src/main/java/rx/Subscriber.java
/**
* 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;
import rx.internal.util.SubscriptionList;
/**
* Provides a mechanism for receiving push-based notifications from Observables, and permits manual
* unsubscribing from these Observables.
* <p>
* After a Subscriber calls an {@link Observable}'s {@link Observable#subscribe subscribe} method, the
* {@link Observable} calls the Subscriber's {@link #onNext} method to emit items. A well-behaved
* {@link Observable} will call a Subscriber's {@link #onCompleted} method exactly once or the Subscriber's
* {@link #onError} method exactly once.
*
* @see <a href="http://reactivex.io/documentation/observable.html">ReactiveX documentation: Observable</a>
* @param <T>
* the type of items the Subscriber expects to observe
*/
public abstract class Subscriber<T> implements Observer<T>, Subscription {
// represents requested not set yet
private static final long NOT_SET = Long.MIN_VALUE;
private final SubscriptionList subscriptions;
private final Subscriber<?> subscriber;
/* protected by `this` */
private Producer producer;
/* protected by `this` */
private long requested = NOT_SET; // default to not set
protected Subscriber() {
this(null, false);
}
/**
* Construct a Subscriber by using another Subscriber for backpressure and
* for holding the subscription list (when <code>this.add(sub)</code> is
* called this will in fact call <code>subscriber.add(sub)</code>).
*
* @param subscriber
* the other Subscriber
*/
protected Subscriber(Subscriber<?> subscriber) {
this(subscriber, true);
}
/**
* Construct a Subscriber by using another Subscriber for backpressure and
* optionally for holding the subscription list (if
* <code>shareSubscriptions</code> is <code>true</code> then when
* <code>this.add(sub)</code> is called this will in fact call
* <code>subscriber.add(sub)</code>).
* <p>
* To retain the chaining of subscribers when setting
* <code>shareSubscriptions</code> to <code>false</code>, add the created
* instance to {@code subscriber} via {@link #add}.
*
* @param subscriber
* the other Subscriber
* @param shareSubscriptions
* {@code true} to share the subscription list in {@code subscriber} with
* this instance
* @since 1.0.6
*/
protected Subscriber(Subscriber<?> subscriber, boolean shareSubscriptions) {
this.subscriber = subscriber;
this.subscriptions = shareSubscriptions && subscriber != null ? subscriber.subscriptions : new SubscriptionList();
}
/**
* Adds a {@link Subscription} to this Subscriber's list of subscriptions if this list is not marked as
* unsubscribed. If the list <em>is</em> marked as unsubscribed, {@code add} will indicate this by
* explicitly unsubscribing the new {@code Subscription} as well.
*
* @param s
* the {@code Subscription} to add
*/
public final void add(Subscription s) {
subscriptions.add(s);
}
@Override
public final void unsubscribe() {
subscriptions.unsubscribe();
}
/**
* Indicates whether this Subscriber has unsubscribed from its list of subscriptions.
*
* @return {@code true} if this Subscriber has unsubscribed from its subscriptions, {@code false} otherwise
*/
@Override
public final boolean isUnsubscribed() {
return subscriptions.isUnsubscribed();
}
/**
* This method is invoked when the Subscriber and Observable have been connected but the Observable has
* not yet begun to emit items or send notifications to the Subscriber. Override this method to add any
* useful initialization to your subscription, for instance to initiate backpressure.
*/
public void onStart() {
// do nothing by default
}
/**
* Request a certain maximum number of emitted items from the Observable this Subscriber is subscribed to.
* This is a way of requesting backpressure. To disable backpressure, pass {@code Long.MAX_VALUE} to this
* method.
* <p>
* Requests are additive but if a sequence of requests totals more than {@code Long.MAX_VALUE} then
* {@code Long.MAX_VALUE} requests will be actioned and the extras <i>may</i> be ignored. Arriving at
* {@code Long.MAX_VALUE} by addition of requests cannot be assumed to disable backpressure. For example,
* the code below may result in {@code Long.MAX_VALUE} requests being actioned only.
*
* <pre>
* request(100);
* request(Long.MAX_VALUE-1);
* </pre>
*
* @param n the maximum number of items you want the Observable to emit to the Subscriber at this time, or
* {@code Long.MAX_VALUE} if you want the Observable to emit items at its own pace
* @throws IllegalArgumentException
* if {@code n} is negative
*/
protected final void request(long n) {
if (n < 0) {
throw new IllegalArgumentException("number requested cannot be negative: " + n);
}
// if producer is set then we will request from it
// otherwise we increase the requested count by n
Producer producerToRequestFrom;
synchronized (this) {
if (producer != null) {
producerToRequestFrom = producer;
} else {
addToRequested(n);
return;
}
}
// after releasing lock (we should not make requests holding a lock)
producerToRequestFrom.request(n);
}
private void addToRequested(long n) {
if (requested == NOT_SET) {
requested = n;
} else {
final long total = requested + n;
// check if overflow occurred
if (total < 0) {
requested = Long.MAX_VALUE;
} else {
requested = total;
}
}
}
/**
* If other subscriber is set (by calling constructor
* {@link #Subscriber(Subscriber)} or
* {@link #Subscriber(Subscriber, boolean)}) then this method calls
* <code>setProducer</code> on the other subscriber. If the other subscriber
* is not set and no requests have been made to this subscriber then
* <code>p.request(Long.MAX_VALUE)</code> is called. If the other subscriber
* is not set and some requests have been made to this subscriber then
* <code>p.request(n)</code> is called where n is the accumulated requests
* to this subscriber.
*
* @param p
* producer to be used by this subscriber or the other subscriber
* (or recursively its other subscriber) to make requests from
*/
public void setProducer(Producer p) {
long toRequest;
boolean passToSubscriber = false;
synchronized (this) {
toRequest = requested;
producer = p;
if (subscriber != null) {
// middle operator ... we pass through unless a request has been made
if (toRequest == NOT_SET) {
// we pass through to the next producer as nothing has been requested
passToSubscriber = true;
}
}
}
// do after releasing lock
if (passToSubscriber) {
subscriber.setProducer(producer);
} else {
// we execute the request with whatever has been requested (or Long.MAX_VALUE)
if (toRequest == NOT_SET) {
producer.request(Long.MAX_VALUE);
} else {
producer.request(toRequest);
}
}
}
}