设计模式第六集——命令模式（Command）

什么时候使用命令模式？当需要将发出请求的对象和执行请求的对象解耦的时候，使用命令模式。

命令模式：将请求封装成对象。

Client创建一个ConcreteCommend，并设置接受者。Command为所有命令生命了一个接口，调用命令对象的execute()方法，可以让接收者进行相关动作。Invoker是命令最终的调用者。

具体例子，现在设计一个遥控器，这个遥控器来控制房间内所有电器的开关。为这个遥控器编程。

以电风扇为例，RemoteLoader创建很多命令对象，然后将他们加载到遥控起的插槽中。每个命令对象都封装了某个家电自动化的一项要求。RemoteControl管理一组命令对象，每个按钮按下的时候就调用相应的onButtonWasPushed()或者offButtonWasPushed()，进而调用execute()方法被调用。

package com;

public interface Command {
    public void execute();
    public void undo();
}

package com;

public class FanHighCommand implements Command {
    Fan fan;
    int prevSpeed;

    public FanHighCommand(Fan fan) {
        this.fan = fan;
    }

    public void execute() {
        prevSpeed = fan.getSpeed();

    }

    @Override
    public void undo() {
        if (prevSpeed == Fan.High) {
            fan.high();
        } else if (prevSpeed == Fan.Medium) {
            fan.medium();
        } else if (prevSpeed == Fan.Low) {
            fan.low();
        } else if (prevSpeed == Fan.Off) {
            fan.off();
        }
    }
}

package com;

public class NoCommand implements Command{

    @Override
    public void execute() {
        // TODO Auto-generated method stub
        
    }

    @Override
    public void undo() {
        // TODO Auto-generated method stub
        
    }

}

package com;

public class Fan {
    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 Fan(String location) {
        this.location = location;
        speed=Off;
    }
    //设置吊扇速度
    public void high(){
        speed=High;
    }
    public void medium(){
        speed=Medium;
    }
    public void low(){
        speed=Low;
    }
    public void off(){
        speed=Off;
    }
    //返回当前吊扇速度，为undo使用
    public int getSpeed(){
        return speed;
    }
}


package com;

public class RemoteControl {
    Command[] onCommands;
    Command[] offCommands;
    Command undoCommand;
    
    public RemoteControl(){
        onCommands=new Command[3];//假设共控制三种电器
        offCommands=new Command[3];
        
        Command noCommand=new NoCommand();
        for(int i=0;i<3;i++){
            onCommands[i]=noCommand;
            offCommands[i]=noCommand;            
        }
        undoCommand=noCommand;
    }
    public void setCommand(int slot,Command onCommand,Command offCommand){
        onCommands[slot]=onCommand;
        offCommands[slot]=offCommand;
    }
    public void onButtonWasPushed(int slot){
        onCommands[slot].execute();
        undoCommand=onCommands[slot];
    }
    public void offButtonWasPushed(int slot){
        offCommands[slot].execute();
        undoCommand=offCommands[slot];
    }
    public void undoButtonWashPushed(){
        undoCommand.undo();
    }
}