命令模式

《Head First设计模式》Eric Freeman;Elisabeth Freeman;Kathy Sierra;Bert Bates
中国电力出版社
ISBN：9787508353937

命令模式-将“请求”封装成对象，以便使用不同的请求、队列或日志来参数化其他对象。命令模式也支持可撤销的操作。

命令模式代码示例：
Command接口

package net.dp.command.simpleremote;

public interface Command {
    public void execute();
}

所要封装的两个事物GarageDoor和Light(Receiver)

package net.dp.command.simpleremote;

public class GarageDoor {

    public GarageDoor() {
    }

    public void up() {
        System.out.println("Garage Door is Open");
    }

    public void down() {
        System.out.println("Garage Door is Closed");
    }

    public void stop() {
        System.out.println("Garage Door is Stopped");
    }

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

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

package net.dp.command.simpleremote;

public class Light {

    public Light() {
    }

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

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

封装的具体命令(ConcreteCommand)

package net.dp.command.simpleremote;

public class GarageDoorOpenCommand implements Command {
    GarageDoor garageDoor;

    public GarageDoorOpenCommand(GarageDoor garageDoor) {
        this.garageDoor = garageDoor;
    }

    public void execute() {
        garageDoor.up();
    }
}

package net.dp.command.simpleremote;

public class LightOnCommand implements Command {
    Light light;

    public LightOnCommand(Light light) {
        this.light = light;
    }

    public void execute() {
        light.on();
    }
}

package net.dp.command.simpleremote;

public class LightOffCommand implements Command {
    Light light;

    public LightOffCommand(Light light) {
        this.light = light;
    }

    public void execute() {
        light.off();
    }
}

下面是Invoker

package net.dp.command.simpleremote;

//
// This is the invoker
//
public class SimpleRemoteControl {
    Command slot;

    public SimpleRemoteControl() {}

    public void setCommand(Command command) {
        slot = command;
    }

    public void buttonWasPressed() {
        slot.execute();
    }
}

调用关系

package net.dp.command.simpleremote;

public class RemoteControlTest {
    public static void main(String[] args) {
        SimpleRemoteControl remote = new SimpleRemoteControl();
        Light light = new Light();
        GarageDoor garageDoor = new GarageDoor();
        LightOnCommand lightOn = new LightOnCommand(light);
        GarageDoorOpenCommand garageOpen = new GarageDoorOpenCommand(garageDoor);

        remote.setCommand(lightOn);
        remote.buttonWasPressed();
        remote.setCommand(garageOpen);
        remote.buttonWasPressed();
    }
}

下面是带有undo功能的命令模式示例
Command接口

package net.dp.command.undo;

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

所要封装的两个事物CeilingFan和Light(Receiver)

package net.dp.command.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;
        System.out.println(location + " ceiling fan is on high");
    }

    public void medium() {
        speed = MEDIUM;
        System.out.println(location + " ceiling fan is on medium");
    }

    public void low() {
        speed = LOW;
        System.out.println(location + " ceiling fan is on low");
    }

    public void off() {
        speed = OFF;
        System.out.println(location + " ceiling fan is off");
    }

    public int getSpeed() {
        return speed;
    }
}

package net.dp.command.undo;

public class Light {
    String location;
    int level;

    public Light(String location) {
        this.location = location;
    }

    public void on() {
        level = 100;
        System.out.println("Light is on");
    }

    public void off() {
        level = 0;
        System.out.println("Light is off");
    }

    public void dim(int level) {
        this.level = level;
        if (level == 0) {
            off();
        }
        else {
            System.out.println("Light is dimmed to " + level + "%");
        }
    }

    public int getLevel() {
        return level;
    }
}

封装的具体命令(ConcreteCommand)
这里为了实现undo()，在execute()执行之前，会保存之前的对象状态。

package net.dp.command.undo;

public class CeilingFanHighCommand implements Command {
    CeilingFan ceilingFan;
    int prevSpeed;

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

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

    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();
        }
    }
}

package net.dp.command.undo;

public class CeilingFanMediumCommand implements Command {
    CeilingFan ceilingFan;
    int prevSpeed;

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

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

    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();
        }
    }
}

package net.dp.command.undo;

public class CeilingFanLowCommand implements Command {
    CeilingFan ceilingFan;
    int prevSpeed;

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

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

    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();
        }
    }
}

package net.dp.command.undo;

public class CeilingFanOnCommand implements Command {
    CeilingFan ceilingFan;

    public CeilingFanOnCommand(CeilingFan ceilingFan) {
        this.ceilingFan = ceilingFan;
    }
    public void execute() {
        ceilingFan.high();
    }
    public void undo() {
        ceilingFan.off();
    }
}

package net.dp.command.undo;

public class CeilingFanOffCommand implements Command {
    CeilingFan ceilingFan;
    int prevSpeed;

