/**
* Copyright 2010 Google 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 org.waveprotocol.wave.client.common.util;
import com.google.common.base.Preconditions;
import org.waveprotocol.wave.model.util.CollectionUtils;
import java.util.Queue;
/**
* Something that holds a value, revealed by potentially asynchronous access.
*
* @param <T> type of held value
*/
public interface AsyncHolder<T> {
/** Accessor of a value in this holder. */
public interface Accessor<T> {
void use(T x);
}
/**
* Reveals the value held by this holder to a consumer. The value may be
* revealed synchronously or asynchronously.
*
* @param accessor procedure to apply
*/
void call(Accessor<T> accessor);
public abstract class Impl<T> implements AsyncHolder<T>, Accessor<T> {
private Queue<Accessor<T>> waiting;
private T value;
@Override
public final void call(Accessor<T> accessor) {
if (value == null) {
addWaiter(accessor);
} else {
accessor.use(value);
}
}
private void addWaiter(Accessor<T> accessor) {
assert value == null;
if (waiting == null) {
waiting = CollectionUtils.createQueue();
waiting.add(accessor);
create(this);
} else {
waiting.add(accessor);
}
}
protected abstract void create(Accessor<T> whenReady);
@Override
public void use(T x) {
Preconditions.checkState(value == null && waiting != null);
value = x;
for (Accessor<T> waiter : waiting) {
waiter.use(value);
}
waiting = null;
}
}
}