content: // Copyright 2014 The Bazel Authors. All rights reserved.
  //
  // 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 com.google.android.apps.common.testing.broker.shell;
  
  /**
   * <p>A simple implementation of {@link KillableObserver} which can be told
   * explicitly to kill its {@link Killable} by calling {@link #kill()}. This
   * is the sort of functionality that callers might expect to find available
   * on the {@link Command} class.</p>
   *
   * <p>Note that this class can only observe one {@link Killable} at a time;
   * multiple instances should be used for concurrent calls to
   * {@link Command#execute(byte[], KillableObserver, boolean)}.</p>
   */
  public final class SimpleKillableObserver implements KillableObserver {
  
    private Killable killable;
  
    /**
     * Does nothing except store a reference to the given {@link Killable}.
     *
     * @param killable {@link Killable} to kill
     */
    @Override
    public synchronized void startObserving(final Killable killable) {
      this.killable = killable;
    }
  
    /**
     * ts reference to {@link Killable} provided to
     * {@link #startObserving(Killable)}
     */
    @Override
    public synchronized void stopObserving(final Killable killable) {
      if (!this.killable.equals(killable)) {
        throw new IllegalStateException("start/stopObservering called with " +
                                        "different Killables");
      }
      this.killable = null;
    }
  
    /**
     * Calls {@link Killable#kill()} on the saved {@link Killable}.
     */
    public synchronized void kill() {
      if (killable != null) {
        killable.kill();
      }
    }
  }
  