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

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

    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();
        }
    }
}

package net.dp.command.undo;

public class DimmerLightOnCommand implements Command {
    Light light;
    int prevLevel;

    public DimmerLightOnCommand(Light light) {
        this.light = light;
    }

    public void execute() {
        prevLevel = light.getLevel();
        light.dim(75);
    }

    public void undo() {
        light.dim(prevLevel);
    }
}

package net.dp.command.undo;

public class DimmerLightOffCommand implements Command {
    Light light;
    int prevLevel;

    public DimmerLightOffCommand(Light light) {
        this.light = light;
        prevLevel = 100;
    }

    public void execute() {
        prevLevel = light.getLevel();
        light.off();
    }

    public void undo() {
        light.dim(prevLevel);
    }
}

package net.dp.command.undo;

public class LightOnCommand implements Command {
    Light light;

    public LightOnCommand(Light light) {
        this.light = light;
    }

    public void execute() {
        light.on();
    }

    public void undo() {
        light.off();
    }
}

package net.dp.command.undo;

public class LightOffCommand implements Command {
    Light light;

    public LightOffCommand(Light light) {
        this.light = light;
    }

    public void execute() {
        light.off();
    }

    public void undo() {
        light.on();
    }
}

下面是一个空对象(null object)。可以将处理null的责任转移给空对象。举例来说，遥控器不可能一出厂就设置了有意义的命令对象，所以提供了NoCommand对象作为代用品，当调用它的execute()时，它不会做任何事情。

package net.dp.command.undo;

public class NoCommand implements Command {
    public void execute() { }
    public void undo() { }
}

下面是Invoker

package net.dp.command.undo;


//
// This is the invoker
//
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;
    }

    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();
    }

    public String toString() {
        StringBuffer stringBuff = new StringBuffer();
        stringBuff.append("\n------ Remote Control -------\n");
        for (int i = 0; i < onCommands.length; i++) {
            stringBuff.append("[slot " + i + "] " + onCommands[i].getClass().getName()
                + "    " + offCommands[i].getClass().getName() + "\n");
        }
        stringBuff.append("[undo] " + undoCommand.getClass().getName() + "\n");
        return stringBuff.toString();
    }
}

调用关系

package net.dp.command.undo;

public class RemoteLoader {

    public static void main(String[] args) {
        RemoteControlWithUndo remoteControl = new RemoteControlWithUndo();

        Light livingRoomLight = new Light("Living Room");

        LightOnCommand livingRoomLightOn =
                new LightOnCommand(livingRoomLight);
        LightOffCommand livingRoomLightOff =
                new LightOffCommand(livingRoomLight);

        remoteControl.setCommand(0, livingRoomLightOn, livingRoomLightOff);

        remoteControl.onButtonWasPushed(0);
        remoteControl.offButtonWasPushed(0);
        System.out.println(remoteControl);
        remoteControl.undoButtonWasPushed();
        remoteControl.offButtonWasPushed(0);
        remoteControl.onButtonWasPushed(0);
        System.out.println(remoteControl);
        remoteControl.undoButtonWasPushed();

        CeilingFan ceilingFan = new CeilingFan("Living Room");

        CeilingFanMediumCommand ceilingFanMedium =
                new CeilingFanMediumCommand(ceilingFan);
        CeilingFanHighCommand ceilingFanHigh =
                new CeilingFanHighCommand(ceilingFan);
        CeilingFanOffCommand ceilingFanOff =
                new CeilingFanOffCommand(ceilingFan);

        remoteControl.setCommand(0, ceilingFanMedium, ceilingFanOff);
        remoteControl.setCommand(1, ceilingFanHigh, ceilingFanOff);

        remoteControl.onButtonWasPushed(0);
        remoteControl.offButtonWasPushed(0);
        System.out.println(remoteControl);
        remoteControl.undoButtonWasPushed();

        remoteControl.onButtonWasPushed(1);
        System.out.println(remoteControl);
        remoteControl.undoButtonWasPushed();
    }
}

设置宏命令，可以将部分命令组合在一起依次连续触发或关闭

package net.dp.command.party;

public class MacroCommand implements Command {
    Command[] commands;

    public MacroCommand(Command[] commands) {
        this.commands = commands;
    }

    public void execute() {
        for (int i = 0; i < commands.length; i++) {
            commands[i].execute();
        }
    }

    public void undo() {
        for (int i = 0; i < commands.length; i++) {
            commands[i].undo();
        }
    }
}

调用关系

package net.dp.command.party;

public class RemoteLoader {

    public static void main(String[] args) {

        RemoteControl remoteControl = new RemoteControl();

        Light light = new Light("Living Room");
        TV tv = new TV("Living Room");
        Stereo stereo = new Stereo("Living Room");
        Hottub hottub = new Hottub();

        LightOnCommand lightOn = new LightOnCommand(light);
        StereoOnCommand stereoOn = new StereoOnCommand(stereo);
        TVOnCommand tvOn = new TVOnCommand(tv);
        HottubOnCommand hottubOn = new HottubOnCommand(hottub);
        LightOffCommand lightOff = new LightOffCommand(light);
        StereoOffCommand stereoOff = new StereoOffCommand(stereo);
        TVOffCommand tvOff = new TVOffCommand(tv);
        HottubOffCommand hottubOff = new HottubOffCommand(hottub);

        Command[] partyOn = { lightOn, stereoOn, tvOn, hottubOn};
        Command[] partyOff = { lightOff, stereoOff, tvOff, hottubOff};

        MacroCommand partyOnMacro = new MacroCommand(partyOn);
        MacroCommand partyOffMacro = new MacroCommand(partyOff);

        remoteControl.setCommand(0, partyOnMacro, partyOffMacro);

        System.out.println(remoteControl);
        System.out.println("--- Pushing Macro On---");
        remoteControl.onButtonWasPushed(0);
        System.out.println("--- Pushing Macro Off---");
        remoteControl.offButtonWasPushed(0);
    }
}

《Head First Design Pattern》源代码下载 http://zhangjunhd.blog.51cto.com/113473/69908

本文出自 “子 孑” 博客，请务必保留此出处http://zhangjunhd.blog.51cto.com/113473/409996