一、命令模式簡介（Brief Introduction）
命令模式（Command Pattern）將請求封裝為一個對象，從而使你用不同的請求對客戶進 行參數化，對請求排隊或紀錄請求日志，以及支持可撤銷的操作。
Encapsulate a request as an object, thereby letting you parameterize clients with different requests, queue or log requests, and support undoable operations
二、解決的問題（What To Solve）
當需要有撤銷或者恢復操作時，可以考慮使用命令模式。
三、命令模式分析（Analysis）1、命令模式結構
Command抽象類：聲明一個接口，用於執行操作，declares an interface for executing an operation。
ConcreteCommand實現類：將一個接收者對象綁定到一個動作。調用接收者相應的操作， 以實現Execute 。
defines a binding between a Receiver object and an action
implements Execute by invoking the corresponding operation(s) on Receiver。
Receiver類：知道如何執行一個請求相關的操作。knows how to perform the operations associated with carrying out the request.
Invoker類：要求命令執行一個請求。asks the command to carry out the request 。
2、源代碼
1、Command抽象類：聲明一個接口，用於執行操作
abstract class Command
{
protected Receiver receiver;
public Command(Receiver receiver)
{
this.receiver = receiver;
}
public abstract void Execute();
}
2、ConcreteCommand具體實現類：將一個接收者對象綁定到一個動作
class ConcreteCommand : Command
{
// Constructor
public ConcreteCommand(Receiver receiver)
:base(receiver)
{
}
public override void Execute()
{
receiver.Action();
}
}
3、Receiver類：知道如何執行一個請求相關的操作
class Receiver
{
public void Action()
{
Console.WriteLine("Called Receiver.Action()");
}
}
4、Invoker類：要求命令執行一個請求
class Invoker
{
private Command _command;
public void SetCommand(Command command)
{
this._command = command;
}
public void ExecuteCommand()
{
_command.Execute();
}
}
4、客戶端代碼
static void Main(string[] args)
{
// Create receiver, command, and invoker
Receiver receiver = new Receiver();
Command command = new ConcreteCommand(receiver);
Invoker invoker = new Invoker();
// Set and execute command
invoker.SetCommand(command);
invoker.ExecuteCommand();
Console.ReadKey();
}
3、程序運行結果
四．案例分析（Example）
1、場景
使用命令模式進行計算器計算，可以是加減乘除等運算，可以進行Undo操作和Rodo操作 。如下圖所示
Command抽象命令類：聲明一個接口，用於執行操作。
CalculatorCommand具體實現類：將一個接收者對象綁定到一個動作。調用接收者相應的 操作，以實現Execute 。
UnExecute方法：執行Undo操作。
Calculator 類-Operation方法：執行加減乘除操作。
User類：要求命令Calculator執行一個計算請求。
Compute方法：加減乘除等計算操作
Undo方法：撤銷操作。Redo方法：重復操作。
2、代碼
1、抽象命令類Command及其計算器類CalculatorCommand
/// <summary>
/// The 'Command' abstract class
/// </summary>
abstract class Command
{
public abstract void Execute();
public abstract void UnExecute();
}
/// <summary>
/// The 'ConcreteCommand' class
/// </summary>
class CalculatorCommand : Command
{
private char _operator;
private int _operand;
private Calculator _calculator;
// Constructor
public CalculatorCommand(Calculator calculator,char @operator, int operand)
{
this._calculator = calculator;
this._operator = @operator;
this._operand = operand;
}
// Gets operator
public char Operator
{
set { _operator = value; }
}
// Get operand
public int Operand
{
set { _operand = value; }
}
// Execute new command
public override void Execute()
{
_calculator.Operation(_operator, _operand);
}
// Unexecute last command
public override void UnExecute()
{
_calculator.Operation(Undo(_operator), _operand);
}
// Returns opposite operator for given operator
private char Undo(char @operator)
{
switch (@operator)
{
case '+': return '-';
case '-': return '+';
case '*': return '/';
case '/': return '*';
default: throw new
ArgumentException("@operator");
}
}
}
2、計算器類Calculator
/// <summary>
/// The 'Receiver' class
/// </summary>
class Calculator
{
private int _curr = 0;
public void Operation(char @operator, int operand)
{
switch (@operator)
{
case '+': _curr += operand; break;
case '-': _curr -= operand; break;
case '*': _curr *= operand; break;
case '/': _curr /= operand; break;
}
Console.WriteLine(
"Current value = {0,3} (following {1} {2})",
_curr, @operator, operand);
}
}
3、請求類User
/// <summary>
/// The 'Invoker' class
/// </summary>
class User
{
// Initializers
private Calculator _calculator = new Calculator();
private List<Command> _commands = new List<Command> ();
private int _current = 0;
public void Redo(int levels)
{
Console.WriteLine("\n---- Redo {0} levels ", levels);
// Perform redo operations
for (int i = 0; i < levels; i++)
{
if (_current < _commands.Count - 1)
{
Command command = _commands [_current++];
command.Execute();
}
}
}
public void Undo(int levels)
{
Console.WriteLine("\n---- Undo {0} levels ", levels);
// Perform undo operations
for (int i = 0; i < levels; i++)
{
if (_current > 0)
{
Command command = _commands[--_current] as Command;
command.UnExecute();
}
}
}
public void Compute(char @operator, int operand)
{
// Create command operation and execute it
Command command = new CalculatorCommand(
_calculator, @operator, operand);
command.Execute();
// Add command to undo list
_commands.Add(command);
_current++;
}
4、客戶端代碼
static void Main(string[] args)
{
// Create user and let her compute
User user = new User();
// User presses calculator buttons
user.Compute('+', 100);
user.Compute('-', 50);
user.Compute('*', 10);
user.Compute('/', 2);
// Undo 4 commands
user.Undo(4);
// Redo 3 commands
user.Redo(3);
Console.ReadKey();
}
3、程序運行結果
五、總結（Summary）
命令模式（Command Pattern）將請求封裝為一個對象，從而使你用不同的請求對客戶進 行參數化，對請求排隊或紀錄請求日志，以及支持可撤銷的操作。當需要有撤銷或者恢復操 作時，可以考慮使用命令模式。
出處：http://www.cnblogs.com/ywqu
