/**
* 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.internal.operators;
import java.util.concurrent.ConcurrentLinkedQueue;
import java.util.concurrent.atomic.AtomicIntegerFieldUpdater;
import java.util.concurrent.atomic.AtomicReferenceFieldUpdater;
import rx.Observer;
import rx.Subscriber;
import rx.functions.Action0;
import rx.observers.Subscribers;
import rx.subjects.Subject;
import rx.subscriptions.Subscriptions;
/**
* A solution to the "time gap" problem that occurs with {@code groupBy} and {@code pivot}.
* <p>
* This currently has temporary unbounded buffers. It needs to become bounded and then do one of two things:
* <ol>
* <li>blow up and make the user do something about it</li>
* <li>work with the backpressure solution ... still to be implemented (such as co-routines)</li>
* </ol><p>
* Generally the buffer should be very short lived (milliseconds) and then stops being involved. It can become a
* memory leak though if a {@code GroupedObservable} backed by this class is emitted but never subscribed to
* (such as filtered out). In that case, either a time-bomb to throw away the buffer, or just blowing up and
* making the user do something about it is needed.
* <p>
* For example, to filter out {@code GroupedObservable}s, perhaps they need a silent {@code subscribe()} on them
* to just blackhole the data.
* <p>
* This is an initial start at solving this problem and solves the immediate problem of {@code groupBy} and
* {@code pivot} and trades off the possibility of memory leak for deterministic functionality.
*
* @see <a href="https://github.com/Netflix/RxJava/issues/844">the Github issue describing the time gap problem</a>
* @param <T>
* the type of the items to be buffered
*/
public class BufferUntilSubscriber<T> extends Subject<T, T> {
/**
* @warn create() undescribed
* @return
*/
public static <T> BufferUntilSubscriber<T> create() {
State<T> state = new State<T>();
return new BufferUntilSubscriber<T>(state);
}
/** The common state. */
static final class State<T> {
/** The first observer or the one which buffers until the first arrives. */
volatile Observer<? super T> observerRef = new BufferedObserver<T>();
/** Allow a single subscriber only. */
volatile int first;
/** Field updater for observerRef. */
@SuppressWarnings("rawtypes")
static final AtomicReferenceFieldUpdater<State, Observer> OBSERVER_UPDATER
= AtomicReferenceFieldUpdater.newUpdater(State.class, Observer.class, "observerRef");
/** Field updater for first. */
@SuppressWarnings("rawtypes")
static final AtomicIntegerFieldUpdater<State> FIRST_UPDATER
= AtomicIntegerFieldUpdater.newUpdater(State.class, "first");
boolean casFirst(int expected, int next) {
return FIRST_UPDATER.compareAndSet(this, expected, next);
}
void setObserverRef(Observer<? super T> o) {
observerRef = o;
}
}
static final class OnSubscribeAction<T> implements OnSubscribe<T> {
final State<T> state;
public OnSubscribeAction(State<T> state) {
this.state = state;
}
@Override
public void call(final Subscriber<? super T> s) {
if (state.casFirst(0, 1)) {
final NotificationLite<T> nl = NotificationLite.instance();
// drain queued notifications before subscription
// we do this here before PassThruObserver so the consuming thread can do this before putting itself in the line of the producer
BufferedObserver<? super T> buffered = (BufferedObserver<? super T>)state.observerRef;
Object o;
while ((o = buffered.buffer.poll()) != null) {
nl.accept(s, o);
}
// register real observer for pass-thru ... and drain any further events received on first notification
state.setObserverRef(new PassThruObserver<T>(s, buffered.buffer, state));
s.add(Subscriptions.create(new Action0() {
@Override
public void call() {
state.setObserverRef(Subscribers.empty());
}
}));
} else {
s.onError(new IllegalStateException("Only one subscriber allowed!"));
}
}
}
final State<T> state;
private BufferUntilSubscriber(State<T> state) {
super(new OnSubscribeAction<T>(state));
this.state = state;
}
@Override
public void onCompleted() {
state.observerRef.onCompleted();
}
@Override
public void onError(Throwable e) {
state.observerRef.onError(e);
}
@Override
public void onNext(T t) {
state.observerRef.onNext(t);
}
/**
* This is a temporary observer between buffering and the actual that gets into the line of notifications
* from the producer and will drain the queue of any items received during the race of the initial drain and
* switching this.
*
* It will then immediately swap itself out for the actual (after a single notification), but since this is
* now being done on the same producer thread no further buffering will occur.
*/
private static final class PassThruObserver<T> extends Subscriber<T> {
private final Observer<? super T> actual;
// this assumes single threaded synchronous notifications (the Rx contract for a single Observer)
private final ConcurrentLinkedQueue<Object> buffer;
private final State<T> state;
PassThruObserver(Observer<? super T> actual, ConcurrentLinkedQueue<Object> buffer,
State<T> state) {
this.actual = actual;
this.buffer = buffer;
this.state = state;
}
@Override
public void onCompleted() {
drainIfNeededAndSwitchToActual();
actual.onCompleted();
}
@Override
public void onError(Throwable e) {
drainIfNeededAndSwitchToActual();
actual.onError(e);
}
@Override
public void onNext(T t) {
drainIfNeededAndSwitchToActual();
actual.onNext(t);
}
private void drainIfNeededAndSwitchToActual() {
final NotificationLite<T> nl = NotificationLite.instance();
Object o;
while ((o = buffer.poll()) != null) {
nl.accept(this, o);
}
// now we can safely change over to the actual and get rid of the pass-thru
// but only if not unsubscribed
state.setObserverRef(actual);
}
}
private static final class BufferedObserver<T> extends Subscriber<T> {
private final ConcurrentLinkedQueue<Object> buffer = new ConcurrentLinkedQueue<Object>();
private static final NotificationLite<Object> nl = NotificationLite.instance();
@Override
public void onCompleted() {
buffer.add(nl.completed());
}
@Override
public void onError(Throwable e) {
buffer.add(nl.error(e));
}
@Override
public void onNext(T t) {
buffer.add(nl.next(t));
}
}
}