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.CheckReturnValue;
import io.reactivex.annotations.Nullable;
import io.reactivex.annotations.NonNull;
import java.util.concurrent.atomic.*;

import io.reactivex.Observer;
import io.reactivex.disposables.Disposable;
import io.reactivex.internal.functions.ObjectHelper;
import io.reactivex.plugins.RxJavaPlugins;

A Subject that emits (multicasts) items to currently subscribed Observers and terminal events to current or late Observers. 


 This subject does not have a public constructor by design; a new empty instance of this PublishSubject 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 a PublishSubject is an Observable, it does not support backpressure. 
 When this PublishSubject is terminated via onError(Throwable) or onComplete(), late Observers only receive the respective terminal event. 
 Unlike a BehaviorSubject, a PublishSubject doesn't retain/cache items, therefore, a new Observer won't receive any past items. 
 Even though PublishSubject 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 PublishSubject 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 PublishSubject supports the standard state-peeking methods hasComplete(), hasThrowable(), getThrowable() and hasObservers(). 

 
Scheduler:
 
PublishSubject does not operate by default on a particular Scheduler and the Observers get notified on the thread the respective onXXX methods were invoked.
 
Error handling:
 
When the onError(Throwable) is called, the PublishSubject 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 PublishSubject when the onError() was called, the global error handler is not invoked. 


Example usage:
 
PublishSubject<Object> subject = PublishSubject.create();
// observer1 will receive all onNext and onComplete events
subject.subscribe(observer1);
subject.onNext("one");
subject.onNext("two");
// observer2 will only receive "three" and onComplete
subject.subscribe(observer2);
subject.onNext("three");
subject.onComplete();
// late Observers only receive the terminal event
subject.test().assertEmpty();
 
Type parameters:
<T> – 
         the type of items observed and emitted by the Subject/**
 * A Subject that emits (multicasts) items to currently subscribed {@link Observer}s and terminal events to current
 * or late {@code Observer}s.
 * <p>
 * <img width="640" height="281" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/PublishSubject.png" alt="">
 * <p>
 * This subject does not have a public constructor by design; a new empty instance of this
 * {@code PublishSubject} 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 a {@code PublishSubject} is an {@link io.reactivex.Observable}, it does not support backpressure.
 * <p>
 * When this {@code PublishSubject} is terminated via {@link #onError(Throwable)} or {@link #onComplete()},
 * late {@link io.reactivex.Observer}s only receive the respective terminal event.
 * <p>
 * Unlike a {@link BehaviorSubject}, a {@code PublishSubject} doesn't retain/cache items, therefore, a new
 * {@code Observer} won't receive any past items.
 * <p>
 * Even though {@code PublishSubject} 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 PublishSubject} 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).
 * <p>
 * This {@code PublishSubject} supports the standard state-peeking methods {@link #hasComplete()}, {@link #hasThrowable()},
 * {@link #getThrowable()} and {@link #hasObservers()}.
 * <dl>
 *  <dt><b>Scheduler:</b></dt>
 *  <dd>{@code PublishSubject} does not operate by default on a particular {@link io.reactivex.Scheduler} and
 *  the {@code Observer}s get notified on the thread the respective {@code onXXX} methods were invoked.</dd>
 *  <dt><b>Error handling:</b></dt>
 *  <dd>When the {@link #onError(Throwable)} is called, the {@code PublishSubject} 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 PublishSubject} when the {@code onError()}
 *  was called, the global error handler is not invoked.
 *  </dd>
 * </dl>
 * <p>
 * Example usage:
 * <pre> {@code

  PublishSubject<Object> subject = PublishSubject.create();
  // observer1 will receive all onNext and onComplete events
  subject.subscribe(observer1);
  subject.onNext("one");
  subject.onNext("two");
  // observer2 will only receive "three" and onComplete
  subject.subscribe(observer2);
  subject.onNext("three");
  subject.onComplete();

  // late Observers only receive the terminal event
  subject.test().assertEmpty();
  } </pre>
 *
 * @param <T>
 *          the type of items observed and emitted by the Subject
 */
public final class PublishSubject<T> extends Subject<T> {
    
The terminated indicator for the subscribers array. /** The terminated indicator for the subscribers array. */
    @SuppressWarnings("rawtypes")
    static final PublishDisposable[] TERMINATED = new PublishDisposable[0];
    
