外观(Facade)模式

May 1, 2016

简介

外观模式应用场景:

外观模式提供了一个统一的接口, 用来访问子系统中的一群接口. 外观定义了一个高层接口, 让子系统更容易使用

外观模式的使用模板如下:

design-pattern-facade-template

应用例子

我是一个电影迷, 所以我要建立一个属于自己的家庭影院. 通过一番研究比较, 我组装了一套杀手级的系统, 内含DVD播放器, 投影机, 自动屏幕, 环绕立体声, 甚至还有爆米花机. 这些组件的组成如下:

design-pattern-adapter-amplifier

场景一

花了好几个星期布线, 挂上投影机, 连接所有的装置并进行微调. 现在躺下来正准备享受一部电影……

愉快地挑选了一部DVD影片, 放松, 准备开始感受电影的魔幻魅力. OMG, 忘了一件事: 想看电影必须先执行一些任务.

  • 打开爆米花机
  • 开始爆米花
  • 将灯光调暗
  • 放下屏幕
  • 打开投影机
  • 将投影机的输入切换到DVD
  • 将投影机设置宽屏模式
  • 打开功放
  • 将功放的输入切换到DVD
  • 将功放设置到环绕立体声
  • 将功放音量调到中
  • 打开DVD播放器
  • 开始播放DVD

想到这里, 心累, 但是还不止如此.

  • 看完电影还需要把这一切都关掉
  • 以后想要听CD或者广播, 也是这么麻烦

怎么办? 使用家庭影院竞变得如此复杂! 接下来看看外观模式是如何简化我们的需求的:

design-pattern-adapter-amplifier1

现在, 我就可以直接调用watchMovie()来观看电影, 灯光, DVD播放器, 投影机, 功放, 屏幕, 爆米花, 一口气全部搞定.

外观模式除了简化接口之外,, 还将客户从组件的子系统中解耦.

例如我中了彩票, 想要更新我的家庭影院, 很多设备的接口都改变了, 那么唯一需要改变的只是外观代码. 我依旧直接使用watchMovie()观看电影, 也就是客户代码不需要改变.

代码设计

  • 功放Amplifier

      public class Amplifier {
      String description;
      Tuner tuner;
      DvdPlayer dvd;
      CdPlayer cd;

      public Amplifier(String description) {
          this.description = description;
      }

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

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

      public void setStereoSound() {
          System.out.println(description + " stereo mode on");
      }

      public void setSurroundSound() {
          System.out.println(description + " surround sound on (5 speakers, 1 subwoofer)");
      }

      public void setVolume(int level) {
          System.out.println(description + " setting volume to " + level);
      }

      public void setTuner(Tuner tuner) {
          System.out.println(description + " setting tuner to " + dvd);
          this.tuner = tuner;
      }

      public void setDvd(DvdPlayer dvd) {
          System.out.println(description + " setting DVD player to " + dvd);
          this.dvd = dvd;
      }

      public void setCd(CdPlayer cd) {
          System.out.println(description + " setting CD player to " + cd);
          this.cd = cd;
      }

      public String toString() {
          return description;
      }
  }

  • CD播放器CdPlayer

      public class CdPlayer {
      String description;
      int currentTrack;
      Amplifier amplifier;
      String title;

      public CdPlayer(String description, Amplifier amplifier) {
          this.description = description;
          this.amplifier = amplifier;
      }

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

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

      public void eject() {
          title = null;
          System.out.println(description + " eject");
      }

      public void play(String title) {
          this.title = title;
          currentTrack = 0;
          System.out.println(description + " playing \"" + title + "\"");
      }

      public void play(int track) {
          if (title == null) {
              System.out.println(description + " can't play track " + currentTrack + 
                      ", no cd inserted");
          } else {
              currentTrack = track;
              System.out.println(description + " playing track " + currentTrack);
          }
      }

      public void stop() {
          currentTrack = 0;
          System.out.println(description + " stopped");
      }

      public void pause() {
          System.out.println(description + " paused \"" + title + "\"");
      }

      public String toString() {
          return description;
      }
  }

  • DVD播放器DvdPlayer

      public class DvdPlayer {
      String description;
      int currentTrack;
      Amplifier amplifier;
      String movie;

      public DvdPlayer(String description, Amplifier amplifier) {
          this.description = description;
          this.amplifier = amplifier;
      }

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

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

          public void eject() {
          movie = null;
                  System.out.println(description + " eject");
          }

      public void play(String movie) {
          this.movie = movie;
          currentTrack = 0;
          System.out.println(description + " playing \"" + movie + "\"");
      }

      public void play(int track) {
          if (movie == null) {
              System.out.println(description + " can't play track " + track + " no dvd inserted");
          } else {
              currentTrack = track;
              System.out.println(description + " playing track " + currentTrack + " of \"" + movie + "\"");
          }
      }

      public void stop() {
          currentTrack = 0;
          System.out.println(description + " stopped \"" + movie + "\"");
      }

      public void pause() {
          System.out.println(description + " paused \"" + movie + "\"");
      }

      public void setTwoChannelAudio() {
          System.out.println(description + " set two channel audio");
      }

      public void setSurroundAudio() {
          System.out.println(description + " set surround audio");
      }

      public String toString() {
          return description;
      }
  }

  • 灯光TheaterLight

      public class TheaterLights {
      String description;

      public TheaterLights(String description) {
          this.description = description;
      }

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

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

      public void dim(int level) {
          System.out.println(description + " dimming to " + level  + "%");
      }

      public String toString() {
          return description;
      }
  }

  • 爆米花机PopcornPopper

      public class PopcornPopper {
      String description;

      public PopcornPopper(String description) {
          this.description = description;
      }

