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Reactive Extensions for Java
There is a newer version: 2.2.21
/**
 * 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.internal.operators.observable;

import java.util.*;
import java.util.concurrent.Semaphore;
import java.util.concurrent.atomic.AtomicReference;

import io.reactivex.*;
import io.reactivex.Observable;
import io.reactivex.internal.util.*;
import io.reactivex.observers.DisposableObserver;
import io.reactivex.plugins.RxJavaPlugins;

/**
 * Wait for and iterate over the latest values of the source observable. If the source works faster than the
 * iterator, values may be skipped, but not the {@code onError} or {@code onComplete} events.
 * @param  the value type
 */
public final class BlockingObservableLatest implements Iterable {

    final ObservableSource source;

    public BlockingObservableLatest(ObservableSource source) {
        this.source = source;
    }

    @Override
    public Iterator iterator() {
        BlockingObservableLatestIterator lio = new BlockingObservableLatestIterator();

        Observable> materialized = Observable.wrap(source).materialize();

        materialized.subscribe(lio);
        return lio;
    }

    static final class BlockingObservableLatestIterator extends DisposableObserver> implements Iterator {
        // iterator's notification
        Notification iteratorNotification;

        final Semaphore notify = new Semaphore(0);
        // observer's notification
        final AtomicReference> value = new AtomicReference>();

        @Override
        public void onNext(Notification args) {
            boolean wasNotAvailable = value.getAndSet(args) == null;
            if (wasNotAvailable) {
                notify.release();
            }
        }

        @Override
        public void onError(Throwable e) {
            RxJavaPlugins.onError(e);
        }

        @Override
        public void onComplete() {
            // not expected
        }

        @Override
        public boolean hasNext() {
            if (iteratorNotification != null && iteratorNotification.isOnError()) {
                throw ExceptionHelper.wrapOrThrow(iteratorNotification.getError());
            }
            if (iteratorNotification == null) {
                try {
                    BlockingHelper.verifyNonBlocking();
                    notify.acquire();
                } catch (InterruptedException ex) {
                    dispose();
                    iteratorNotification = Notification.createOnError(ex);
                    throw ExceptionHelper.wrapOrThrow(ex);
                }

                Notification n = value.getAndSet(null);
                iteratorNotification = n;
                if (n.isOnError()) {
                    throw ExceptionHelper.wrapOrThrow(n.getError());
                }
            }
            return iteratorNotification.isOnNext();
        }

        @Override
        public T next() {
            if (hasNext()) {
                T v = iteratorNotification.getValue();
                iteratorNotification = null;
                return v;
            }
            throw new NoSuchElementException();
        }

        @Override
        public void remove() {
            throw new UnsupportedOperationException("Read-only iterator.");
        }
    }
}