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SerializedObserver.java
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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.observers;
import rx.Observer;
import rx.exceptions.*;
import rx.internal.operators.NotificationLite;
/**
* Enforces single-threaded, serialized, ordered execution of {@link #onNext}, {@link #onCompleted}, and
* {@link #onError}.
* <p>
* When multiple threads are emitting and/or notifying they will be serialized by:
* </p><ul>
* <li>Allowing only one thread at a time to emit</li>
* <li>Adding notifications to a queue if another thread is already emitting</li>
* <li>Not holding any locks or blocking any threads while emitting</li>
* </ul>
*
* @param <T>
* the type of items expected to be observed by the {@code Observer}
*/
public class SerializedObserver<T> implements Observer<T> {
private final Observer<? super T> actual;
private boolean emitting;
/** Set to true if a terminal event was received. */
private volatile boolean terminated;
/** If not null, it indicates more work. */
private FastList queue;
static final class FastList {
Object[] array;
int size;
public void add(Object o) {
int s = size;
Object[] a = array;
if (a == null) {
a = new Object[16];
array = a;
} else if (s == a.length) {
Object[] array2 = new Object[s + (s >> 2)];
System.arraycopy(a, 0, array2, 0, s);
a = array2;
array = a;
}
a[s] = o;
size = s + 1;
}
}
public SerializedObserver(Observer<? super T> s) {
this.actual = s;
}
@Override
public void onNext(T t) {
if (terminated) {
return;
}
synchronized (this) {
if (terminated) {
return;
}
if (emitting) {
FastList list = queue;
if (list == null) {
list = new FastList();
queue = list;
}
list.add(NotificationLite.next(t));
return;
}
emitting = true;
}
try {
actual.onNext(t);
} catch (Throwable e) {
terminated = true;
Exceptions.throwOrReport(e, actual, t);
return;
}
for (;;) {
FastList list;
synchronized (this) {
list = queue;
if (list == null) {
emitting = false;
return;
}
queue = null;
}
for (Object o : list.array) {
if (o == null) {
break;
}
try {
if (NotificationLite.accept(actual, o)) {
terminated = true;
return;
}
} catch (Throwable e) {
terminated = true;
Exceptions.throwIfFatal(e);
actual.onError(OnErrorThrowable.addValueAsLastCause(e, t));
return;
}
}
}
}
@Override
public void onError(final Throwable e) {
Exceptions.throwIfFatal(e);
if (terminated) {
return;
}
synchronized (this) {
if (terminated) {
return;
}
terminated = true;
if (emitting) {
/*
* FIXME: generally, errors jump the queue but this wasn't true
* for SerializedObserver and may break existing expectations.
*/
FastList list = queue;
if (list == null) {
list = new FastList();
queue = list;
}
list.add(NotificationLite.error(e));
return;
}
emitting = true;
}
actual.onError(e);
}
@Override
public void onCompleted() {
if (terminated) {
return;
}
synchronized (this) {
if (terminated) {
return;
}
terminated = true;
if (emitting) {
FastList list = queue;
if (list == null) {
list = new FastList();
queue = list;
}
list.add(NotificationLite.completed());
return;
}
emitting = true;
}
actual.onCompleted();
}
}