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

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

      public void pop() {
          System.out.println(description + " popping popcorn!");
      }


          public String toString() {
                  return description;
          }
  }

  • 屏幕Screen

      public class Screen {
      String description;

      public Screen(String description) {
          this.description = description;
      }

      public void up() {
          System.out.println(description + " going up");
      }

      public void down() {
          System.out.println(description + " going down");
      }


      public String toString() {
          return description;
      }
  }

  • 投影仪Projector

      public class Projector {
      String description;
      DvdPlayer dvdPlayer;

      public Projector(String description, DvdPlayer dvdPlayer) {
          this.description = description;
          this.dvdPlayer = dvdPlayer;
      }

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

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

      public void wideScreenMode() {
          System.out.println(description + " in widescreen mode (16x9 aspect ratio)");
      }

      public void tvMode() {
          System.out.println(description + " in tv mode (4x3 aspect ratio)");
      }

          public String toString() {
                  return description;
          }
  }

  • 收音机Tuner

      public class Tuner {
      String description;
      Amplifier amplifier;
      double frequency;

      public Tuner(String description, Amplifier amplifier) {
          this.description = description;
      }

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

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

      public void setFrequency(double frequency) {
          System.out.println(description + " setting frequency to " + frequency);
          this.frequency = frequency;
      }

      public void setAm() {
          System.out.println(description + " setting AM mode");
      }

      public void setFm() {
          System.out.println(description + " setting FM mode");
      }

      public String toString() {
          return description;
      }
  }

  • 家庭影院外观模式

      public class HomeTheaterFacade {
      Amplifier amp;
      Tuner tuner;
      DvdPlayer dvd;
      CdPlayer cd;
      Projector projector;
      TheaterLights lights;
      Screen screen;
      PopcornPopper popper;

      public HomeTheaterFacade(Amplifier amp, 
                   Tuner tuner, 
                   DvdPlayer dvd, 
                   CdPlayer cd, 
                   Projector projector, 
                   Screen screen,
                   TheaterLights lights,
                   PopcornPopper popper) {

          this.amp = amp;
          this.tuner = tuner;
          this.dvd = dvd;
          this.cd = cd;
          this.projector = projector;
          this.screen = screen;
          this.lights = lights;
          this.popper = popper;
      }

      public void watchMovie(String movie) {
          System.out.println("Get ready to watch a movie...");
          popper.on();
          popper.pop();
          lights.dim(10);
          screen.down();
          projector.on();
          projector.wideScreenMode();
          amp.on();
          amp.setDvd(dvd);
          amp.setSurroundSound();
          amp.setVolume(5);
          dvd.on();
          dvd.play(movie);
      }


      public void endMovie() {
          System.out.println("Shutting movie theater down...");
          popper.off();
          lights.on();
          screen.up();
          projector.off();
          amp.off();
          dvd.stop();
          dvd.eject();
          dvd.off();
      }

      public void listenToCd(String cdTitle) {
          System.out.println("Get ready for an audiopile experence...");
          lights.on();
          amp.on();
          amp.setVolume(5);
          amp.setCd(cd);
          amp.setStereoSound();
          cd.on();
          cd.play(cdTitle);
      }

      public void endCd() {
          System.out.println("Shutting down CD...");
          amp.off();
          amp.setCd(cd);
          cd.eject();
          cd.off();
      }

      public void listenToRadio(double frequency) {
          System.out.println("Tuning in the airwaves...");
          tuner.on();
          tuner.setFrequency(frequency);
          amp.on();
          amp.setVolume(5);
          amp.setTuner(tuner);
      }

      public void endRadio() {
          System.out.println("Shutting down the tuner...");
          tuner.off();
          amp.off();
      }
  }

  • 测试代码

      public class HomeTheaterTestDrive {
      public static void main(String[] args) {
          Amplifier amp = new Amplifier("Top-O-Line Amplifier");
          Tuner tuner = new Tuner("Top-O-Line AM/FM Tuner", amp);
          DvdPlayer dvd = new DvdPlayer("Top-O-Line DVD Player", amp);
          CdPlayer cd = new CdPlayer("Top-O-Line CD Player", amp);
          Projector projector = new Projector("Top-O-Line Projector", dvd);
          TheaterLights lights = new TheaterLights("Theater Ceiling Lights");
          Screen screen = new Screen("Theater Screen");
          PopcornPopper popper = new PopcornPopper("Popcorn Popper");

          HomeTheaterFacade homeTheater = 
                  new HomeTheaterFacade(amp, tuner, dvd, cd, 
                          projector, screen, lights, popper);

          homeTheater.watchMovie("Raiders of the Lost Ark");
          homeTheater.endMovie();
      }
  }
  // 测试结果:
  // watchMovie()
  // Get ready to watch a movie...
  // Popcorn Popper on
  // Popcorn Popper popping popcorn!
  // Theater Ceiling Lights dimming to 10%
  // Theater Screen going down
  // Top-O-Line Projector on
  // Top-O-Line Projector in widescreen mode (16x9 aspect ratio)
  // Top-O-Line Amplifier on
  // Top-O-Line Amplifier setting DVD player to Top-O-Line DVD Player
  // Top-O-Line Amplifier surround sound on (5 speakers, 1 subwoofer)
  // Top-O-Line Amplifier setting volume to 5
  // Top-O-Line DVD Player on
  // Top-O-Line DVD Player playing "Raiders of the Lost Ark"

  // endMovie()
  // Shutting movie theater down...
  // Popcorn Popper off
  // Theater Ceiling Lights on
  // Theater Screen going up
  // Top-O-Line Projector off
  // Top-O-Line Amplifier off
  // Top-O-Line DVD Player stopped "Raiders of the Lost Ark"
  // Top-O-Line DVD Player eject
  // Top-O-Line DVD Player off
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