/*
 * Copyright (C) 2009 The Guava Authors
 *
 * 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.common.util.concurrent;

import com.google.common.base.Preconditions;

import java.util.concurrent.Executor;

/**
 * A {@link ListenableFuture} which forwards all its method calls to another
 * future. Subclasses should override one or more methods to modify the behavior
 * of the backing future as desired per the <a
 * href="http://en.wikipedia.org/wiki/Decorator_pattern">decorator pattern</a>.
 *
 * <p>Most subclasses can just use {@link SimpleForwardingListenableFuture}.
 *
 * @param <V> The result type returned by this Future's {@code get} method
 * 
 * @author Shardul Deo
 * @since 4.0
 */
public abstract class ForwardingListenableFuture<V> extends ForwardingFuture<V>
    implements ListenableFuture<V> {

  /** Constructor for use by subclasses. */
  protected ForwardingListenableFuture() {}

  @Override
  protected abstract ListenableFuture<V> delegate();

  @Override
  public void addListener(Runnable listener, Executor exec) {
    delegate().addListener(listener, exec);
  }

  /*
   * TODO(cpovirk): Use standard Javadoc form for SimpleForwarding* class and
   * constructor
   */
  /**
   * A simplified version of {@link ForwardingListenableFuture} where subclasses
   * can pass in an already constructed {@link ListenableFuture} 
   * as the delegate.
   * 
   * @since 9.0
   */
  public abstract static class SimpleForwardingListenableFuture<V>
      extends ForwardingListenableFuture<V> {
    private final ListenableFuture<V> delegate;

    protected SimpleForwardingListenableFuture(ListenableFuture<V> delegate) {
      this.delegate = Preconditions.checkNotNull(delegate);
    }

    @Override
    protected final ListenableFuture<V> delegate() {
      return delegate;
    }
  }
}