外观(Facade)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。 外观模式很容易理解,它允许我们让客户和子系统之间避免紧耦合。这里先给出它的类图
外观模式改变接口的原因是为了简化接口,之所以取这个名字,是因为它将一个或数个类的复杂的一切都隐藏在背后,只显露出一个干净美好的外观。
下面用一个实例来说明。我们模拟了一个家庭影院,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。我们需要将系统中的每一个组成部分写成一个类。这里以DVD播放器为例,编写一个类
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;
}
}
我们在开始观赏电影前需要逐一打开这些组件,这些任务将被写成很多类和方法的调用。在看完电影后,你还要把一切都关掉,如果反向地把这一切动作再进行一次,在代码编写上将非常复杂。 为此我们将采用外观模式,通过实现一个提供更合理的接口的外观类,我们可以将一个复杂的子系统变得容易使用。
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();
}
}
这样一来,我们在测试程序中编写观看电影的代码时,将十分轻松:
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");//简化的接口
homeTheater.endMovie();
}
}
设计原则
外观模式体现了又一个设计原则——最少知识原则:只和你的密友谈话。这个原则告诉我们要减少对象之间的交互,只留下几个“密友”,即在设计中,不要让太多的类耦合在一起。
这个原则为我们提供了一些方针:就任何对象而言,在该对象的方法内,我们只应该调用属于以下范围的方法:
- 该对象本身
- 被当作方法的参数而传递进来的对象
- 此方法所创建或实例化的任何对象
- 此对象的任何组件
值得一提的是,这一原则虽然减少了对象之间的依赖,减少了软件的维护成本;但是采用这个原则也会导致更多的“包装”类被制造出来,以处理和其他组件的沟通,这可能会导致复杂度和开发时间的增加,并降低运行时的性能。在开发过程中,我们需要根据实际情况进行取舍,在本文的例子中,很明显采用最少知识原则带来的软件维护的便利远大于其弊端。