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/*
 * 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.rxjava3.processors;

import java.util.concurrent.atomic.AtomicReference;

import org.reactivestreams.*;

import io.reactivex.rxjava3.annotations.*;
import io.reactivex.rxjava3.internal.subscriptions.DeferredScalarSubscription;
import io.reactivex.rxjava3.internal.util.ExceptionHelper;
import io.reactivex.rxjava3.plugins.RxJavaPlugins;

/**
 * Processor that emits the very last value followed by a completion event or the received error
 * to {@link Subscriber}s.
 * 

* *

* 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. *

* Since an {@code AsyncProcessor} is a Reactive Streams {@code Processor} type, * {@code null}s are not allowed (Rule 2.13) * 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. *

* {@code AsyncProcessor} is a {@link io.reactivex.rxjava3.core.Flowable} as well as a {@link FlowableProcessor} and supports backpressure from the downstream but * its {@link Subscriber}-side consumes items in an unbounded manner. *

* 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. *

* 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. *

* Even though {@code AsyncProcessor} implements the {@link Subscriber} interface, calling * {@code onSubscribe} is not required (Rule 2.12) * 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. *

* 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. *

* 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()} or {@link #getValue()}. *

*
Backpressure:
*
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.rxjava3.core.Flowable}, the processor consumes this * {@code Flowable} in an unbounded manner (requesting {@link Long#MAX_VALUE}) as only the very last upstream item is * retained by it. *
*
Scheduler:
*
{@code AsyncProcessor} does not operate by default on a particular {@link io.reactivex.rxjava3.core.Scheduler} and * the {@code Subscriber}s get notified on the thread where the terminating {@code onError} or {@code onComplete} * methods were invoked.
*
Error handling:
*
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.rxjava3.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. *
*
*

* 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);
 * 
* @param the value type */ public final class AsyncProcessor<@NonNull T> extends FlowableProcessor { @SuppressWarnings("rawtypes") static final AsyncSubscription[] EMPTY = new AsyncSubscription[0]; @SuppressWarnings("rawtypes") static final AsyncSubscription[] TERMINATED = new AsyncSubscription[0]; final AtomicReference[]> subscribers; /** Write before updating subscribers, read after reading subscribers as TERMINATED. */ Throwable error; /** Write before updating subscribers, read after reading subscribers as TERMINATED. */ T value; /** * Creates a new AsyncProcessor. * @param the value type to be received and emitted * @return the new AsyncProcessor instance */ @CheckReturnValue @NonNull public static AsyncProcessor create() { return new AsyncProcessor<>(); } /** * Constructs an AsyncProcessor. * @since 2.0 */ @SuppressWarnings("unchecked") AsyncProcessor() { this.subscribers = new AtomicReference<>(EMPTY); } @Override public void onSubscribe(@NonNull 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(@NonNull T t) { ExceptionHelper.nullCheck(t, "onNext called with a null value."); if (subscribers.get() == TERMINATED) { return; } value = t; } @SuppressWarnings("unchecked") @Override public void onError(@NonNull Throwable t) { ExceptionHelper.nullCheck(t, "onError called with a null Throwable."); if (subscribers.get() == TERMINATED) { RxJavaPlugins.onError(t); return; } value = null; error = t; for (AsyncSubscription as : subscribers.getAndSet(TERMINATED)) { as.onError(t); } } @SuppressWarnings("unchecked") @Override public void onComplete() { if (subscribers.get() == TERMINATED) { return; } T v = value; AsyncSubscription[] array = subscribers.getAndSet(TERMINATED); if (v == null) { for (AsyncSubscription as : array) { as.onComplete(); } } else { for (AsyncSubscription as : array) { as.complete(v); } } } @Override @CheckReturnValue public boolean hasSubscribers() { return subscribers.get().length != 0; } @Override @CheckReturnValue public boolean hasThrowable() { return subscribers.get() == TERMINATED && error != null; } @Override @CheckReturnValue public boolean hasComplete() { return subscribers.get() == TERMINATED && error == null; } @Override @Nullable @CheckReturnValue public Throwable getThrowable() { return subscribers.get() == TERMINATED ? error : null; } @Override protected void subscribeActual(@NonNull Subscriber s) { AsyncSubscription as = new AsyncSubscription<>(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. * @param ps the subscriber to add * @return true if successful, false if the processor has terminated */ boolean add(AsyncSubscription ps) { for (;;) { AsyncSubscription[] a = subscribers.get(); if (a == TERMINATED) { return false; } int n = a.length; @SuppressWarnings("unchecked") AsyncSubscription[] 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. * @param ps the subscriber's subscription wrapper to remove */ @SuppressWarnings("unchecked") void remove(AsyncSubscription ps) { for (;;) { AsyncSubscription[] 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[] 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. * @return true if this processor has any value */ @CheckReturnValue 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. * @return a single value this processor currently has or null if no such value exists */ @Nullable @CheckReturnValue public T getValue() { return subscribers.get() == TERMINATED ? value : null; } static final class AsyncSubscription<@NonNull T> extends DeferredScalarSubscription { private static final long serialVersionUID = 5629876084736248016L; final AsyncProcessor parent; AsyncSubscription(Subscriber actual, AsyncProcessor 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); } } } }





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