[设计模式]之命令模式（Command）

命令模式的定义为：把一个请求封装成一个对象，因此可以使用不同的请求来参数化别的对象，将请求加入队列或者记录请求日志，并且支持撤销操作。
官方定义为：The Command Pattern encapsulates a request as an object, thereby letting you parameterize other objects with different request, queue or log requests, and support undoable operations.

Command中比较重要的几个参与者：
Command
- declares an inte易做图ce for executing an operation.
ConcreteCommand
- defines a binding between a Receiver object and an action
- implements Execute by invoking the corresponding operation(s) on Receiver.
Client
- Create a ConcreteCommand ojbect and sets its receiver.
Invoker
- asks the command to carry out the request.
Receiver
- konws how to perform the oprations associated with carrying out a request, Any class may serve as a Receiver.

Command模式将命令的发送者与接受者分离，Command对象即为两者间的桥梁。
首先必须定义一个命令的接口(Command)，接口中定义命令所支持的操作，然后定义具体的命令类(ConcreteCommand)，并指定命令的接收着（Receiver)，有时候接收者与命令可以合二为一。
利用多态机制，调用者(Invoker)只需要定义命令的接口（Command）对象便可，命令的细节对调用着是透明的。

还有一个比较重要的特点就是undoable，
undo的过程就是Execute的逆过程，比较负责的撤销动作则用到了Memento模式。

命令对象不一定只是一个单一的命令，也可以是很多命令的集合，这样命令就可以不用定义Invoker。

《Header First Design Patterns》中的示例代码如下：

[java]
public inte易做图ce Command {
    public void execute();
    public void undo();
}

public class CeilingFanCommand implements Command {
    protected CeilingFan ceilingFan;
    protected int prevSpeed;

    public CeilingFanCommand(CeilingFan ceilingFan) {
        this.ceilingFan = ceilingFan;
    }

    public void execute() {
        prevSpeed = ceilingFan.getSpeed();
    }

    public void undo() {
        if (prevSpeed == CeilingFan.HIGH) {
            ceilingFan.high();
        } else if (prevSpeed == CeilingFan.MEDIUM) {
            ceilingFan.medium();
        } else if (prevSpeed == CeilingFan.LOW) {
            ceilingFan.low();
        } else if (prevSpeed == CeilingFan.OFF) {
            ceilingFan.off();
        }
    }
}
public class CeilingFanHighCommand extends CeilingFanCommand {
    public CeilingFanHighCommand(CeilingFan ceilingFan) {
        super(ceilingFan);
    }

    @Override
    public void execute() {
        super.execute();
        ceilingFan.high();
    }
}
public class CeilingFanMediumCommand extends CeilingFanCommand {
    public CeilingFanMediumCommand(CeilingFan ceilingFan) {
        super(ceilingFan);
    }

    @Override
    public void execute() {
        super.execute();
        ceilingFan.medium();
    }
}

public class CeilingFanOffCommand extends CeilingFanCommand {
    public CeilingFanOffCommand(CeilingFan ceilingFan) {
        super(ceilingFan);
    }

    @Override
    public void execute() {
        super.execute();
        ceilingFan.low();
    }
}

public class NoCommand implements Command {
    public NoCommand() { }

    @Override
    public void execute() { }

    @Override
    public void undo() { }
}

public class CeilingFan {
    public static final int HIGH = 3;
    public static final int MEDIUM = 2;
    public static final int LOW = 1;
    public static final int OFF = 0;
    String location;
    int speed;

    public CeilingFan(String location) {
        this.location = location;
        speed = OFF;
    }

    public void high() {
        speed = HIGH;
    }

    public void medium() {
        speed = MEDIUM;
    }

    public void low() {
        speed = LOW;
    }

    public void off() {
        speed = OFF;
    }

    public int getSpeed() {
        return speed;
    }
}

public class RemoteControlWithUndo {
    Command[] onCommands;
    Command[] offCommands;
    Command undoCommand;

    public RemoteControlWithUndo() {
        onCommands = new Command[7];
        offCommands = new Command[7];

        Command noCommand = new NoCommand();

        for (int i = 0; i < 7; i++) {
            onCommands[i] = noCommand;
            offCommands[i] = noCommand;
        }
        undoCommand = noCommand;
    }

&nb

补充：软件开发 , Java ,