/**
* 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.CancellationException;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.Future;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.TimeoutException;
import java.util.concurrent.atomic.AtomicReference;
import rx.Observable;
import rx.Subscriber;
import rx.Subscription;
/**
* Returns a Future representing the single value emitted by an Observable.
* <p>
* <img width="640" src="https://github.com/Netflix/RxJava/wiki/images/rx-operators/B.toFuture.png" alt="">
* <p>
* The toFuture operation throws an exception if the Observable emits more than one item. If the
* Observable may emit more than item, use <code>toList().toFuture()</code>.
*/
public class BlockingOperatorToFuture {
/**
* Returns a Future that expects a single item from the observable.
*
* @param that
* an observable sequence to get a Future for.
* @param <T>
* the type of source.
* @return the Future to retrieve a single elements from an Observable
*/
public static <T> Future<T> toFuture(Observable<? extends T> that) {
final CountDownLatch finished = new CountDownLatch(1);
final AtomicReference<T> value = new AtomicReference<T>();
final AtomicReference<Throwable> error = new AtomicReference<Throwable>();
final Subscription s = that.single().subscribe(new Subscriber<T>() {
@Override
public void onCompleted() {
finished.countDown();
}
@Override
public void onError(Throwable e) {
error.compareAndSet(null, e);
finished.countDown();
}
@Override
public void onNext(T v) {
// "single" guarantees there is only one "onNext"
value.set(v);
}
});
return new Future<T>() {
private volatile boolean cancelled = false;
@Override
public boolean cancel(boolean mayInterruptIfRunning) {
if (finished.getCount() > 0) {
cancelled = true;
s.unsubscribe();
// release the latch (a race condition may have already released it by now)
finished.countDown();
return true;
} else {
// can't cancel
return false;
}
}
@Override
public boolean isCancelled() {
return cancelled;
}
@Override
public boolean isDone() {
return finished.getCount() == 0;
}
@Override
public T get() throws InterruptedException, ExecutionException {
finished.await();
return getValue();
}
@Override
public T get(long timeout, TimeUnit unit) throws InterruptedException, ExecutionException, TimeoutException {
if (finished.await(timeout, unit)) {
return getValue();
} else {
throw new TimeoutException("Timed out after " + unit.toMillis(timeout) + "ms waiting for underlying Observable.");
}
}
private T getValue() throws ExecutionException {
if (error.get() != null) {
throw new ExecutionException("Observable onError", error.get());
} else if (cancelled) {
// Contract of Future.get() requires us to throw this:
throw new CancellationException("Subscription unsubscribed");
} else {
return value.get();
}
}
};
}
}