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。