设计模式（7）命令模式

在软件设计中，我们经常需要向某些对象发送请求，但是并不知道请求的接收者是谁，也不知道被请求的操作是哪个，我们只需在程序运行时指定具体的请求接收者即可，此时，可以使用命令模式来进行设计，使得请求发送者与请求接收者消除彼此之间的耦合，让对象之间的调用关系更加灵活。

假设现在我们需要制作一个万能遥控器，去控制日光灯、音响和电视的开关，我们使用遥控器进行开关操作的时候，并不关心遥控器具体请求是哪个，我们只知道要进行开关操作，这时候就可以用命令模式来进行设计。

首先我们先定义命令接口，使所有的命令都继承这个接口，如开灯命令、关灯命令等。

public interface Command {
    // 命令
    public void excute();

}

然后我们定义一个遥控器，它有着开关的功能，也就是两个命令，开启命令和关闭命令。

public class RemoteControl {

    private Command onCommand;

    private Command offCommand;

    public RemoteControl() {
        this.onCommand = new NoCommd();
        this.offCommand = new NoCommd();
    }

    public void setCommand(Command onCommand, Command offCommand) {
        this.onCommand = onCommand;
        this.offCommand = offCommand;
    }

    public void onButtonWasPushed() {
        this.onCommand.excute();
    }

    public void offButtonWasPushed() {
        this.offCommand.excute();
    }
}

接下来是音响、日光灯、电视机这三个实体类

public class Audio {
    public void on() {
        System.out.println("Audio is on!");
    }

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

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

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

public class Television {
    public void on() {
        System.out.println("Television is on!");
    }

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

然后是实现各个电器开关命令的命令类

public class AudioOffCommand implements Command {

    private Audio audio;

    public AudioOffCommand(Audio audio) {
        this.audio = audio;
    }

    @Override
    public void excute() {
        audio.off();
    }
}

public class AudioOnCommand implements Command {

    private Audio audio;

    public AudioOnCommand(Audio audio) {
        this.audio = audio;
    }

    @Override
    public void excute() {
        audio.on();
    }
}

public class LightOffCommand implements Command {

    private Light light;

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

    @Override
    public void excute() {
        light.off();
    }
}

public class LightOnCommand implements Command {

    private Light light;

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

    @Override
    public void excute() {
        light.on();
    }
}

public class TelevisionOffCommand implements Command {

    private Television television;

    public TelevisionOffCommand(Television television) {
        this.television = television;
    }

    @Override
    public void excute() {
        television.off();
    }
}

public class TelevisionOnCommand implements Command {

    private Television television;

    public TelevisionOnCommand(Television television) {
        this.television = television;
    }

    @Override
    public void excute() {
        television.on();
    }
}

测试

public static void main(String[] args) {
        RemoteControl remoteControl = new RemoteControl();
        Light light = new Light();
        LightOnCommand lightOnCommand = new LightOnCommand(light);
        LightOffCommand lightOffCommand = new LightOffCommand(light);
        System.out.println("使用万能遥控器控制日光灯");
        remoteControl.setCommand(lightOnCommand, lightOffCommand);
        remoteControl.onButtonWasPushed();
        remoteControl.offButtonWasPushed();

        Audio audio = new Audio();
        AudioOnCommand audioOnCommand = new AudioOnCommand(audio);
        AudioOffCommand audioOffCommand = new AudioOffCommand(audio);
        System.out.println("使用万能遥控器控制音响");
        remoteControl.setCommand(audioOnCommand, audioOffCommand);
        remoteControl.onButtonWasPushed();
        remoteControl.offButtonWasPushed();

        Television television = new Television();
        TelevisionOnCommand televisionOnCommand = new TelevisionOnCommand(television);
        TelevisionOffCommand televisionOffCommand = new TelevisionOffCommand(television);
        System.out.println("使用万能遥控器控制电视");
        remoteControl.setCommand(televisionOnCommand, televisionOffCommand);
        remoteControl.onButtonWasPushed();
        remoteControl.offButtonWasPushed();
    }

结果

这样就用命令模式实现了万能遥控器的功能。

命令模式将发出请求的万能遥控器和执行请求的对象解耦了，降低了耦合度。新的命令可以很容易地加入到其中，如果需要控制电风扇，只需要创建开启和关闭电风扇的命令，然后绑定到万能遥控器中即可。

但是每个命令都需要创建一个命令对象，这就导致了具体命令类的数量会很多，使用的时候需要谨慎。