推荐：介绍一个UndoFramework

　　由于其他工作，好多天又没有对MetaModelEngine进行思考了，这两天又腾出时间可以思考一下了，本篇介绍一下在图形编辑器中对操作如何实现Undo操作。

　　在图形设计器操作中，每个操作按钮都对应到一个命令，很多情况下我们都应该允许用户执行操作后回滚这些操作，或者回滚后又再次执行。在我做的报表引擎中，我是在每次操作后把设计文件都保留下来，这个在报表设计中是没有问题，但是在毕竟不是很好的设计。接下来要考虑对OpenExpressApp提供建模支持了，所以也需要考虑一下如何让图形设计器更好的支持这种Undo操作。

　　在公司的一个项目组中内部是使用命令模式，只是在传统命令模式中增加了一个UnExecute方法，这个方法就是用来做Undo操作的。在codeplex上我找到了一类似的轻量级UndoFramework，后期准备就用它了，在这里我就给大家介绍一下。

UndoFramework项目

Codeplex网站地址：http://undo.codeplex.com/

下载地址：http://undo.codeplex.com/releases/view/29440

项目描述：

　　It's a simple framework to add Undo/Redo functionality to your applications, based on the classical Command design pattern. It supports merging actions, nested transactions, delayed execution (execution on top-level transaction commit) and possible non-linear undo history (where you can have a choice of multiple actions to redo).

　　The status of the project is Stable (released). I might add more stuff to it later, but right now it fully satisfies my needs. It's implemented in C# 3.0 (Visual Studio 2008) and I can build it for both desktop and Silverlight. The release has both binaries.

现有应用

A good example of where this framework is used is the Live Geometry project (http://livegeometry.codeplex.com). It defines several actions such as AddFigureAction, RemoveFigureAction, MoveAction and SetPropertyAction.

如何使用

　　我学习这些东西一般都喜欢先看如何使用，因为从使用方式就能看出封装得是否简单易用。

　　以下是一个控制台的演示程序，代码如下：

using System;
using GuiLabs.Undo;
 
namespace MinimalSample
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("Original color");
 
            SetConsoleColor(ConsoleColor.Green);
            Console.WriteLine("New color");
 
            actionManager.Undo();
            Console.WriteLine("Old color again");
 
            using (Transaction.Create(actionManager))
            {
                SetConsoleColor(ConsoleColor.Red); // you never see Red
                Console.WriteLine("Still didn't change to Red because of lazy evaluation");
                SetConsoleColor(ConsoleColor.Blue);
            }
            Console.WriteLine("Changed two colors at once");
 
            actionManager.Undo();
            Console.WriteLine("Back to original");
 
            actionManager.Redo();
            Console.WriteLine("Blue again");
            Console.ReadKey();
        }
 
        static void SetConsoleColor(ConsoleColor color)
        {
            SetConsoleColorAction action = new SetConsoleColorAction(color);
            actionManager.RecordAction(action);
        }
 
        static ActionManager actionManager = new ActionManager();
    }
 
    class SetConsoleColorAction : AbstractAction
    {
        public SetConsoleColorAction(ConsoleColor newColor)
        {
            color = newColor;
        }
 
        ConsoleColor color;
        ConsoleColor oldColor;
 
        protected override void ExecuteCore()
        {
            oldColor = Console.ForegroundColor;
            Console.ForegroundColor = color;
        }
 
        protected override void UnExecuteCore()
        {
            Console.ForegroundColor = oldColor;
        }
    }
}

运行后界面如下：

下载代码你会看到，它还自带一个Form的例子，感兴趣可以自己去看看

Actions

　　所有操作都从 IAction继承下来，必须实现两个操作：一个是执行操作，一个是反执行操作

/// <summary>
/// Encapsulates a user action (actually two actions: Do and Undo)
/// Can be anything.
/// You can give your implementation any information it needs to be able to
/// execute and rollback what it needs.
/// </summary>
public interface IAction
{
    /// <summary>
    /// Apply changes encapsulated by this object.
    /// </summary>
    void Execute();
 
    /// <summary>
    /// Undo changes made by a previous Execute call.
    /// </summary>
    void UnExecute();
 
    /// <summary>
    /// For most Actions, CanExecute is true when ExecuteCount = 0 (not yet executed)
    /// and false when ExecuteCount = 1 (already executed once)
    /// </summary>
    /// <returns>true if an encapsulated action can be applied</returns>
    bool CanExecute();
 
    /// <returns>true if an action was already executed and can be undone</returns>
    bool CanUnExecute();
 
