Java设计模式（二） 观察者模式

观察者模式： 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，他的所有依赖者都会受到通知并自动更新。

1，定义事件源接口

package com.pattern.observer;

public interface Subject {

	public void registerObserver(Observer o);

	public void removeObserver(Observer o);

	public void notifyObserver();
}

2，定义事件源

package com.pattern.observer;

import java.util.ArrayList;

/**
 * 事件源
 */
public class WeatherData implements Subject {

	private ArrayList observers;
	private float temperature;
	private float humidity;
	private float pressure;

	public WeatherData() {
		observers = new ArrayList();
	}

	/**
	 * @see com.pattern.observer.Subject#registerObserver(com.pattern.observer.Observer)
	 */
	@Override
	public void registerObserver(Observer o) {
		observers.add(o);
	}

	/**
	 * @see com.pattern.observer.Subject#removeObserver(com.pattern.observer.Observer)
	 */
	@Override
	public void removeObserver(Observer o) {
		int i = observers.indexOf(o);
		if (i >= 0) {
			observers.remove(i);
		}
	}

	/**
	 * @see com.pattern.observer.Subject#notifyObserver()
	 */
	@Override
	public void notifyObserver() {
		for(int i=0;i<observers.size();i++){
			Observer observer = (Observer)observers.get(i);
			observer.update(temperature, humidity, pressure);
		}
	}

	public void measurementsChanged(){
		notifyObserver();
	}

	public void setMeasurements(float temperature,float humidity,float pressure){
		this.temperature  = temperature;
		this.humidity = humidity;
		this.pressure = pressure;
		measurementsChanged();
	}

}

3，定义观察者接口

package com.pattern.observer;

public interface Observer {

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

}

4，定义显示接口

package com.pattern.observer;

public interface DisplayElement {

	public void display();

}

5，观察者实现类

package com.pattern.observer;

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);
	}

	/**
	 * @see com.pattern.observer.DisplayElement#display()
	 */
	@Override
	public void display() {
		System.out.println("Current conditions: "+temperature +"F degrees and "+humidity+"% humidity");
	}

	/**
	 * @see com.pattern.observer.Observer#update(float, float, float)
	 */
	@Override
	public void update(float temperature, float humidity, float pressure) {
		this.temperature = temperature;
		this.humidity = humidity;
		display();
	}

}

6,测试

package com.pattern.observer;

public class WeatherStation {

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

		CurrentConditionsDisplay currentDisplay =  new CurrentConditionsDisplay(weatherData);

		weatherData.setMeasurements(80, 65, 30.4f);
	}
}

类图如下所示：