外观模式

• 定义：
  Facade（外观）模式为子系统中的一组接口提供一个一致的界面，定义一个高层接口，这个接口使得这一子系统更加容易使用。

• UML:

  
  image.png
• 模型：
  家庭影院，包含DVD播放器，投影仪，自动屏幕，环绕立体声，爆米花机等，这些社保都有开关设置功能。最后我们用遥控器控制各个社保的开关，实现播放家庭影院：

实现播放家庭影院步骤：
1开爆米花机
2.放下屏幕
3.开投影仪
4.开音响
5.开DVD，选dvd
6.去拿爆米花
7.调节灯光
总体来看我们想播放家庭影院需要很多的复杂的步骤，并且这些步骤都是固定的。而对于用户并不关心这些步骤的操作过程，只要最后有爆米花，有的dvd看就行。外观模式就是做这种事情的。我们通过外观的包装，使用户只能看到外观对象，而不会看到具体的细节对象，这样无疑会降低应用程序的复杂度，并且提高了程序的可维护性。

image.png

public class DVDPlayer {
    private static DVDPlayer instance = null;
    private DVDPlayer() {}
    public static DVDPlayer getInstance() {
        if (instance == null) {
            instance = new DVDPlayer();
        }
        return instance;
    }
    public void on() {
        System.out.println("DVDPlayer On");
    }
    public void off() {
        System.out.println("DVDPlayer Off");
    }
    public void play() {
        System.out.println("DVDPlayer is playing");
    }   
    public void pause() {
        System.out.println("DVDPlayer  pause");
    }
    public void setdvd() {
        System.out.println("DVDPlayer  is setting dvd");
    }
}
//爆米花机
public class Popcorn {
    private static Popcorn instance = null;
    private Popcorn() {}
    public static Popcorn getInstance() {
        if (instance == null) {
            instance = new Popcorn();
        }
        return instance;
    }
    public void on() {
        System.out.println("Popcorn On");
    }
    public void off() {
        System.out.println("Popcorn Off");
    }
    public void pop() {
        System.out.println("Popcorn is popping");
    }   
}
//环绕立体声
public class Stereo {
    private static Stereo instance = null;
    private int volume = 5;
    private Stereo() {}
    public static Stereo getInstance() {
        if (instance == null) {
            instance = new Stereo();
        }
        return instance;
    }
    public void on() {
        System.out.println("Stereo On");
    }
    public void off() {
        System.out.println("Stereo Off");
    }
    public void setVolume(int vol) {
        volume = vol;
        System.out.println("the volume of Stereo is set to  " + volume);
    }
    public void addVolume() {
        if (volume < 11) {
            volume++;
            setVolume(volume);
        }
    }
    public void subVolume() {
        if (volume > 0) {
            volume--;
            setVolume(volume);
        }
    }
}
//投影机
public class Projector {
    private int size=5;
    private static Projector instance = null;
    private Projector() {}
    public static Projector getInstance() {
        if (instance == null) {
            instance = new Projector();
        }
        return instance;
    }
    public void on() {
        System.out.println("Projector On");
    }
    public void off() {
        System.out.println("Projector Off");
    }
    public void focus() {
        System.out.println("Popcorn is focus");
    }
    public void zoom(int size) {
        this.size=size;
        System.out.println("Popcorn zoom to"+size);
    }
}
//屏幕
public class Screen {
    private static Screen instance = null;
    private Screen() {}
    public static Screen getInstance() {
        if (instance == null) {
            instance = new Screen();
        }
        return instance;
    }
    public void up() {
        System.out.println("Screen  up");
    }
    public void down() {
        System.out.println("Screen  down");
    }   
}
//灯
public class TheaterLights {
    private static TheaterLights instance = null;
    private TheaterLights() {   }
    public static TheaterLights getInstance() {
        if (instance == null) {
            instance = new TheaterLights();
        }
        return instance;
    }
    public void on() {
        System.out.println("TheaterLights On");
    }
    public void off() {
        System.out.println("TheaterLights Off");
    }
     //调节亮度
    public void dim(int d) {
        System.out.println("TheaterLights dim to " + d + "%");
    }
    public void bright() {
        dim(100);
        System.out.println("TheaterLights bright");
    }
}
//包装的外观类
public class HomeTheaterFacade {
    private TheaterLights mTheaterLights;
    private Popcorn mPopcorn;
    private Stereo mStereo;
    private Projector mProjector;
    private Screen mScreen;
    private DVDPlayer mDVDPlayer;
        //注入各个设备对象
    public HomeTheaterFacade() {
        mTheaterLights = TheaterLights.getInstance();
        mPopcorn = Popcorn.getInstance();
        mStereo = Stereo.getInstance();
        mProjector = Projector.getInstance();
        mScreen = Screen.getInstance();
        mDVDPlayer = DVDPlayer.getInstance();
    }
        //固定的准备流程
    public void ready() {
        mPopcorn.on();
        mPopcorn.pop();
        mScreen.down();
        mProjector.on();
        mStereo.on();
        mDVDPlayer.on();
        mDVDPlayer.setdvd();
        mTheaterLights.dim(10);
    }
         //固定的关闭流程
    public void end() {
        mPopcorn.off();
        mTheaterLights.bright();
        mScreen.up();
        mProjector.off();
        mStereo.off();
        mDVDPlayer.setdvd();
        mDVDPlayer.off();
    }
    public void play() {
        mDVDPlayer.play();
    }
    public void pause() {
        mDVDPlayer.pause();
    }
}

使用包装的外观对象操作家庭影院：

public class MainTest {
    public static void main(String[] args) {
        HomeTheaterFacade mHomeTheaterFacade=new HomeTheaterFacade();
        mHomeTheaterFacade.ready();
        mHomeTheaterFacade.play();
    }
}

通过这样的包装，我们可以把外观对象做成遥控器，这样我们就能通过遥控器来实现家庭影院的控制。