HMCL/HMCLAPI/src/main/java/rx/operators/OperationMerge.java

/**
 * Copyright 2013 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 rx.operators;

import java.util.Arrays;
import java.util.List;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.atomic.AtomicBoolean;

import rx.Observable;
import rx.Observer;
import rx.Subscription;
import rx.util.AtomicObservableSubscription;
import rx.util.SynchronizedObserver;
import rx.util.functions.Func1;

public final class OperationMerge {

    /**
     * Flattens the observable sequences from the list of Observables into one
     * observable sequence without any transformation.
     *
     * @param source An observable sequence of elements to project.
     *
     * @return An observable sequence whose elements are the result of
     *         flattening the output from the list of Observables.
     *
     * @see http://msdn.microsoft.com/en-us/library/hh229099(v=vs.103).aspx
     */
    public static <T> Func1<Observer<T>, Subscription> merge(final Observable<Observable<T>> source) {
        // wrap in a Func so that if a chain is built up, then asynchronously subscribed to twice we will have 2 instances of Take<T> rather than 1 handing both, which is not thread-safe.
        return new MergeObservable<T>(source)::call;
    }

    public static <T> Func1<Observer<T>, Subscription> merge(final Observable<T>... sequences) {
        return merge(Arrays.asList(sequences));
    }

    public static <T> Func1<Observer<T>, Subscription> merge(final List<Observable<T>> sequences) {
        return merge(Observable.create(new Func1<Observer<Observable<T>>, Subscription>() {

            private volatile boolean unsubscribed = false;

            @Override
            public Subscription call(Observer<Observable<T>> observer) {
                for (Observable<T> o : sequences)
                    if (!unsubscribed)
                        observer.onNext(o);
                    else
                        // break out of the loop if we are unsubscribed
                        break;
                if (!unsubscribed)
                    observer.onCompleted();

                return () -> {
                    unsubscribed = true;
                };
            }
        }));
    }

    /**
     * This class is NOT thread-safe if invoked and referenced multiple times.
     * In other words, don't subscribe to it multiple times from different
     * threads.
     * <p>
     * It IS thread-safe from within it while receiving onNext events from
     * multiple threads.
     * <p>
     * This should all be fine as long as it's kept as a private class and a new
     * instance created from static factory method above.
     * <p>
     * Note how the take() factory method above protects us from a single
     * instance being exposed with the Observable wrapper handling the subscribe
     * flow.
     *
     * @param <T>
     */
    private static final class MergeObservable<T> implements Func1<Observer<T>, Subscription> {

        private final Observable<Observable<T>> sequences;
        private final MergeSubscription ourSubscription = new MergeSubscription();
        private final AtomicBoolean stopped = new AtomicBoolean(false);
        private volatile boolean parentCompleted = false;
        private final ConcurrentHashMap<ChildObserver, ChildObserver> childObservers = new ConcurrentHashMap<>();
        private final ConcurrentHashMap<ChildObserver, Subscription> childSubscriptions = new ConcurrentHashMap<>();

        private MergeObservable(Observable<Observable<T>> sequences) {
            this.sequences = sequences;
        }

        @Override
        public Subscription call(Observer<T> actualObserver) {

            /**
             * We must synchronize a merge because we subscribe to multiple
             * sequences in parallel that will each be emitting.
             * <p>
             * The calls from each sequence must be serialized.
             * <p>
             * Bug report: https://github.com/Netflix/RxJava/issues/200
             */
            AtomicObservableSubscription subscription = new AtomicObservableSubscription(ourSubscription);
            SynchronizedObserver<T> synchronizedObserver = new SynchronizedObserver<>(actualObserver, subscription);

            /**
             * Subscribe to the parent Observable to get to the children
             * Observables
             */
            sequences.subscribe(new ParentObserver(synchronizedObserver));

            /*
             * return our subscription to allow unsubscribing
             */
            return subscription;
        }

        /**
         * Manage the internal subscription with a thread-safe means of
         * stopping/unsubscribing so we don't unsubscribe twice.
         * <p>
         * Also has the stop() method returning a boolean so callers know if
         * their thread "won" and should perform further actions.
         */
        private class MergeSubscription implements Subscription {

            @Override
            public void unsubscribe() {
                stop();
            }

            public boolean stop() {
                // try setting to false unless another thread beat us
                boolean didSet = stopped.compareAndSet(false, true);
                if (didSet) {
                    // this thread won the race to stop, so unsubscribe from the actualSubscription
                    for (Subscription _s : childSubscriptions.values())
                        _s.unsubscribe();
                    return true;
                } else
                    // another thread beat us
                    return false;
            }
        }

        /**
         * Subscribe to the top level Observable to receive the sequence of
         * Observable<T> children.
         *
         * @param <T>
         */
        private class ParentObserver implements Observer<Observable<T>> {

            private final Observer<T> actualObserver;

            public ParentObserver(Observer<T> actualObserver) {
                this.actualObserver = actualObserver;
            }

            @Override
            public void onCompleted() {
                parentCompleted = true;
                // this *can* occur before the children are done, so if it does we won't send onCompleted
                // but will let the child worry about it
                // if however this completes and there are no children processing, then we will send onCompleted

                if (childObservers.isEmpty())
                    if (!stopped.get())
                        if (ourSubscription.stop())
                            actualObserver.onCompleted();
            }

            @Override
            public void onError(Exception e) {
                actualObserver.onError(e);
            }

            @Override
            public void onNext(Observable<T> childObservable) {
                if (stopped.get())
                    // we won't act on any further items
                    return;

                if (childObservable == null)
                    throw new IllegalArgumentException("Observable<T> can not be null.");

                /**
                 * For each child Observable we receive we'll subscribe with a
                 * separate Observer that will each then forward their sequences
                 * to the actualObserver.
                 * <p>
                 * We use separate child Observers for each sequence to simplify
                 * the onComplete/onError handling so each sequence has its own
                 * lifecycle.
                 */
                ChildObserver _w = new ChildObserver(actualObserver);
                childObservers.put(_w, _w);
                Subscription _subscription = childObservable.subscribe(_w);
                // remember this Observer and the subscription from it
                childSubscriptions.put(_w, _subscription);
            }
        }

        /**
         * Subscribe to each child Observable<T> and forward their sequence of
         * data to the actualObserver
         *
         */
        private class ChildObserver implements Observer<T> {

            private final Observer<T> actualObserver;

            public ChildObserver(Observer<T> actualObserver) {
                this.actualObserver = actualObserver;
            }

            @Override
            public void onCompleted() {
                // remove self from map of Observers
                childObservers.remove(this);
                // if there are now 0 Observers left, so if the parent is also completed we send the onComplete to the actualObserver
                // if the parent is not complete that means there is another sequence (and child Observer) to come
                if (!stopped.get())
                    if (childObservers.isEmpty() && parentCompleted)
                        if (ourSubscription.stop())
                            // this thread 'won' the race to unsubscribe/stop so let's send onCompleted
                            actualObserver.onCompleted();
            }

            @Override
            public void onError(Exception e) {
                if (!stopped.get())
                    if (ourSubscription.stop())
                        // this thread 'won' the race to unsubscribe/stop so let's send the error
                        actualObserver.onError(e);
            }

            @Override
            public void onNext(T args) {
                // in case the Observable is poorly behaved and doesn't listen to the unsubscribe request
                // we'll ignore anything that comes in after we've unsubscribed
                if (!stopped.get())
                    actualObserver.onNext(args);
            }

        }
    }
}