理解命令模式

概念理解

一个命令有命令的发出者和执行者，命令模式的主要目的是将这两者主体解耦。简单理解，发出的命令（不管是什么）是一个对象，对象中仅有做和不做两个方法。命令如何执行，有具体的命令实现对象调用命令接收者来具体执行。

需求描述

遥控器需要控制家中电灯的开关，以及冰箱的开关和设置温度。

设计一：直接方式

从直接实现的结构可以看出，发出命令者和执行命令者存在着紧耦合，这种结构不利于扩展。

public class Fridge {
    private int temperature=10;

    public void on() {
        System.out.println("fridge is on ");
    }

    public void off() {
        System.out.println("fridge is  off");
    }

    public void temperatureUp() {
        this.temperature++;
        System.out.println(" up temperature：" + temperature + " is set");
    }

    public void temperatureDown() {
        this.temperature--;
        System.out.println(" down temperature：" + temperature + " is set");
    }
}

public class Light {

    public Light() {
    }

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

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

}

public class Controller {
    private Light light = new Light();
    private Fridge fridge = new Fridge();

    public void invoke(int slot) {
        switch (slot) {
            case 1:
                light.on();
            case 2:
                light.off();
            case 3:
                fridge.on();
            case 4:
                fridge.off();
            case 5:
                fridge.temperatureUp();
            case 6:
                fridge.temperatureDown();
            default:
                break;
        }
    }
}

命令模式的主要目的是加入中间层，命令对象，将命令发出者和执行者解耦。

设计二：命令模式

命令模式实际上就是在命令发出者和接受者之间加入了中间层——命令对象。

挑选几个代码实例

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

public class LightOnCommander implements Commander {
    private Light light;

    public LightOnCommander() {
        this.light = new Light();
    }

    @Override
    public void execute() {
        light.on();

    }

    @Override
    public void undo() {
        light.off();

    }
}

public class LightOffCommander implements  Commander{
    private Light light;

    public LightOffCommander(){
        this.light = new Light();
    }

    @Override
    public void execute() {
        light.off();

    }

    @Override
    public void undo() {
        light.on();

    }
}

public class ControlPannel {
    private int slot;
    private Commander[] onCommanders;
    private Commander[] offCommanders = new Commander[5];

    private Deque<Commander> stack;

    public ControlPannel() {
        onCommanders = new Commander[5];
        offCommanders = new Commander[5];
        Commander noCommander = new NoCommander();
        for (int i = 0; i < 5; i++) {
            onCommanders[i] = noCommander;
            offCommanders[i] = noCommander;
        }
        stack = new ArrayDeque<Commander>();
    }

    public void setCommanders(Commander onCommande, Commander offCommander, int slot) {
        onCommanders[slot] = onCommande;
        offCommanders[slot] = offCommander;
    }

    public void onButton(int slot) {
        onCommanders[slot].execute();
        stack.push(onCommanders[slot]);
    }

    public void offButton(int slot) {
        offCommanders[slot].execute();
        stack.push(offCommanders[slot]);
    }

    public void undoCommander() {
        stack.pop().undo();
    }
}

public class TestApp {

    public static void main(String[] args) {
        ControlPannel panel = initPanel();
        panel.onButton(1);
        panel.onButton(2);
        panel.undoCommander();
        panel.undoCommander();
    }

    public static ControlPannel initPanel() {
        ControlPannel pannel = new ControlPannel();
        pannel.setCommanders(new LightOnCommander(), new LightOffCommander(), 0);
        pannel.setCommanders(new FridgeOnCommander(), new FridgeOffCommander(), 1);
        pannel.setCommanders(new FridgeTmeperatureOnCommander(), new FrigdeTemperatureDownCommander(), 2);
        return pannel;
    }
}

注意点

优点：命令模式的主要目的是解耦，层次分明，各司其职，方便扩展
缺点：命令模式因为其层次结构的存在，如果存在多个命令，则会造成命令类过于庞大。

具体代码实例参见https://github.com/jxl198/designPattern/tree/master/commander