/**
* Copyright 2009 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 com.google.gwt.user.client.Command;
/**
* Simple synchronization tool that waits for some fixed number of calls to
* {@link #tick()}, and invokes a command on the last one. Any further calls to
* {@link #tick()} produce an error.
* <p>
* The intended use case for this class is a gate that waits for some set of
* asynchronous tasks to complete.
*
*/
public final class CountdownLatch {
/** Command to execute when the all ticks have occurred. */
private final Command whenZero;
/** Number of expected ticks. */
private int count;
private CountdownLatch(int count, Command whenZero) {
this.whenZero = whenZero;
this.count = count;
}
/**
* Creates a countdown latch.
*
* @param count number of ticks
* @param whenZero command to execute after {@code count} ticks
* @return a new latch.
*/
public static CountdownLatch create(int count, Command whenZero) {
return new CountdownLatch(count, whenZero);
}
/**
* Ticks this counter.
*
* @throws IllegalStateException if this counter has already been ticked the
* expected number of times.
*/
public void tick() {
Preconditions.checkState(count > 0);
count--;
if (count == 0) {
whenZero.execute();
}
}
}