学习笔记之观察者模式

观察者（observer）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，它的所有依赖者都会收到通知并自动更新。

observer模式中有两个关键要素，一是Observer，一是Subject，
Subject就是被观察者，它一般是事件的来源，
Observer是观察者，它一般是事件发生的时候应该进行相应的操作的对象。

package org.kangsg219.intf;

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

package org.kangsg219.intf;

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

package org.kangsg219.intf;

public interface DisplayElement {
	public void display();

}

package org.kangsg219.impl;

import java.util.ArrayList;
import java.util.List;

import org.kangsg219.intf.Observer;
import org.kangsg219.intf.Subject;

public class WeatherData implements Subject {

	private List<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(int i=0;i<observers.size();i++){
			Observer tempo=observers.get(i);
			tempo.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();
	}

}

package org.kangsg219.impl;

import org.kangsg219.intf.DisplayElement;
import org.kangsg219.intf.Observer;
import org.kangsg219.intf.Subject;

public class CurrentConditionDisplay implements Observer,DisplayElement {

	private float temperature;

	private float humidity;

	private float pressure;
	
	@SuppressWarnings("unused")
	private Subject weatherData;
	
	public CurrentConditionDisplay(Subject weatherData){
		this.weatherData=weatherData;
		weatherData.registerObserver(this);
	}
	
	public void update(float temperature, float humidity, float pressure) {
		this.temperature=temperature;
		this.humidity=humidity;
		this.pressure=pressure;
		display();

	}

	public void display() {
		System.out.println("CurrentConditionDisplay:");
		System.out.println("temperature:"+temperature+" "+"humidity:"+humidity+" "+"pressure:"+pressure);
		System.out.println("=============================================");
	}

}

package org.kangsg219.impl;

import org.kangsg219.intf.DisplayElement;
import org.kangsg219.intf.Observer;
import org.kangsg219.intf.Subject;

public class CurrentConditionDisplay implements Observer,DisplayElement {

	private float temperature;

	private float humidity;

	private float pressure;
	
	@SuppressWarnings("unused")
	private Subject weatherData;
	
	public CurrentConditionDisplay(Subject weatherData){
		this.weatherData=weatherData;
		weatherData.registerObserver(this);
	}
	
	public void update(float temperature, float humidity, float pressure) {
		this.temperature=temperature;
		this.humidity=humidity;
		this.pressure=pressure;
		display();

	}

	public void display() {
		System.out.println("CurrentConditionDisplay:");
		System.out.println("temperature:"+temperature+" "+"humidity:"+humidity+" "+"pressure:"+pressure);
		System.out.println("=============================================");
	}

}

package org.kangsg219.impl;

import org.kangsg219.intf.DisplayElement;
import org.kangsg219.intf.Observer;
import org.kangsg219.intf.Subject;

public class StatisticsDisplay implements Observer, DisplayElement {
	
	private float temperature;

	private float humidity;

	private float pressure;
	
	@SuppressWarnings("unused")
	private Subject weatherData;
	
	public StatisticsDisplay(Subject weatherData){
		this.weatherData=weatherData;
		weatherData.registerObserver(this);
	}
	
	public void update(float temperature, float humidity, float pressure) {
		this.temperature=temperature;
		this.humidity=humidity;
		this.pressure=pressure;
		display();

	}

	public void display() {
		System.out.println("StatisticsDisplay:");
		System.out.println("temperature:"+temperature+" "+"humidity:"+humidity+" "+"pressure:"+pressure);
		System.out.println("=============================================");
	}

}

package org.kangsg219.test;

import org.kangsg219.impl.CurrentConditionDisplay;
import org.kangsg219.impl.ForecastDisplay;
import org.kangsg219.impl.StatisticsDisplay;
import org.kangsg219.impl.WeatherData;

public class WeatherStation {

	public static void main(String[] args) {
		WeatherData weatherData = new WeatherData();
		CurrentConditionDisplay ccd=new CurrentConditionDisplay(weatherData);
		new StatisticsDisplay(weatherData);
		new ForecastDisplay(weatherData);
		weatherData.setMeasurements(33, 30, 30.4F);
		weatherData.removeObserver(ccd);
		weatherData.setMeasurements(34, 35, 30.4F);
	}

}

java API中内置了观察者模式

package org.kangsg219.intf;

import java.util.Observer;

public interface IObserver extends Observer {
	public void display();
}

package org.kangsg219.impl;

import java.util.Observable;

public class WeatherData2 extends Observable {
	
	
	private float temperature;

	private float humidity;

	private float pressure;
	
	public void measurementsChanged(){
		setChanged();
System.out.println("before notifyObservers,hasChanged="+hasChanged());		
		notifyObservers();
System.out.println("after notifyObservers,hasChanged="+hasChanged());		
	}
	
	public void setMeasurements(float temperature,float humidity,float pressure){
		this.temperature=temperature;
		this.humidity=humidity;
		this.pressure=pressure;
		measurementsChanged();
	}

	public float getHumidity() {
		return humidity;
	}

	public float getPressure() {
		return pressure;
	}

	public float getTemperature() {
		return temperature;
	}	
}

package org.kangsg219.impl;

import java.util.Observable;

import org.kangsg219.intf.IObserver;

public class WeatherDisplay implements IObserver {

	private float temperature;

	private float humidity;

	private float pressure;
	
	@SuppressWarnings("unused")
	private Observable observerable;
	
	public WeatherDisplay(Observable observerable){
		this.observerable=observerable;
		observerable.addObserver(this);
	}
	
	public void update(Observable o, Object arg) {
		if(o instanceof WeatherData2){
			WeatherData2 weatherdata2=(WeatherData2)o;
			this.humidity=weatherdata2.getHumidity();
			this.temperature=weatherdata2.getTemperature();
			this.pressure=weatherdata2.getPressure();
			display();
		}
		
	}

	public void display(){
		System.out.println("WeatherDisplay:");
		System.out.println("temperature:"+temperature+" "+"humidity:"+humidity+" "+"pressure:"+pressure);
		System.out.println("=============================================");
	}
}

package org.kangsg219.test;

import org.kangsg219.impl.WeatherData2;
import org.kangsg219.impl.WeatherDisplay;

public class WeatherStation2 {

	
	public static void main(String[] args) {
		WeatherData2 weatherdata2=new WeatherData2();
		WeatherDisplay wdisplay=new WeatherDisplay(weatherdata2);
		new WeatherDisplay(weatherdata2);
		weatherdata2.setMeasurements(30, 30, 30F);
		weatherdata2.deleteObserver(wdisplay);
		weatherdata2.setMeasurements(33, 33, 33.3F);
	}

}

java api 中内置的观察者模式是怎么实现的我们并不清楚，它对结果的输出顺序我们也无法把握，有必要的话建议自己实现这个模式！

评论

1 楼 kangsg219 2007-10-19  

代码参考《Head First 设计模式》