Command模式实现的redo/undo功能(以前在闪吧发过)

Command—俺有一个mm家里管得特别严，没法见面，只好借助于她弟弟在我们俩之间
传送信息，她对我有什么指示，就写一张纸条让她弟弟带给我。这不，她弟弟又传送过来
一个command，为了感谢他，我请他吃了碗杂酱面，哪知道他说：“我同时给我姐姐三个
男朋友送command，就数你最小气，才请我吃面。”，

命令模式：命令模式把一个请求或者操作封装到一个对象中。命令模式把发出命令的责任
和执行命令的责任分割开，委派给不同的对象。命令模式允许请求的一方和发送的一方独
立开来，使得请求的一方不必知道接收请求的一方的接口，更不必知道请求是怎么被接收
，以及操作是否执行，何时被执行以及是怎么被执行的。系统支持命令的撤消。

这是Command模式的UML图 

class Command
{
private function Command()
{
}
// Methods
        public function Execute():Void
{
}
public function UnExecute():Void
{
}
}
// "ConcreteCommand"
class CalculatorCommand extends Command
{
// Fields
        private var $operator:String;
private var operand:Number;
private var calculator:Calculator;
// Constructor
        public function CalculatorCommand(calculator:Calculator, $operator:String, operand:Number)
{
this.calculator = calculator;
this.$operator = $operator;
this.operand = operand;
}
// Properties
        public function set Operator(value:String):Void
{
$operator = value;
}
public function set Operand(value:Number):Void
{
operand = value;
}
// Methods
        public function Execute():Void
{
calculator.Operation($operator, operand);
}
public function UnExecute():Void
{
calculator.Operation(Undo($operator), operand);
}
// Private helper function
        private function Undo($operator:String):String
{
var undo:String ="";
switch ($operator)
{
case "+" :
undo = "-";
break;
case "-" :
undo = "+";
break;
case "*" :
undo = "/";
break;
case "/" :
undo = "*";
break;
}
return undo;
}
}
// "Receiver"
class Calculator
{
// Fields
        private var total:Number = 0;
// Methods
        public function Operation($operator:String, operand:Number):Void
{
switch ($operator)
{
case "+" :
total += operand;
break;
case "-" :
total -= operand;
break;
case "*" :
total *= operand;
break;
case "/" :
total /= operand;
break;
}
trace("Total =" + total + " (following" + $operator + " " + operand + ")");
}
}
// "Invoker"
class User
{
// Fields
        private var calculator:Calculator;
private var commands:Array = new Array();
private var current:Number = 0;
public function User()
{
calculator = new Calculator();
}
// Methods
        public function Redo(levels:Number):Void
{
//trace(commands[current++] instanceof Command);
                trace("---- Redo " + levels + " levels ");
// Perform redo operations
                for (var i = 0; i < levels; i++)
{
if (current < commands.length - 1)
{
commands[current++].Execute();
}
}
}
public function Undo(levels:Number):Void
{
trace("---- Undo" + levels + " levels ");
// Perform undo operations
                for (var i = 0; i < levels; i++)
{
if (current > 0)
{
commands[--current].UnExecute();
}
}
}
public function Compute($operator:String, operand:Number):Void
{
// Create command operation and execute it
                var command:Command = new CalculatorCommand(calculator, $operator, operand);
command.Execute();
// Add command to undo list
                commands.push(command);
current++;
}
}
client.fla
// Create user and let her compute
var user:User = new User();
user.Compute('+', 100);
user.Compute('-', 50);
user.Compute('*', 10);
user.Compute('/', 2);
// Undo and then redo some commands
user.Undo(4);
user.Redo(3);

output:
Total =100 (following+ 100)
Total =50 (following- 50)
Total =500 (following* 10)
Total =250 (following/ 2)
---- Undo4 levels 
Total =500 (following* 2)
Total =50 (following/ 10)
Total =100 (following+ 50)
Total =0 (following- 100)
---- Redo 3 levels 
Total =100 (following+ 100)
Total =50 (following- 50)
Total =500 (following* 10)