rx.internal.operators.OperatorTake Maven / Gradle / Ivy
/**
* 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.atomic.AtomicLong;
import rx.*;
import rx.Observable.Operator;
/**
* An {@code Observable} that emits the first {@code num} items emitted by the source {@code Observable}.
*
* 
*
* You can choose to pay attention only to the first {@code num} items emitted by an {@code Observable} by using
* the {@code take} operator. This operator returns an {@code Observable} that will invoke a subscriber's
* {@link Subscriber#onNext onNext} function a maximum of {@code num} times before invoking
* {@link Subscriber#onCompleted onCompleted}.
* @param the value type
*/
public final class OperatorTake implements Operator {
final int limit;
public OperatorTake(int limit) {
if (limit < 0) {
throw new IllegalArgumentException("limit >= 0 required but it was " + limit);
}
this.limit = limit;
}
@Override
public Subscriber call(final Subscriber child) {
final Subscriber parent = new Subscriber() {
int count;
boolean completed;
@Override
public void onCompleted() {
if (!completed) {
completed = true;
child.onCompleted();
}
}
@Override
public void onError(Throwable e) {
if (!completed) {
completed = true;
try {
child.onError(e);
} finally {
unsubscribe();
}
}
}
@Override
public void onNext(T i) {
if (!isUnsubscribed() && count++ < limit) {
boolean stop = count == limit;
child.onNext(i);
if (stop && !completed) {
completed = true;
try {
child.onCompleted();
} finally {
unsubscribe();
}
}
}
}
/**
* We want to adjust the requested values based on the `take` count.
*/
@Override
public void setProducer(final Producer producer) {
child.setProducer(new Producer() {
// keeps track of requests up to maximum of `limit`
final AtomicLong requested = new AtomicLong(0);
@Override
public void request(long n) {
if (n > 0 && !completed) {
// because requests may happen concurrently use a CAS loop to
// ensure we only request as much as needed, no more no less
while (true) {
long r = requested.get();
long c = Math.min(n, limit - r);
if (c == 0) {
break;
} else if (requested.compareAndSet(r, r + c)) {
producer.request(c);
break;
}
}
}
}
});
}
};
if (limit == 0) {
child.onCompleted();
parent.unsubscribe();
}
/*
* We decouple the parent and child subscription so there can be multiple take() in a chain such as for
* the groupBy Observer use case where you may take(1) on groups and take(20) on the children.
*
* Thus, we only unsubscribe UPWARDS to the parent and an onComplete DOWNSTREAM.
*
* However, if we receive an unsubscribe from the child we still want to propagate it upwards so we
* register 'parent' with 'child'
*/
child.add(parent);
return parent;
}
}