PublishSubject.java

/**
 * Copyright 2016 Netflix, Inc.
 * 
 * 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 java.util.concurrent.atomic.*;

import io.reactivex.Observer;
import io.reactivex.disposables.Disposable;
import io.reactivex.plugins.RxJavaPlugins;

/**
 * Subject that, once an {@link Observer} has subscribed, emits all subsequently observed items to the
 * subscriber.
 * <p>
 * <img width="640" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/S.PublishSubject.png" alt="">
 * <p>
 * Example usage:
 * <p>
 * <pre> {@code

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

  } </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. */
    @SuppressWarnings("rawtypes")
    static final PublishDisposable[] TERMINATED = new PublishDisposable[0];
    /** 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. */
    final AtomicReference<PublishDisposable<T>[]> subscribers;

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

    /**
     * Constructs a PublishSubject.
     * @param <T> the value type
     * @return the new PublishSubject
     */
    public static <T> PublishSubject<T> create() {
        return new PublishSubject<T>();
    }
    
    /**
     * Constructs a PublishSubject.
     * @since 2.0
     */
    @SuppressWarnings("unchecked")
    PublishSubject() {
        subscribers = new AtomicReference<PublishDisposable<T>[]>(EMPTY);
    }


    @Override
    public 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.
     * @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.
     * @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 s) {
        if (subscribers.get() == TERMINATED) {
            s.dispose();
            return;
        }
    }
    
    @Override
    public void onNext(T t) {
        if (subscribers.get() == TERMINATED) {
            return;
        }
        if (t == null) {
            onError(new NullPointerException());
            return;
        }
        for (PublishDisposable<T> s : subscribers.get()) {
            s.onNext(t);
        }
    }
    
    @SuppressWarnings("unchecked")
    @Override
    public void onError(Throwable t) {
        if (subscribers.get() == TERMINATED) {
            RxJavaPlugins.onError(t);
            return;
        }
        if (t == null) {
            t = new NullPointerException();
        }
        error = t;
        
        for (PublishDisposable<T> s : subscribers.getAndSet(TERMINATED)) {
            s.onError(t);
        }
    }
    
    @SuppressWarnings("unchecked")
    @Override
    public void onComplete() {
        if (subscribers.get() == TERMINATED) {
            return;
        }
        for (PublishDisposable<T> s : subscribers.getAndSet(TERMINATED)) {
            s.onComplete();
        }
    }
    
    @Override
    public boolean hasObservers() {
        return subscribers.get().length != 0;
    }
    
    @Override
    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.
     *
     * @param <T> the value type
     */
    static final class PublishDisposable<T> extends AtomicBoolean implements Disposable {
        /** */
        private static final long serialVersionUID = 3562861878281475070L;
        /** The actual subscriber. */
        final Observer<? super T> actual;
        /** The subject state. */
        final PublishSubject<T> parent;
        
        /**
         * Constructs a PublishSubscriber, wraps the actual subscriber and the state.
         * @param actual the actual subscriber
         * @param parent the parent PublishProcessor
         */
        public PublishDisposable(Observer<? super T> actual, PublishSubject<T> parent) {
            this.actual = actual;
            this.parent = parent;
        }
        
        public void onNext(T t) {
            if (!get()) {
                actual.onNext(t);
            }
        }
        
        public void onError(Throwable t) {
            if (get()) {
                RxJavaPlugins.onError(t);
            } else {
                actual.onError(t);
            }
        }
        
        public void onComplete() {
            if (!get()) {
                actual.onComplete();
            }
        }
        
        @Override
        public void dispose() {
            if (compareAndSet(false, true)) {
                parent.remove(this);
            }
        }
        
        @Override
        public boolean isDisposed() {
            return get();
        }
    }
}