学习head first 设计模式之观察者模式

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，它的所有依赖者都会收到通知并自动更新

主题和观察者定义了一对多关系，观察者于此主题，只要主题状态一有变化，观察者就会被通知。根据通知的风格，观察者可能因此新值而更新。

观察者和主题是松耦合的

设计原则：为了交互对象之间的松耦合设计而努力

实现气象站

subject接口（主题）

package headfirst.designpatterns.observer.weather;

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

observer接口（观察者）

package headfirst.designpatterns.observer.weather;

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

dispalyElement接口（布告板接口）

package headfirst.designpatterns.observer.weather;

public interface DisplayElement {
	public void display();
}

类CurrentConditionsDisplay布告板实现

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");
	}
}

WeatherData主题实现类

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;
	}
	
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);
	}
}

public float getPressure() {return pressure;}}

测试类WeatherStation