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.processors;

import java.util.Arrays;
import java.util.concurrent.atomic.AtomicReference;

import org.reactivestreams.*;

import io.reactivex.annotations.*;
import io.reactivex.internal.functions.ObjectHelper;
import io.reactivex.internal.subscriptions.DeferredScalarSubscription;
import io.reactivex.plugins.RxJavaPlugins;


Processor that emits the very last value followed by a completion event or the received error to Subscribers. 



 This processor does not have a public constructor by design; a new empty instance of this AsyncProcessor can be created via the create() method. 
 Since an AsyncProcessor is a Reactive Streams Processor type, 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 processor's state is not changed. 
 AsyncProcessor is a Flowable as well as a FlowableProcessor and supports backpressure from the downstream but its Subscriber-side consumes items in an unbounded manner. 
 When this AsyncProcessor is terminated via onError(Throwable), the last observed item (if any) is cleared and late Subscribers only receive the onError event. 
 The AsyncProcessor caches the latest item internally and it emits this item only when onComplete is called. Therefore, it is not recommended to use this Processor with infinite or never-completing sources. 
 Even though AsyncProcessor implements the Subscriber interface, calling onSubscribe is not required (Rule 2.12) if the processor is used as a standalone source. However, calling onSubscribe after the AsyncProcessor reached its terminal state will result in the given Subscription being canceled 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 FlowableProcessor.toSerialized() method available to all FlowableProcessors provides such serialization and also protects against reentrance (i.e., when a downstream Subscriber consuming this processor also wants to call onNext(Object) on this processor 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 Subscribers. 
 This AsyncProcessor supports the standard state-peeking methods hasComplete(), hasThrowable(), getThrowable() and hasSubscribers() as well as means to read the very last observed value - after this AsyncProcessor has been completed - in a non-blocking and thread-safe manner via hasValue(), getValue(), getValues() or getValues(Object[]). 

 
Backpressure:

 
The AsyncProcessor honors the backpressure of the downstream Subscribers and won't emit its single value to a particular Subscriber until that Subscriber has requested an item. When the AsyncProcessor is subscribed to a Flowable, the processor consumes this Flowable in an unbounded manner (requesting `Long.MAX_VALUE`) as only the very last upstream item is retained by it. 

 
Scheduler:

 
AsyncProcessor does not operate by default on a particular Scheduler and the Subscribers get notified on the thread where the terminating onError or onComplete methods were invoked.

 
Error handling:

 
When the onError(Throwable) is called, the AsyncProcessor enters into a terminal state and emits the same Throwable instance to the last set of Subscribers. During this emission, if one or more Subscribers dispose their respective Subscriptions, the Throwable is delivered to the global error handler via RxJavaPlugins.onError(Throwable) (multiple times if multiple Subscribers cancel at once). If there were no Subscribers subscribed to this AsyncProcessor when the onError() was called, the global error handler is not invoked. 





Example usage:


AsyncProcessor<Object> processor = AsyncProcessor.create();
TestSubscriber<Object> ts1 = processor.test();
ts1.assertEmpty();
processor.onNext(1);
// AsyncProcessor only emits when onComplete was called.
ts1.assertEmpty();
processor.onNext(2);
processor.onComplete();
// onComplete triggers the emission of the last cached item and the onComplete event.
ts1.assertResult(2);
TestSubscriber<Object> ts2 = processor.test();
// late Subscribers receive the last cached item too
ts2.assertResult(2);