    /// <summary>
    /// Attempts to take a new incoming action and instead of recording that one
    /// as a new action, just modify the current one so that it's summary effect is 
    /// a combination of both.
    /// </summary>
    /// <param name="followingAction"></param>
    /// <returns>true if the action agreed to merge, false if we want the followingAction
    /// to be tracked separately</returns>
    bool TryToMerge(IAction followingAction);
 
    /// <summary>
    /// Defines if the action can be merged with the previous one in the Undo buffer
    /// This is useful for long chains of consecutive operations of the same type,
    /// e.g. dragging something or typing some text
    /// </summary>
    bool AllowToMergeWithPrevious { get; set; }
}

ActionManager

ActionManager负责跟踪undo/redo记录，提供RecordAction(IAction)来记录操作步骤，提供ActionManager.Undo(), ActionManager.Redo(), CanUndo(), CanRedo()等其他方法。

其完整代码如下：

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/// <summary>
  /// Action Manager is a central class for the Undo Framework.
  /// Your domain model (business objects) will have an ActionManager reference that would 
  /// take care of executing actions.
  /// 
  /// Here's how it works:
  /// 1. You declare a class that implements IAction
  /// 2. You create an instance of it and give it all necessary info that it needs to know
  ///    to apply or rollback a change
  /// 3. You call ActionManager.RecordAction(yourAction)
  /// 
  /// Then you can also call ActionManager.Undo() or ActionManager.Redo()
  /// </summary>
  public class ActionManager
  {
      public ActionManager()
      {
          History = new SimpleHistory();
      }
 
      #region Events
 
      /// <summary>
      /// Listen to this event to be notified when a new action is added, executed, undone or redone
      /// </summary>
      public event EventHandler CollectionChanged;
      protected void RaiseUndoBufferChanged(object sender, EventArgs e)
      {
          if (CollectionChanged != null)
          {
              CollectionChanged(this, e);
          }
      }
 
      #endregion
 
      #region RecordAction
 
      #region Running
 
      /// <summary>
      /// Currently running action (during an Undo or Redo process)
      /// </summary>
      /// <remarks>null if no Undo or Redo is taking place</remarks>
      public IAction CurrentAction { get; internal set; }
 
      /// <summary>
      /// Checks if we're inside an Undo or Redo operation
      /// </summary>
      public bool ActionIsExecuting
      {
          get
          {
              return CurrentAction != null;
          }
      }
 
      #endregion
 
      /// <summary>
      /// Defines whether we should record an action to the Undo buffer and then execute,
      /// or just execute it without it becoming a part of history
      /// </summary>
      public bool ExecuteImmediatelyWithoutRecording { get; set; }
 
      /// <summary>
      /// Central method to add and execute a new action.
      /// </summary>
      /// <param name="existingAction">An action to be recorded in the buffer and executed</param>
      public void RecordAction(IAction existingAction)
      {
          if (existingAction == null)
          {
              throw new ArgumentNullException(
                  "ActionManager.RecordAction: the existingAction argument is null");
          }
          // make sure we're not inside an Undo or Redo operation
          CheckNotRunningBeforeRecording(existingAction);
 
          // if we don't want to record actions, just run and forget it
          if (ExecuteImmediatelyWithoutRecording
              && existingAction.CanExecute())
          {
              existingAction.Execute();
              return;
          }
 
          // Check if we're inside a transaction that is being recorded
          ITransaction currentTransaction = RecordingTransaction;
          if (currentTransaction != null)
          {
              // if we're inside a transaction, just add the action to the transaction's list
              currentTransaction.AccumulatingAction.Add(existingAction);
              if (!currentTransaction.IsDelayed)
              {
                  existingAction.Execute();
              }
          }
          else
          {
              RunActionDirectly(existingAction);
          }
      }
 
      void CheckNotRunningBeforeRecording(IAction existingAction)
      {
          string existing = existingAction != null ? existingAction.ToString() : "";
 
          if (CurrentAction != null)
          {
              throw new InvalidOperationException
              (
                  string.Format
                  (
                        "ActionManager.RecordActionDirectly: the ActionManager is currently running "
                      + "or undoing an action ({0}), and this action (while being executed) attempted "
                      + "to recursively record another action ({1}), which is not allowed. "
                      + "You can examine the stack trace of this exception to see what the "
                      + "executing action did wrong and change this action not to influence the "
                      + "Undo stack during its execution. Checking if ActionManager.ActionIsExecuting == true "
                      + "before launching another transaction might help to avoid the problem. Thanks and sorry for the inconvenience.",
                      CurrentAction.ToString(),
                      existing
                  )
              );
          }
      }
 
      object recordActionLock = new object();
      /// <summary>
      /// Adds the action to the buffer and runs it
      /// </summary>
      void RunActionDirectly(IAction actionToRun)
      {
          CheckNotRunningBeforeRecording(actionToRun);
 
          lock (recordActionLock)
          {
              CurrentAction = actionToRun;
              if (History.AppendAction(actionToRun))
              {
                  History.MoveForward();
              }
              CurrentAction = null;
          }
      }
 
