设计模式之观察者模式

    某公司有一个气象站应用，有温度、湿度、气压感应装置，我们要做的是实现WeatherData对象，它可以从气象站获得数据并显示，显示方式有三种，对应三种布告板。

首先是一个错误示范：

public class BadWeatherData {//实例变量声明public void measurementsChanged(){float temp=getTemperature();float humidity=getHumidity();float pressure=getPressure();currentConditionsDisplay.update(temp,humidity,pressure);statisticsDisplay.update(temp,humidity,pressure);forecastDisplay.update(temp,humidity,pressure);}//其他方法}

update（temp,humidity,pressure）可以是一个统一的接口，而且针对具体实现编程，会导致我们以后再增加或删除布告板时必须修改程序。改变的地方，需要封装起来。

所以，here comes the observer pattern.观察者模式=出版者（subject）+订阅者（observer）。

观察者模式定义了对象之间的一对多依赖，这样一来，当一个对象改变状态时，他的所有依赖者都会收到通知并自动更新。

它提供了一种对象设计，让主题和观察者之间松耦合。

下面实现气象站：

public interface Subject {public void registerObserver(Observer o);public void removeObserver(Observer o);public void notifyObserver();}

public interface Observer {public void update(float temp,float humidity,float pressure);}

public interface DisplayElement {public void display();}

在WeatherData中实现subject接口：

import java.util.ArrayList;public class WeatherData implements Subject{private ArrayList observers;private float temperature;private float humidity;private float pressure;public WeatherData(){observers=new ArrayList<>();}@Overridepublic void registerObserver(Observer o) {// TODO Auto-generated method stubobservers.add(o);}@Overridepublic void removeObserver(Observer o) {// TODO Auto-generated method stubint i=observers.indexOf(o);observers.remove(i);}@Overridepublic void notifyObserver() {// TODO Auto-generated method stubfor(int i=0;i<observers.size();i++){Observer observer=(Observer)observers.get(i);observer.update(temperature, humidity, pressure);}}public void measurementsChanged(){notifyObserver();}public void setMeasurements(float temperature,float humidity,float pressure){this.temperature=temperature;this.humidity=humidity;this.pressure=pressure;measurementsChanged();}//其他方法}

建立布告板：

public class CurrentConditionsDisplay implements Observer,DisplayElement{private float temperature;private float humidity;private Subject weatherData;public CurrentConditionsDisplay(Subject weatherData){this.weatherData=weatherData;weatherData.registerObserver(this);}@Overridepublic void display() {// TODO Auto-generated method stubSystem.out.println("Current conditions:"+temperature+"F degrees and"+humidity+"% humidity");}@Overridepublic void update(float temp, float humidity, float pressure) {// TODO Auto-generated method stubthis.temperature=temp;this.humidity=humidity;display();}}

public class ForecastDisplay implements Observer,DisplayElement{private float temperature;private float humidity;private float pressure;private Subject weatherData;public ForecastDisplay(Subject weatherData){this.temperature=temperature;this.humidity=humidity;this.pressure=pressure;weatherData.registerObserver(this);}@Overridepublic void display() {// TODO Auto-generated method stubif(temperature>80&&humidity>65&&pressure<29.5f)System.out.println("Improving weather on the way");else if(temperature<70&&pressure>30f)System.out.println("Watch out for cooler,rainy weather");else System.out.println("More of the same");}@Overridepublic void update(float temp, float humidity, float pressure) {// TODO Auto-generated method stubthis.temperature=temp;this.humidity=humidity;this.pressure=pressure;display();}}

public class StatisticsDisplay implements Observer,DisplayElement{private float pressure;private Subject weatherData;public StatisticsDisplay(Subject weatherData){this.weatherData=weatherData;weatherData.registerObserver(this);}@Overridepublic void display() {// TODO Auto-generated method stubSystem.out.println("Pressure  is "+pressure);}@Overridepublic void update(float temp, float humidity, float pressure) {// TODO Auto-generated method stubthis.pressure=pressure;display();}}

启动气象站：

public class WeatherStation {public static void main(String[]args){WeatherData weatherData=new WeatherData();CurrentConditionsDisplay currentConditionsDisplay=new CurrentConditionsDisplay(weatherData);StatisticsDisplay statisticsDisplay=new StatisticsDisplay(weatherData);ForecastDisplay forecastDisplay=new ForecastDisplay(weatherData);weatherData.setMeasurements(90,70,29f);weatherData.setMeasurements(65, 75, 31f);weatherData.setMeasurements(70, 75, 30f);}}

运行程序：

Current conditions:90.0F degrees and70.0% humidityPressure  is 29.0Improving weather on the wayCurrent conditions:65.0F degrees and75.0% humidityPressure  is 31.0Watch out for cooler,rainy weatherCurrent conditions:70.0F degrees and75.0% humidityPressure  is 30.0More of the same

使用Java内置的观察者模式：

java.util包内包含最基本的Observer接口和Observable类。

和以上的实现类似，但有差异，WeatherData现在扩展自Observable类，并继承到一些增加删除通知Observer的方法。

重做气象站：

import java.util.Observable;public class WeatherData extends Observable{private float temperature;private float humidity;private float pressure;public WeatherData(){}public void measurementsChanged(){setChanged();notifyObservers();}public void setMeasurements(float temperature,float humidity,float pressure){this.temperature=temperature;this.humidity=humidity;this.pressure=pressure;measurementsChanged();}public float getTemperature(){return temperature;}public float getHumidity(){return humidity;}public float getPressure(){return pressure;}}

布告栏：

import java.util.Observable;import java.util.Observer;public class CurrentConditionsDisplay implements Observer,DisplayElement{Observable observable;private float temperature;private float humidity;public CurrentConditionsDisplay(Observable observable){this.observable=observable;observable.addObserver(this);}@Overridepublic void display() {// TODO Auto-generated method stubSystem.out.println("Current conditions:"+temperature+"F degrees and"+humidity+"% humidity");}@Overridepublic void update(Observable o, Object arg) {// TODO Auto-generated method stubif(o instanceof WeatherData){WeatherData weatherData=(WeatherData)o;this.temperature=weatherData.getTemperature();this.humidity=weatherData.getHumidity();display();}}}

etc

运行代码：

Improving weather on the wayPressure  is 29.0Current conditions:90.0F degrees and70.0% humidityWatch out for cooler,rainy weatherPressure  is 31.0Current conditions:65.0F degrees and75.0% humidityMore of the samePressure  is 30.0Current conditions:70.0F degrees and75.0% humidity

仔细看会发现输出的次序不一样了。如果我们依赖次序，这样就错了。

所以，java.util.Observable也有局限性，它是一个类不是接口，限制了它的使用和复用。

ps，其实JavaBeans和Swing中，也实现了观察者模式。