Type parameters:
  • <T> – the value type
/**
 * Processor that emits the very last value followed by a completion event or the received error
 * to {@link Subscriber}s.
 * <p>
 * <img width="640" height="239" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/AsyncProcessor.png" alt="">
 * <p>
 * This processor does not have a public constructor by design; a new empty instance of this
 * {@code AsyncProcessor} can be created via the {@link #create()} method.
 * <p>
 * Since an {@code AsyncProcessor} is a Reactive Streams {@code Processor} type,
 * {@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 processor's state is not changed.
 * <p>
 * {@code AsyncProcessor} is a {@link io.reactivex.Flowable} as well as a {@link FlowableProcessor} and supports backpressure from the downstream but
 * its {@link Subscriber}-side consumes items in an unbounded manner.
 * <p>
 * When this {@code AsyncProcessor} is terminated via {@link #onError(Throwable)}, the
 * last observed item (if any) is cleared and late {@link Subscriber}s only receive
 * the {@code onError} event.
 * <p>
 * The {@code AsyncProcessor} 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 Processor} with infinite or never-completing sources.
 * <p>
 * Even though {@code AsyncProcessor} implements the {@link Subscriber} 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 processor is used as a standalone source. However, calling {@code onSubscribe}
 * after the {@code AsyncProcessor} reached its terminal state will result in the
 * given {@link Subscription} being canceled 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 FlowableProcessor}s
 * provides such serialization and also protects against reentrance (i.e., when a downstream {@code Subscriber}
 * consuming this processor also wants to call {@link #onNext(Object)} on this processor recursively).
 * The implementation of {@code onXXX} methods are technically thread-safe but non-serialized calls
 * to them may lead to undefined state in the currently subscribed {@code Subscriber}s.
 * <p>
 * This {@code AsyncProcessor} supports the standard state-peeking methods {@link #hasComplete()}, {@link #hasThrowable()},
 * {@link #getThrowable()} and {@link #hasSubscribers()} as well as means to read the very last observed value -
 * after this {@code AsyncProcessor} 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>Backpressure:</b></dt>
 *  <dd>The {@code AsyncProcessor} honors the backpressure of the downstream {@code Subscriber}s and won't emit
 *  its single value to a particular {@code Subscriber} until that {@code Subscriber} has requested an item.
 *  When the {@code AsyncProcessor} is subscribed to a {@link io.reactivex.Flowable}, the processor consumes this
 *  {@code Flowable} in an unbounded manner (requesting `Long.MAX_VALUE`) as only the very last upstream item is
 *  retained by it.
 *  </dd>
 *  <dt><b>Scheduler:</b></dt>
 *  <dd>{@code AsyncProcessor} does not operate by default on a particular {@link io.reactivex.Scheduler} and
 *  the {@code Subscriber}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 AsyncProcessor} enters into a terminal state
 *  and emits the same {@code Throwable} instance to the last set of {@code Subscriber}s. During this emission,
 *  if one or more {@code Subscriber}s dispose their respective {@code Subscription}s, the
 *  {@code Throwable} is delivered to the global error handler via
 *  {@link io.reactivex.plugins.RxJavaPlugins#onError(Throwable)} (multiple times if multiple {@code Subscriber}s
 *  cancel at once).
 *  If there were no {@code Subscriber}s subscribed to this {@code AsyncProcessor} when the {@code onError()}
 *  was called, the global error handler is not invoked.
 *  </dd>
 * </dl>
 * <p>
 * Example usage:
 * <pre><code>
 * AsyncProcessor&lt;Object&gt; processor = AsyncProcessor.create();
 * 
 * TestSubscriber&lt;Object&gt; ts1 = processor.test();
 *
 * ts1.assertEmpty();
 *
 * processor.onNext(1);
 *
 * // AsyncProcessor only emits when onComplete was called.
 * ts1.assertEmpty();
 *
 * processor.onNext(2);
 * processor.onComplete();
 *
 * // onComplete triggers the emission of the last cached item and the onComplete event.
 * ts1.assertResult(2);
 *
 * TestSubscriber&lt;Object&gt; ts2 = processor.test();
 *
 * // late Subscribers receive the last cached item too
 * ts2.assertResult(2);
 * </code></pre>
 * @param <T> the value type
 */
public final class AsyncProcessor<T> extends FlowableProcessor<T> {

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

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

    final AtomicReference<AsyncSubscription<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> AsyncProcessor<T> create() {
        return new AsyncProcessor<T>();
    }

    
Constructs an AsyncProcessor.

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

    @Override
    public void onSubscribe(Subscription s) {
        if (subscribers.get() == TERMINATED) {
            s.cancel();
            return;
        }
        // AsyncProcessor doesn't bother with request coordination.
        s.request(Long.MAX_VALUE);
    }

    @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 (AsyncSubscription<T> as : subscribers.getAndSet(TERMINATED)) {
            as.onError(t);
        }
    }

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

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

    @Override
    protected void subscribeActual(Subscriber<? super T> s) {
        AsyncSubscription<T> as = new AsyncSubscription<T>(s, this);
        s.onSubscribe(as);
        if (add(as)) {
            if (as.isCancelled()) {
                remove(as);
            }
        } else {
            Throwable ex = error;
            if (ex != null) {
                s.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 processor has terminated.

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

            int n = a.length;
            @SuppressWarnings("unchecked")
            AsyncSubscription<T>[] b = new AsyncSubscription[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 this processor.

Params:
  • ps – the subscriber's subscription wrapper to remove
/**
     * Atomically removes the given subscriber if it is subscribed to this processor.
     * @param ps the subscriber's subscription wrapper to remove
     */
    @SuppressWarnings("unchecked")
    void remove(AsyncSubscription<T> ps) {
        for (;;) {
            AsyncSubscription<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;
            }

            AsyncSubscription<T>[] b;

            if (n == 1) {
                b = EMPTY;
            } else {
                b = new AsyncSubscription[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 this processor has any value.

The method is thread-safe.

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

    
Returns a single value this processor currently has or null if no such value exists.

The method is thread-safe.

Returns:a single value this processor currently has or null if no such value exists
/**
     * Returns a single value this processor currently has or null if no such value exists.
     * <p>The method is thread-safe.
     * @return a single value this processor 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 this processor.

The method is thread-safe.

Returns:the array containing the snapshot of all values of this processor
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 this processor.
     * <p>The method is thread-safe.
     * @return the array containing the snapshot of all values of this processor
     * @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 this processor.

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 this processor.
     * <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 AsyncSubscription<T> extends DeferredScalarSubscription<T> {
        private static final long serialVersionUID = 5629876084736248016L;

        final AsyncProcessor<T> parent;

        AsyncSubscription(Subscriber<? super T> actual, AsyncProcessor<T> parent) {
            super(actual);
            this.parent = parent;
        }

        @Override
        public void cancel() {
            if (super.tryCancel()) {
                parent.remove(this);
            }
        }

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

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