观察者模式

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，他的所有依赖者都会收到通知并自动更新；

系统描述：设计一个气象站，每当温度变化时，显示温度的布告板就会更新；

主题接口：Subject.java

package cn.weather.demo;
/**
 * 主题接口
 * @author wy
 *
 */
public interface Subject {
    //注册观察者
    public void registerObserver(Observer o);
    //移除观察者
    public void removeObserver(Observer o);
    //通知观察者
    public void notifyObserver();
}

观察者接口：

package cn.weather.demo;

/**
 * 观察者接口
 * @author wy
 *
 */
public interface Observer {
	public void update (int temp,int humidity);	
}

布告板接口

package cn.weather.demo;

public interface DisplayData {
	public void display();
}

一个具体的主题，这是被观察的对象，在主题中有一个观察者的集合，

package cn.weather.demo;

import java.util.ArrayList;

public class WeatherData implements Subject {
	private ArrayList<Observer> observers;
	private int temp;
	private int humidity;

	public WeatherData() {
		observers = new ArrayList<Observer>();
	}

	@Override
	public void registerObserver(Observer o) {
		// TODO Auto-generated method stub
		observers.add(o);
	}

	@Override
	public void removeObserver(Observer o) {
		// TODO Auto-generated method stub
		observers.remove(o);
	}

	@Override
	public void notifyObserver() {
		// TODO Auto-generated method stub
		for (Observer observer : observers) {
			observer.update(temp, humidity);
		}
	}

	public void measurementChanged() {
		notifyObserver();
	}

	public void setMeasurements(int temp, int humidity) {
		this.temp = temp;
		this.humidity = humidity;
		measurementChanged();
	}

}

创建一个布告板，也就是观察者，其中包含了主题通知该观察者时所需要做的动作

package cn.weather.demo;

import java.awt.image.ImageConsumer;

public class FirstDisplayer implements Observer, DisplayData {

	private int temp;
	private int humidity;
	private Subject weather;

	public FirstDisplayer(Subject weather) {
		this.weather = weather;
		weather.registerObserver(this);
	}

	@Override
	public void update(int temp, int humidity) {
		this.temp = temp;
		this.humidity = humidity;
		display();
	}

	@Override
	public void display() {
		System.out.println("first displayer:");
		System.out.println("temp:" + temp + "; humidity: " + humidity + "; ");
		System.out.println();
	}
}

在创建2个观察者

package cn.weather.demo;

public class SecondDisplayer implements Observer, DisplayData {

	private int temp, humidity;
	private Subject subject;

	public SecondDisplayer(Subject subject) {
		this.subject = subject;
		subject.registerObserver(this);
	}

	@Override
	public void display() {
		// TODO Auto-generated method stub
		System.out.println("second diplayer: ");
		System.out.println("temp:" + this.temp + "; humidity:" + this.humidity);
		System.out.println();
	}

	@Override
	public void update(int temp, int humidity) {
		// TODO Auto-generated method stub
		this.temp = temp;
		this.humidity = humidity;
		display();
	}

}

package cn.weather.demo;

public class ThirdDisplayer implements Observer {

	private int min;
	private int sec;
	private Subject subject;

	public ThirdDisplayer(Subject subject) {
		this.subject = subject;
		subject.registerObserver(this);
	}

	@Override
	public void update(int min, int sec) {
		this.min = min;
		this.sec = sec;
		show();
	}

	public void show() {
		System.out.println("third displayer:");
		System.out.println("min:" + min + "; sec:" + sec);
		System.out.println();
	}

}

测试：

package cn.weather.demo;

import java.util.Date;

public class Test {
	public static void main(String[] args) {
		 WeatherData weatherData = new WeatherData();
		 FirstDisplayer firstDisplayer = new FirstDisplayer(weatherData);
		 weatherData.setMeasurements(10, 20);
		
		 SecondDisplayer secondDisplayer = new SecondDisplayer(weatherData);
		 weatherData.setMeasurements(500, 200);
		
		 weatherData.setMeasurements(300, 0);
		
	}
}

观察者模式，就是在主题中设置一个观察点方法，每次运行该方法，就会通知所有的观察者，即调用观察者的通知方法，观察者在这个通知方法中进行其他处理。