设计模式之观察者模式

情景：

气象站会实时发布气象数据，要求创建布告板，并把气象站发布的数据显示出来。

布告板会有很多，随时回添加一个或删除一个，而每个布告板显示的格式也不尽相同。

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

被观察对象称作主题（Subject），依赖主题的对象称作观察者（Observer）。

当两个对象之间松耦合，它们依然可以交互，但是不太清除彼此之间的细节。

观察者模式提供了一种对象设计，让主题和观察者之间松耦合。

类图： 

设计原则：为了交互对象之间的松耦合设计而努力。

气象站的实现：

主题接口：

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

观察者接口：

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

主题（气象站）实现：

import java.util.ArrayList;

public class WeatherData implements Subject {
    
    private ArrayList<Observer> observers;
    private float temperature;
    private float humidity;
    private float pressure;
    
    public WeatherData() {
        observers = new ArrayList<>();
    }

    @Override
    public void registerObserver(Observer o) {
        observers.add(o);
    }

    @Override
    public void removeObserver(Observer o) {
        int i = observers.indexOf(o);
        if (i >= 0) {
            observers.remove(i);
        }
    }

    @Override
    public void notifyObservers() {
        for (int i = 0; i < observers.size(); i++) {
            Observer observer = observers.get(i);
            observer.update(temperature, humidity, pressure);
        }
    }
    
    public void measurementsChanged() {
        notifyObservers();
    }
    
    public void setMeasurements(float temperature, float humidity, float pressure) {
        this.temperature = temperature;
        this.humidity = humidity;
        this.pressure = pressure;
        measurementsChanged();
    }
    
}

观察者（布告板）实现：

public interface DisplayElement {
    public void display();
}
……………………

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);
    }

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

    @Override
    public void update(float temperature, float humidity, float pressure) {
        this.temperature = temperature;
        this.humidity = humidity;
        display();
    }

}
……………………
public class ChineseDisplay implements Observer, DisplayElement {
    private float temperature;
    private float humidity;
    private float pressure;
    private Subject weatherData;
    
    public ChineseDisplay(Subject data) {
        this.weatherData = data;
        weatherData.registerObserver(this);
    }

    @Override
    public void display() {
        System.out.println("天气状况: 气温：" + temperature + 
                "F， 湿度：" + humidity + "%， 气压:" + pressure + " Pa。");
    }

    @Override
    public void update(float temperature, float humidity, float pressure) {
        this.temperature = temperature;
        this.humidity = humidity;
        this.pressure = pressure;
        display();
    }
}

测试类：

public class WeatherStation {
    
    public static void main(String[] args) {
        WeatherData weatherData = new WeatherData();
        CurrentConditionsDisplay conditionsDisplay = new CurrentConditionsDisplay(weatherData);
        StatisticsDisplay statisticsDisplay = new StatisticsDisplay(weatherData);
        ChineseDisplay chineseDisplay = new ChineseDisplay(weatherData);
        
        weatherData.setMeasurements(80, 65, 30.4f);
        System.out.println();
        weatherData.setMeasurements(82, 70, 29.2f);
        System.out.println();
        weatherData.setMeasurements(78, 90, 29.2f);
    }
    
}

输出：

Current conditions: 80.0F degrees and 65.0% humdity.
天气状况: 气温：80.0F， 湿度：65.0%， 气压:30.4 Pa。

Current conditions: 82.0F degrees and 70.0% humdity.
天气状况: 气温：82.0F， 湿度：70.0%， 气压:29.2 Pa。

Java中自带Observable类和Observer类，可以通过继承这两个类来实现观察者模式

主题：

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;
    }
    
}

观察Observable源码setChanged方法

protected synchronized void setChanged() {
        changed = true;
}

有时候改变需要自己定义，比如温度改变0.5℃以上才算改变等……

notifyObservers方法的实现：

public void notifyObservers(Object arg) {
        Object[] arrLocal;

        synchronized (this) {
            if (!changed)
                return;
            arrLocal = obs.toArray();
            clearChanged();
        }

        for (int i = arrLocal.length-1; i>=0; i--)
            ((Observer)arrLocal[i]).update(this, arg);
    }

布告板的实现：

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

public class CurrentConditionsDisplay implements Observer, DisplayElement {
    
    private Observable observable;
    private float temperature;
    private float humidity;
    
    public CurrentConditionsDisplay(Observable observable) {
        this.observable = observable;
        observable.addObserver(this);
    }

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

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

}

这里的update()方法和上面的实现不同，上面的实现是push的方法，也就是主题把数据直接送到观察者那里。而这里的是观察者通过getter()方法来自己pull所需要的数据。

但是java.util.Observable和java.util.Observer都是类而不是接口，所以要继承它们就无法继承别的类了。

JDK中的观察者模式

Swing中的JButton就用到了观察者模式。

import java.awt.BorderLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.JButton;
import javax.swing.JFrame;

public class SwingObserverExample {
    JFrame frame;
    
    public static void main(String[] args) {
        SwingObserverExample example = new SwingObserverExample();
        example.go();
    }
    
    public void go() {
        frame = new JFrame();
        
        JButton button = new JButton("Should I do it?");
        button.addActionListener(new AngelListener());
        button.addActionListener(new DevilListener());
        frame.getContentPane().add(BorderLayout.CENTER, button);
        frame.setSize(200, 200);
        frame.setVisible(true);
    }
    // 一个ActionListener就是一个观察者
    class AngelListener implements ActionListener {
        @Override
        public void actionPerformed(ActionEvent e) {
            System.out.println("Dont do it, you might regret it!");
        }
    }
    
    
    class DevilListener implements ActionListener {
        @Override
        public void actionPerformed(ActionEvent e) {
            System.out.println("Come on, do it!");
        }
    }
}