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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.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.Observer;
import rx.Subscription;
/**
* Returns a Future representing the single value emitted by an Observable.
*
* 
*
* The toFuture operation throws an exception if the Observable emits more than one item. If the
* Observable may emit more than item, use toList().toFuture().
*/
public class OperationToFuture {
/**
* Returns a Future that expects a single item from the observable.
*
* @param that
* an observable sequence to get a Future for.
* @param
* the type of source.
* @return the Future to retrieve a single elements from an Observable
*/
public static Future toFuture(Observable that) {
final CountDownLatch finished = new CountDownLatch(1);
final AtomicReference value = new AtomicReference();
final AtomicReference error = new AtomicReference();
final Subscription s = that.subscribe(new Observer() {
@Override
public void onCompleted() {
finished.countDown();
}
@Override
public void onError(Throwable e) {
error.compareAndSet(null, e);
finished.countDown();
}
@Override
public void onNext(T v) {
if (!value.compareAndSet(null, v)) {
// this means we received more than one value and must fail as a Future can handle only a single value
error.compareAndSet(null, new IllegalStateException("Observable.toFuture() only supports sequences with a single value. Use .toList().toFuture() if multiple values are expected."));
finished.countDown();
}
}
});
return new Future() {
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();
}
}
};
}
}