观察者模式（二）--《Head First DesignPattern》

我们用Java中自带的观察者模式接口来重写前面的例子。

先看一下类图：

这里用到了一个setChanged函数，它用来标记状态已经改变的事实，好让notifyObservers()知道当它调用时就应该更新观察者。如果调用notifyObservers()之前没有先调用setChanged()，观察者就不会被通知到。setChanged()方法可以让你在更新观察者时，有更大的弹性，你可以适当地通知观察者。

Observable内部是这样的结构：

setChanged(){
 changed = true;
}

notifyObservers(Object arg){
 if (changed){
 　　for every observer on the list{
  　　　　call update(this, arg)       
　　}
 }
}

notifyObservers(){
 notifyObservers(null);
}

注意这里是继承Observable类，而不是接口

 package headfirst.observer.weatherobservable;

 import java.util.Observable;
 import java.util.Observer;

 public class WeatherData extends Observable {
     private float temperature;
     private float humidity;
     private float pressure;

     public WeatherData() { }

     public void measurementsChanged() {
         //设置changed变量
         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;
     }
 }
     

观察者实现Observer接口，

 package headfirst.observer.weatherobservable;

 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);
     }

     public void update(Observable obs, Object arg) {
         if (obs instanceof WeatherData) {
             WeatherData weatherData = (WeatherData)obs;
             this.temperature = weatherData.getTemperature();
             this.humidity = weatherData.getHumidity();
             display();
         }
     }

     public void display() {
         System.out.println("Current conditions: " + temperature
             + "F degrees and " + humidity + "% humidity");
     }
 }

ForecastDisplay

 package headfirst.observer.weatherobservable;

 import java.util.Observable;
 import java.util.Observer;

 public class ForecastDisplay implements Observer, DisplayElement {
     private float currentPressure = 29.92f;
     private float lastPressure;

     public ForecastDisplay(Observable observable) {
         observable.addObserver(this);
     }

     public void update(Observable observable, Object arg) {
         if (observable instanceof WeatherData) {
             WeatherData weatherData = (WeatherData)observable;
             lastPressure = currentPressure;
             currentPressure = weatherData.getPressure();
             display();
         }
     }

     public void display() {
         System.out.print("Forecast: ");
         if (currentPressure > lastPressure) {
             System.out.println("Improving weather on the way!");
         } else if (currentPressure == lastPressure) {
             System.out.println("More of the same");
         } else if (currentPressure < lastPressure) {
             System.out.println("Watch out for cooler, rainy weather");
         }
     }
 }

 package headfirst.observer.weatherobservable;

 import java.util.Observable;
 import java.util.Observer;

 public class StatisticsDisplay implements Observer, DisplayElement {
     private float maxTemp = 0.0f;
     private float minTemp = 200;
     private float tempSum= 0.0f;
     private int numReadings;

     public StatisticsDisplay(Observable observable) {
         observable.addObserver(this);
     }

     public void update(Observable observable, Object arg) {
         if (observable instanceof WeatherData) {
             WeatherData weatherData = (WeatherData)observable;
             float temp = weatherData.getTemperature();
             tempSum += temp;
             numReadings++;

             if (temp > maxTemp) {
                 maxTemp = temp;
             }

             if (temp < minTemp) {
                 minTemp = temp;
             }

             display();
         }
     }

     public void display() {
         System.out.println("Avg/Max/Min temperature = " + (tempSum / numReadings)
             + "/" + maxTemp + "/" + minTemp);
     }
 }

main函数

 package headfirst.observer.weatherobservable;

 public class WeatherStation {

     public static void main(String[] args) {
         WeatherData weatherData = new WeatherData();
         CurrentConditionsDisplay currentConditions = new CurrentConditionsDisplay(weatherData);
         StatisticsDisplay statisticsDisplay = new StatisticsDisplay(weatherData);
         ForecastDisplay forecastDisplay = new ForecastDisplay(weatherData);

         weatherData.setMeasurements(80, 65, 30.4f);
         weatherData.setMeasurements(82, 70, 29.2f);
         weatherData.setMeasurements(78, 90, 29.2f);
     }
 }

这里还是要注意几点：

1. java.util.Observable是一个“类”而不是一个“接口”，所以我们的类必须继承它，这带来的问题就是Java中只有单继承，所以这限制了Observable的复用能力。而且Observable中的setChanged是被设置为protected的，所以除非你继承自Observable，否则你无法创建Observable实例并把它组合到我们的对象中来。如果实在不行，还是建立我们自己实现一套自己的接口，类似于第一篇所说的。
2. Java中的Swing的API就用到了观察者模式。例如我们有一个JButton对象，然后给他设置监听器。这里的监听器就是一个Observer，而JButton就是一个主题。当在JButton上有对应的事件发生的时候，例如点击，那么就会通知监听器，调用类似于update的方法，其实就是ActionListener中的actionPerformed()方法。