HeadFirst设计模式之观察者模式

一、什么是观察者模式

观察者模式定义了一系列对象间一对多的关系，当主题对象的状态发生变化时，会通知所有观察者

二、自定义观察模式

1.

2.

package headfirst.designpatterns.observer.weather;

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

3.

package headfirst.designpatterns.observer.weather;

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

4.

package headfirst.designpatterns.observer.weather;

import java.util.*;

public class WeatherData implements Subject {
    private ArrayList<Observer> observers;
    private float temperature;
    private float humidity;
    private float pressure;
    
    public WeatherData() {
        observers = new ArrayList<Observer>();
    }
    
    public void registerObserver(Observer o) {
        observers.add(o);
    }
    
    public void removeObserver(Observer o) {
        int i = observers.indexOf(o);
        if (i >= 0) {
            observers.remove(i);
        }
    }
    
    public void notifyObservers() {
        for (Observer observer : observers) {
            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();
    }

    public float getTemperature() {
        return temperature;
    }
    
    public float getHumidity() {
        return humidity;
    }
    
    public float getPressure() {
        return pressure;
    }

}

5.

package headfirst.designpatterns.observer.weather;

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

    public StatisticsDisplay(WeatherData weatherData) {
        this.weatherData = weatherData;
        weatherData.registerObserver(this);
    }

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

6.

package headfirst.designpatterns.observer.weather;

public interface DisplayElement {
    public void display();
}

7.

package headfirst.designpatterns.observer.weather;
    
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);
    }
    
    public void update(float temperature, float humidity, float pressure) {
        this.temperature = temperature;
        this.humidity = humidity;
        display();
    }
    
    public void display() {
        System.out.println("Current conditions: " + temperature 
            + "F degrees and " + humidity + "% humidity");
    }
}

8.

package headfirst.designpatterns.observer.weather;

public class HeatIndexDisplay implements Observer, DisplayElement {
    float heatIndex = 0.0f;
    private WeatherData weatherData;

    public HeatIndexDisplay(WeatherData weatherData) {
        this.weatherData = weatherData;
        weatherData.registerObserver(this);
    }

    public void update(float t, float rh, float pressure) {
        heatIndex = computeHeatIndex(t, rh);
        display();
    }
    
    private float computeHeatIndex(float t, float rh) {
        float index = (float)((16.923 + (0.185212 * t) + (5.37941 * rh) - (0.100254 * t * rh) 
            + (0.00941695 * (t * t)) + (0.00728898 * (rh * rh)) 
            + (0.000345372 * (t * t * rh)) - (0.000814971 * (t * rh * rh)) +
            (0.0000102102 * (t * t * rh * rh)) - (0.000038646 * (t * t * t)) + (0.0000291583 * 
            (rh * rh * rh)) + (0.00000142721 * (t * t * t * rh)) + 
            (0.000000197483 * (t * rh * rh * rh)) - (0.0000000218429 * (t * t * t * rh * rh)) +
            0.000000000843296 * (t * t * rh * rh * rh)) -
            (0.0000000000481975 * (t * t * t * rh * rh * rh)));
        return index;
    }

    public void display() {
        System.out.println("Heat index is " + heatIndex);
    }
}

9.

package headfirst.designpatterns.observer.weather;

public class WeatherStation {

    public static void main(String[] args) {
        WeatherData weatherData = new WeatherData();
    
        CurrentConditionsDisplay currentDisplay = 
            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);
    }
}

10.

package headfirst.designpatterns.observer.weather;

public class WeatherStationHeatIndex {

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

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

三、Java.util.Observable

1.

package headfirst.designpatterns.observer.weatherobservable;

public interface DisplayElement {
    public void display();
}

2.

package headfirst.designpatterns.observer.weatherobservable;
    
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;
    }
}

3.

package headfirst.designpatterns.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);
    }
}

4.

package headfirst.designpatterns.observer.weatherobservable;

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

public class HeatIndexDisplay implements Observer, DisplayElement {
    float heatIndex = 0.0f;

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

    public void update(Observable observable, Object arg) {
        if (observable instanceof WeatherData) {
            WeatherData weatherData = (WeatherData)observable;
            float t = weatherData.getTemperature();
            float rh = weatherData.getHumidity();
            heatIndex = (float)
                (
                (16.923 + (0.185212 * t)) + 
                (5.37941 * rh) - 
                (0.100254 * t * rh) + 
                (0.00941695 * (t * t)) + 
                (0.00728898 * (rh * rh)) + 
                (0.000345372 * (t * t * rh)) - 
                (0.000814971 * (t * rh * rh)) +
                (0.0000102102 * (t * t * rh * rh)) - 
                (0.000038646 * (t * t * t)) + 
                (0.0000291583 * (rh * rh * rh)) +
                (0.00000142721 * (t * t * t * rh)) + 
                (0.000000197483 * (t * rh * rh * rh)) - 
                (0.0000000218429 * (t * t * t * rh * rh)) +
                (0.000000000843296 * (t * t * rh * rh * rh)) -
                (0.0000000000481975 * (t * t * t * rh * rh * rh)));
            display();
        }
    }

    public void display() {
        System.out.println("Heat index is " + heatIndex);
    }
}

5.

package headfirst.designpatterns.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");
        }
    }
}

6.

package headfirst.designpatterns.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);
    }
}

四、Swing中的观察者模式

1.

package headfirst.observer.swing;
    
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import javax.swing.event.*;
    
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);

        // Set frame properties 
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.getContentPane().add(BorderLayout.CENTER, button);
        frame.setSize(300,300);
        frame.setVisible(true);
    }
    
    class AngelListener implements ActionListener {
        public void actionPerformed(ActionEvent event) {
            System.out.println("Don't do it, you might regret it!");
        }
    }

    class DevilListener implements ActionListener {
        public void actionPerformed(ActionEvent event) {
            System.out.println("Come on, do it!");
        }
    }
}