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设计模式——命令模式

时间:2016-01-14 15:56:44      阅读:218      评论:0      收藏:0      [点我收藏+]

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     命令模式的定义:将“请求”封装成对象,以便使用不同的请求、队列或者日志来参数化其他对象。命令模式也支持撤销的操作。注意命令模式是将请求封装成对象!

    其实简单的说,命令模式就是把方法调用封装起来了,通过封装方法调用,可以把运算块包装成型,所以调用此运算的对象不需要关心事情是如何进行的,只要知道如何使用包装成型的方法来完成它就可以。

    命令模式的使用场景:当需要将发出请求的对象和执行请求的对象解耦的时候,请考虑使用命令模式吧。

    场景举例:设计一款多功能遥控器,该遥控器可以控制家里各个电器的状态,例如电灯的开关,电视的开关,电扇的速度,车库门的开关等等,下面使用命令模式实现这个遥控器。在这里,发出请求的对象是遥控器,执行请求的对象是各种电器。

    首先需要让所有的命令对象实现同一个命令接口

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

     接口中有两个方法,一个是执行动作的方法execute,一个是撤销动作的方法undo

    下面实现电灯打开关闭命令前先声明一个电灯类

public class Light {
    
    protected void on() {
        System.out.println("light is on");
    }

    protected void off() {
        System.out.println("light is off");
    }
}

    电灯类只有两个方法on—>打开,off—>关闭;然后实现打开电灯的命令

public class LightOnCommand implements Command {

    Light light;

    public LightOnCommand(Light light) {
        // TODO Auto-generated constructor stub
        this.light = light;
    }

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

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

}

    注意undo的操作正好是与execute的操作相反的;在写一个电灯关闭的命令

public class LightOffCommand implements Command {

    Light light;

    public LightOffCommand(Light light) {
        // TODO Auto-generated constructor stub
        this.light = light;
    }

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

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

}

    再增加一个车库门的开启和关闭

public class GarageDoor {
    public void up() {
        System.out.println("the garagedoor is up!");
    }

    public void down() {
        System.out.println("the garagedoor is down!");
    }

    public void lightOn() {
        System.out.println("the garagedoor‘s light is on!");
    }

    public void lightOff() {
        System.out.println("the garagedoor‘s light is off!");
    }
}
public class GarageDoorOpenCommand implements Command {
    GarageDoor garageDoor;

    public GarageDoorOpenCommand(GarageDoor garageDoor) {
        // TODO Auto-generated constructor stub
        this.garageDoor = garageDoor;
    }

    @Override
    public void execute() {
        // TODO Auto-generated method stub
        garageDoor.up();
        garageDoor.lightOn();
    }

    @Override
    public void undo() {
        // TODO Auto-generated method stub
        garageDoor.down();
        garageDoor.lightOff();
    }

}
public class GarageDoorCloseCommand implements Command {
    GarageDoor garageDoor;

    public GarageDoorCloseCommand(GarageDoor garageDoor) {
        // TODO Auto-generated constructor stub
        this.garageDoor = garageDoor;
    }

    @Override
    public void execute() {
        // TODO Auto-generated method stub
        garageDoor.down();
        garageDoor.lightOff();
    }

    @Override
    public void undo() {
        // TODO Auto-generated method stub
        garageDoor.up();
        garageDoor.lightOn();
    }

}

    然后就是遥控器的实现了

public class RemoteControl {
    /*
     * 由于对应多个电器这里采用数组记录这些命令
     */
    Command[] onCommands;
    Command[] offCommands;
    Command undoCommand;//用来记录撤销命令

    public RemoteControl() {
        // TODO Auto-generated constructor stub
        onCommands = new Command[7];
        offCommands = new Command[7];
        Command noCommand = new NoCommand();//初始化是都是空命令
        for (int i = 0; i < 7; 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 undoButtonWasPushed(){
        undoCommand.undo();
    }
}

    空命令在这里什么也不做,只是保证上的按钮均有命令对象存在,空命令的实现

public class NoCommand implements Command {

    @Override
    public void execute() {
        // TODO Auto-generated method stub
        System.out.println("no command!");
    }

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

}

    开始测试一下遥控器

public class RemoteLoader {

    public static void main(String[] args) {
        // TODO Auto-generated method stub
        RemoteControl remoteControl=new RemoteControl();
        Light light=new Light();
        LightOnCommand lightOn=new LightOnCommand(light);
        LightOffCommand lightOff =new LightOffCommand(light);
        
        GarageDoor garageDoor=new GarageDoor();
        GarageDoorOpenCommand garageDoorOpen=new GarageDoorOpenCommand(garageDoor);
        GarageDoorCloseCommand garageDoorClose=new GarageDoorCloseCommand(garageDoor);
        
        remoteControl.setCommand(0, lightOn, lightOff);
        remoteControl.setCommand(1, garageDoorOpen, garageDoorClose);
        
        remoteControl.onButtonWasPushed(0);
        remoteControl.onButtonWasPushed(1);
        remoteControl.offButtonWasPushed(0);
        remoteControl.offButtonWasPushed(1);
    }

}

    打印结果:

light is on
the garagedoor is up!
the garagedoor‘s light is on!
light is off
the garagedoor is down!
the garagedoor‘s light is off!

    遥控器实现完毕,测试一下撤销命令

public class RemoteLoader {

    public static void main(String[] args) {
        // TODO Auto-generated method stub
        RemoteControl remoteControl=new RemoteControl();
        Light light=new Light();
        LightOnCommand lightOn=new LightOnCommand(light);
        LightOffCommand lightOff =new LightOffCommand(light);
        
        remoteControl.setCommand(0, lightOn, lightOff);
        
        remoteControl.onButtonWasPushed(0);
        System.out.println("****************");
        remoteControl.undoButtonWasPushed();
    }

}

    打印结果

light is on
****************
light is off

    总结:

    (1)命令模式将发出请求的对象和执行请求的对象解耦;

    (2)在被解耦的两者之间是通过命令对象进行沟通的。命令对象封装了接受者和一个或一组动作。

    (3)调用者通过调用命令对象的execute()发出请求,这会使得接受者的动作被调用。

    (4)调用者可以接受命令当作参数,甚至在运行时动态的进行。

    (5)命令可以支持撤销,做法是实现一个undo()方法来回到上一个execute()被执行前的状态。

   

设计模式——命令模式

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原文地址:http://www.cnblogs.com/shiwei-bai/p/5066824.html

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