Subject
Deprecated:in 2.1.14; put the result of getValue() into an array manually, will be removed in 3.x
/**
     * Returns an Object array containing snapshot all values of the Subject.
     * <p>The method is thread-safe.
     * @return the array containing the snapshot of all values of the Subject
     * @deprecated in 2.1.14; put the result of {@link #getValue()} into an array manually, will be removed in 3.x
     */
    @Deprecated
    public Object[] getValues() {
        T v = getValue();
        return v != null ? new Object[] { v } : new Object[0];
    }

    
Returns a typed array containing a snapshot of all values of the Subject.

The method follows the conventions of Collection.toArray by setting the array element
after the last value to null (if the capacity permits).

The method is thread-safe.

Params:
  • array – the target array to copy values into if it fits
Returns:the given array if the values fit into it or a new array containing all values
Deprecated:in 2.1.14; put the result of getValue() into an array manually, will be removed in 3.x
/**
     * Returns a typed array containing a snapshot of all values of the Subject.
     * <p>The method follows the conventions of Collection.toArray by setting the array element
     * after the last value to null (if the capacity permits).
     * <p>The method is thread-safe.
     * @param array the target array to copy values into if it fits
     * @return the given array if the values fit into it or a new array containing all values
     * @deprecated in 2.1.14; put the result of {@link #getValue()} into an array manually, will be removed in 3.x
     */
    @Deprecated
    public T[] getValues(T[] array) {
        T v = getValue();
        if (v == null) {
            if (array.length != 0) {
                array[0] = null;
            }
            return array;
        }
        if (array.length == 0) {
            array = Arrays.copyOf(array, 1);
        }
        array[0] = v;
        if (array.length != 1) {
            array[1] = null;
        }
        return array;
    }

    static final class AsyncDisposable<T> extends DeferredScalarDisposable<T> {
        private static final long serialVersionUID = 5629876084736248016L;

        final AsyncSubject<T> parent;

        AsyncDisposable(Observer<? super T> actual, AsyncSubject<T> parent) {
            super(actual);
            this.parent = parent;
        }

        @Override
        public void dispose() {
            if (super.tryDispose()) {
                parent.remove(this);
            }
        }

        void onComplete() {
            if (!isDisposed()) {
                downstream.onComplete();
            }
        }

        void onError(Throwable t) {
            if (isDisposed()) {
                RxJavaPlugins.onError(t);
            } else {
                downstream.onError(t);
            }
        }
    }
}