外观设计模式解读

1) 外观模式可以理解为转换一群接口，客户只要调用一个接口，而不用调用多个接口才能达到目的。比如：在 pc上安装软件的时候经常有一键安装选项（省去选择安装目录、安装的组件等等），还有就是手机的重启功能（把关机和启动合为一个操作）。
2) 外观模式就是解决多个复杂接口带来的使用困难，起到简化用户操作的作用

外观模式应用实例

1) 应用实例要求
2) 使用外观模式来完成家庭影院项目
3) 思路分析和图解(类图)

DVDPlayer

public class DVDPlayer {

    //使用单例模式, 使用饿汉式
    private static DVDPlayer instance = new DVDPlayer();

    public static DVDPlayer getInstanc() {
        return instance;
    }

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

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

    public void play() {
        System.out.println(" dvd is playing ");
    }

    public void pause() {
        System.out.println(" dvd pause ..");
    }
}

Popcorn

public class Popcorn {
    private static Popcorn instance = new Popcorn();
    public static Popcorn getInstance() {
        return instance;
    }
    public void on() {
        System.out.println(" popcorn on ");
    }
    public void off() {
        System.out.println(" popcorn ff ");
    }
    public void pop() {
        System.out.println(" popcorn is poping ");
    }

}

Projector

public class Projector {
    private static Projector instance = new Projector();
    public static Projector getInstance() {
        return instance;
    }
    public void on() {
        System.out.println(" Projector on ");
    }
    public void off() {
        System.out.println(" Projector ff ");
    }
    public void focus() {
        System.out.println(" Projector is Projector ");
    }

}

Stereo

public class Stereo {
    private static Stereo instance = new Stereo();
    public static Stereo getInstance() {
        return instance;
    }
    public void on() {
        System.out.println(" Stereo on ");
    }
    public void off() {
        System.out.println(" Screen off ");
    }
    public void up(){
        System.out.println(" Screen up.. ");
    }
}

TheaterLight

public class TheaterLight {
    private static TheaterLight instance = new TheaterLight();
    public static TheaterLight getInstance() {
        return instance;
    }
    public void on() {
        System.out.println(" TheaterLight on ");
    }
    public void off() {
        System.out.println(" TheaterLight off ");
    }
    public void dim() {
        System.out.println(" TheaterLight dim.. ");
    }
    public void bright() {
        System.out.println(" TheaterLight bright.. ");
    }

}

Screen

public class Screen {
    private static Screen instance = new Screen();
    public static Screen getInstance() {
        return instance;
    }
    public void up() {
        System.out.println(" Screen up ");
    }
    public void down() {
        System.out.println(" Screen down ");
    }
}

HomeTheaterFacade

public class HomeTheaterFacade {
    //定义各个子系统对象
    private TheaterLight theaterLight;
    private Popcorn popcorn;
    private Stereo stereo;
    private Projector projector;
    private Screen screen;
    private DVDPlayer dVDPlayer;
    //构造器
    public HomeTheaterFacade() {
        super();
        this.theaterLight = TheaterLight.getInstance();
        this.popcorn = Popcorn.getInstance();
        this.stereo = Stereo.getInstance();
        this.projector = Projector.getInstance();
        this.screen = Screen.getInstance();
        this.dVDPlayer = DVDPlayer.getInstanc();
    }
    //操作分成 4 步
    public void ready() {
        popcorn.on();
        popcorn.pop();
        screen.down();
        projector.on();
        stereo.on();
        dVDPlayer.on();
        theaterLight.dim();
    }
    public void play() {
        dVDPlayer.play();
    }
    public void pause() {
        dVDPlayer.pause();
    }
    public void end() {
        popcorn.off();
        theaterLight.bright();
        screen.up();
        projector.off();
        stereo.off();
        dVDPlayer.off();
    }
}

Client

public class Client {
    public static void main(String[] args) {

        HomeTheaterFacade homeTheaterFacade = new HomeTheaterFacade();
        homeTheaterFacade.ready();
        homeTheaterFacade.play();
        homeTheaterFacade.end();
    }
}

外观模式的注意事项和细节

1) 外观模式对外屏蔽了子系统的细节，因此外观模式降低了客户端对子系统使用的复杂性
2) 外观模式对客户端与子系统的耦合关系 - 解耦，让子系统内部的模块更易维护和扩展
3) 通过合理的使用外观模式，可以帮我们更好的划分访问的层次
4) 当系统需要进行分层设计时，可以考虑使用 Facade 模式
5) 在维护一个遗留的大型系统时，可能这个系统已经变得非常难以维护和扩展，此时可以考虑为新系统开发一个Facade 类，来提供遗留系统的比较清晰简单的接口，让新系统与 Facade 类交互，提高复用性
6) 不能过多的或者不合理的使用外观模式，使用外观模式好，还是直接调用模块好。要以让系统有层次，利于维护为目的