      #endregion
 
      #region Transactions
 
      public Transaction CreateTransaction()
      {
          return Transaction.Create(this);
      }
 
      public Transaction CreateTransaction(bool delayed)
      {
          return Transaction.Create(this, delayed);
      }
 
      private Stack<ITransaction> mTransactionStack = new Stack<ITransaction>();
      public Stack<ITransaction> TransactionStack
      {
          get
          {
              return mTransactionStack;
          }
          set
          {
              mTransactionStack = value;
          }
      }
 
      public ITransaction RecordingTransaction
      {
          get
          {
              if (TransactionStack.Count > 0)
              {
                  return TransactionStack.Peek();
              }
              return null;
          }
      }
 
      public void OpenTransaction(ITransaction t)
      {
          TransactionStack.Push(t);
      }
 
      public void CommitTransaction()
      {
          if (TransactionStack.Count == 0)
          {
              throw new InvalidOperationException(
                  "ActionManager.CommitTransaction was called"
                  + " when there is no open transaction (TransactionStack is empty)."
                  + " Please examine the stack trace of this exception to find code"
                  + " which called CommitTransaction one time too many."
                  + " Normally you don't call OpenTransaction and CommitTransaction directly,"
                  + " but use using(var t = Transaction.Create(Root)) instead.");
          }
 
          ITransaction committing = TransactionStack.Pop();
 
          if (committing.AccumulatingAction.Count > 0)
          {
              RecordAction(committing.AccumulatingAction);
          }
      }
 
      public void RollBackTransaction()
      {
          if (TransactionStack.Count != 0)
          {
              var topLevelTransaction = TransactionStack.Peek();
              if (topLevelTransaction != null && topLevelTransaction.AccumulatingAction != null)
              {
                  topLevelTransaction.AccumulatingAction.UnExecute();
              }
 
              TransactionStack.Clear();
          }
      }
 
      #endregion
 
      #region Undo, Redo
 
      public void Undo()
      {
          if (!CanUndo)
          {
              return;
          }
          if (ActionIsExecuting)
          {
              throw new InvalidOperationException(string.Format("ActionManager is currently busy"
                  + " executing a transaction ({0}). This transaction has called Undo()"
                  + " which is not allowed until the transaction ends."
                  + " Please examine the stack trace of this exception to see"
                  + " what part of your code called Undo.", CurrentAction));
          }
          CurrentAction = History.CurrentState.PreviousAction;
          History.MoveBack();
          CurrentAction = null;
      }
 
      public void Redo()
      {
          if (!CanRedo)
          {
              return;
          }
          if (ActionIsExecuting)
          {
              throw new InvalidOperationException(string.Format("ActionManager is currently busy"
                  + " executing a transaction ({0}). This transaction has called Redo()"
                  + " which is not allowed until the transaction ends."
                  + " Please examine the stack trace of this exception to see"
                  + " what part of your code called Redo.", CurrentAction));
          }
          CurrentAction = History.CurrentState.NextAction;
          History.MoveForward();
          CurrentAction = null;
      }
 
      public bool CanUndo
      {
          get
          {
              return History.CanMoveBack;
          }
      }
 
      public bool CanRedo
      {
          get
          {
              return History.CanMoveForward;
          }
      }
 
      #endregion
 
      #region Buffer
 
      public void Clear()
      {
          History.Clear();
          CurrentAction = null;
      }
 
      public IEnumerable<IAction> EnumUndoableActions()
      {
          return History.EnumUndoableActions();
      }
 
      private IActionHistory mHistory;
      internal IActionHistory History
      {
          get
          {
              return mHistory;
          }
          set
          {
              if (mHistory != null)
              {
                  mHistory.CollectionChanged -= RaiseUndoBufferChanged;
              }
              mHistory = value;
              if (mHistory != null)
              {
                  mHistory.CollectionChanged += RaiseUndoBufferChanged;
              }
          }
      }
 
      #endregion
  }