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命令模式可以很轻松的实现撤销（Undo）功能。
命令的接受者：
unit uReceiveObject;interfacetype TLight = class(TObject) public procedure Open; procedure Off; end;implementation{ TLight }procedure TLight.Off;begin Writeln(‘Light is off.‘);end;procedure TLight.Open;begin Writeln(‘Light is on.‘);end;end.
命令对象：
unit uCommandObject;interfaceuses uReceiveObject;type TCommand = class(TObject) public procedure Execute; virtual; abstract; procedure Undo; virtual; abstract; end; TLightOnCommand = class(TCommand) private FLight: TLight; public constructor Create(aLight: TLight); procedure Execute; override; procedure Undo; override; end; TLightOffCommand = class(TCommand) private FLight: TLight; public constructor Create(aLight: TLight); procedure Execute; override; procedure Undo; override; end;implementation{ TLightOnCommand }constructor TLightOnCommand.Create(aLight: TLight);begin FLight := aLight;end;procedure TLightOnCommand.Execute;begin FLight.Open;end;procedure TLightOnCommand.Undo;begin FLight.Off;end;{ TLightOffCommand }constructor TLightOffCommand.Create(aLight: TLight);begin FLight := aLight;end;procedure TLightOffCommand.Execute;begin FLight.Off;end;procedure TLightOffCommand.Undo;begin FLight.Open;end;end.
命令的请求者：
unit uSimpleRemoteWithUndo;interfaceuses uCommandObject;type TSimpleRemoteWithUndo = class(TObject) private FOnCommand : TCommand; FOffCommand : TCommand; FUndoCommand: TCommand; public procedure SetCommand(aOnCommand, aOffCommand: TCommand); procedure OnButtonWasPressed; procedure OffButtonWasPressed; procedure UndoButtonWasPressed; end; implementation{ TSimpleRemoteWithUndo }procedure TSimpleRemoteWithUndo.OffButtonWasPressed;begin FOffCommand.Execute; FUndoCommand := FOffCommand;end;procedure TSimpleRemoteWithUndo.OnButtonWasPressed;begin FOnCommand.Execute; FUndoCommand := FOnCommand;end;procedure TSimpleRemoteWithUndo.SetCommand(aOnCommand, aOffCommand: TCommand);begin FOnCommand := aOnCommand; FOffCommand := aOffCommand;end;procedure TSimpleRemoteWithUndo.UndoButtonWasPressed;begin FUndoCommand.Undo;end;end.
客户端，创建具体的命令对象：
program pSimpleRemoteWithUndoTest;{$APPTYPE CONSOLE}uses uSimpleRemoteWithUndo in ‘uSimpleRemoteWithUndo.pas‘, uCommandObject in ‘uCommandObject.pas‘, uReceiveObject in ‘uReceiveObject.pas‘;var Remote: TSimpleRemoteWithUndo; Light : TLight; LightOnCommand : TCommand; LightOffCommand: TCommand; begin Remote := TSimpleRemoteWithUndo.Create; Light := TLight.Create; LightOnCommand := TLightOnCommand.Create(Light); LightOffCommand := TLightOffCommand.Create(Light); Remote.SetCommand(LightOnCommand, LightOffCommand); Remote.OnButtonWasPressed; Remote.OffButtonWasPressed; Remote.UndoButtonWasPressed; Writeln; Remote.OffButtonWasPressed; Remote.OnButtonWasPressed; Remote.UndoButtonWasPressed; Remote.Free; Light.Free; LightOnCommand.Free; LightOffCommand.Free; Readln;end.
运行结果：
Delphi 设计模式：《HeadFirst设计模式》Delphi7代码---命令模式之SimpleRemoteWithUndoTest[转]
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原文地址：http://www.cnblogs.com/0x2D-0x22/p/4076159.html
