/**
 * 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.model.undo;

/**
 * An undo manager.
 *
 *
 * @param <T> The type of operations.
 */
public interface UndoManager<T> {

  /**
   * Places into the undo manager an operation that should be undone by undos.
   *
   * @param op the operation that should be undone by an appropriate undo
   */
  void undoableOp(T op);

  /**
   * Places into the undo manager an operation that should not be undone by undos.
   *
   * @param op the operation that should not be undone by any undos
   */
  void nonUndoableOp(T op);

  /**
   * Places an undo checkpoint.
   */
  void checkpoint();

  /**
   * Effects an undo.  Returns null if there are no operations to undo.
   *
   * @return the operation that will effect an undo
   */
  T undo();

  /**
   * Effects a redo.  Returns null if there are no operations to redo.
   *
   * @return the operation that will effect a redo
   */
  T redo();
}