An empty subscribers array to avoid allocating it all the time. /** An empty subscribers array to avoid allocating it all the time. */
    @SuppressWarnings("rawtypes")
    static final PublishDisposable[] EMPTY = new PublishDisposable[0];

    
The array of currently subscribed subscribers. /** The array of currently subscribed subscribers. */
    final AtomicReference<PublishDisposable<T>[]> subscribers;

    
The error, write before terminating and read after checking subscribers. /** The error, write before terminating and read after checking subscribers. */
    Throwable error;

    
Constructs a PublishSubject.
Type parameters:
<T> – the value type
Returns:
the new PublishSubject/**
     * Constructs a PublishSubject.
     * @param <T> the value type
     * @return the new PublishSubject
     */
    @CheckReturnValue
    @NonNull
    public static <T> PublishSubject<T> create() {
        return new PublishSubject<T>();
    }

    
Constructs a PublishSubject.
Since:
2.0/**
     * Constructs a PublishSubject.
     * @since 2.0
     */
    @SuppressWarnings("unchecked")
    PublishSubject() {
        subscribers = new AtomicReference<PublishDisposable<T>[]>(EMPTY);
    }

    @Override
    protected void subscribeActual(Observer<? super T> t) {
        PublishDisposable<T> ps = new PublishDisposable<T>(t, this);
        t.onSubscribe(ps);
        if (add(ps)) {
            // if cancellation happened while a successful add, the remove() didn't work
            // so we need to do it again
            if (ps.isDisposed()) {
                remove(ps);
            }
        } else {
            Throwable ex = error;
            if (ex != null) {
                t.onError(ex);
            } else {
                t.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(PublishDisposable<T> ps) {
        for (;;) {
            PublishDisposable<T>[] a = subscribers.get();
            if (a == TERMINATED) {
                return false;
            }

            int n = a.length;
            @SuppressWarnings("unchecked")
            PublishDisposable<T>[] b = new PublishDisposable[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(PublishDisposable<T> ps) {
        for (;;) {
            PublishDisposable<T>[] a = subscribers.get();
            if (a == TERMINATED || a == EMPTY) {
                return;
            }

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

            if (j < 0) {
                return;
            }

            PublishDisposable<T>[] b;

            if (n == 1) {
                b = EMPTY;
            } else {
                b = new PublishDisposable[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;
            }
        }
    }

    @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.");
        for (PublishDisposable<T> pd : subscribers.get()) {
            pd.onNext(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;
        }
        error = t;

        for (PublishDisposable<T> pd : subscribers.getAndSet(TERMINATED)) {
            pd.onError(t);
        }
    }

    @SuppressWarnings("unchecked")
    @Override
    public void onComplete() {
        if (subscribers.get() == TERMINATED) {
            return;
        }
        for (PublishDisposable<T> pd : subscribers.getAndSet(TERMINATED)) {
            pd.onComplete();
        }
    }

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

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

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

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

    
Wraps the actual subscriber, tracks its requests and makes cancellation
to remove itself from the current subscribers array.
Type parameters:
<T> – the value type/**
     * Wraps the actual subscriber, tracks its requests and makes cancellation
     * to remove itself from the current subscribers array.
     *
     * @param <T> the value type
     */
    static final class PublishDisposable<T> extends AtomicBoolean implements Disposable {

        private static final long serialVersionUID = 3562861878281475070L;
        
The actual subscriber. /** The actual subscriber. */
        final Observer<? super T> downstream;
        
The subject state. /** The subject state. */
        final PublishSubject<T> parent;

        
Constructs a PublishSubscriber, wraps the actual subscriber and the state.
Params:
actual – the actual subscriber
parent – the parent PublishProcessor/**
         * Constructs a PublishSubscriber, wraps the actual subscriber and the state.
         * @param actual the actual subscriber
         * @param parent the parent PublishProcessor
         */
        PublishDisposable(Observer<? super T> actual, PublishSubject<T> parent) {
            this.downstream = actual;
            this.parent = parent;
        }

        public void onNext(T t) {
            if (!get()) {
                downstream.onNext(t);
            }
        }

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

        public void onComplete() {
            if (!get()) {
                downstream.onComplete();
            }
        }

        @Override
        public void dispose() {
            if (compareAndSet(false, true)) {
                parent.remove(this);
            }
        }

        @Override
        public boolean isDisposed() {
            return get();
        }
    }
}