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.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 onError or onComplete events. 
Type parameters:
<T> – the value type/**
 * 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 <T> the value type
 */
public final class BlockingObservableLatest<T> implements Iterable<T> {

    final ObservableSource<T> source;

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

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

        Observable<Notification<T>> materialized = Observable.wrap(source).materialize();

        materialized.subscribe(lio);
        return lio;
    }

    static final class BlockingObservableLatestIterator<T> extends DisposableObserver<Notification<T>> implements Iterator<T> {
        // iterator's notification
        Notification<T> iteratorNotification;

        final Semaphore notify = new Semaphore(0);
        // observer's notification
        final AtomicReference<Notification<T>> value = new AtomicReference<Notification<T>>();

        @Override
        public void onNext(Notification<T> 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<T> 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.");
        }
    }
}