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rxjava-core developed by Netflix

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/**
 * Copyright 2014 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.concurrent.ConcurrentLinkedQueue;
import java.util.concurrent.atomic.AtomicLong;

import rx.Observable.Operator;
import rx.Scheduler;
import rx.Scheduler.Inner;
import rx.Subscriber;
import rx.functions.Action1;
import rx.schedulers.ImmediateScheduler;
import rx.schedulers.TrampolineScheduler;

/**
 * Delivers events on the specified Scheduler asynchronously via an unbounded buffer.
 * 
 * 
 */
public class OperatorObserveOn implements Operator {

    private final Scheduler scheduler;

    /**
     * @param scheduler
     */
    public OperatorObserveOn(Scheduler scheduler) {
        this.scheduler = scheduler;
    }

    @Override
    public Subscriber call(Subscriber child) {
        if (scheduler instanceof ImmediateScheduler) {
            // avoid overhead, execute directly
            return child;
        } else if (scheduler instanceof TrampolineScheduler) {
            // avoid overhead, execute directly
            return child;
        } else {
            return new ObserveOnSubscriber(child);
        }
    }

    private static class Sentinel {

    }

    private static Sentinel NULL_SENTINEL = new Sentinel();
    private static Sentinel COMPLETE_SENTINEL = new Sentinel();

    private static class ErrorSentinel extends Sentinel {
        final Throwable e;

        ErrorSentinel(Throwable e) {
            this.e = e;
        }
    }

    /** Observe through individual queue per observer. */
    private class ObserveOnSubscriber extends Subscriber {
        final Subscriber observer;
        private volatile Scheduler.Inner recursiveScheduler;

        private final ConcurrentLinkedQueue