Head First 设计模式2 观察者模式

观察者模式：

1，观察者模式定义了对象之间一对多的关系

2，主题（也就是可观察者）用一个共同的接口来更新观察者

3，观察者和可观察者之间用松耦合方式结合，可观察者不知道观察者细节，只知道观察者实现了观察者接口

4，使用此模式你可以从被观察者处“推”或者“拉”数据，“推”的方式被认为更正确

5，有多个观察者时，不可以依赖特定的通知次序

设计原则

1，找出程序中变化的方面，然后将其和固定不变的方面相分离

2，针对接口编程，不针对实现编程

3，多用组合，少用继承

设计图：

代码实现：

1接口：

public interface Subject {

public void registerObserver(Observer o);

public void removeObserver(Observer o);

public void notifyObserver();

}

public interface Observer {

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

}

public interface Observer {

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

}

2接口实现

public class WeatherData implements Subject {

private ArrayList observers;//建立一个数组来存储观察者

private float temperature;

private float humidity;

private float pressure;

public WeatherData(){

observers = new ArrayList();

}

//对观察者的注册和删除

@Override

public void registerObserver(Observer o) {

observers.add(o);

}

@Override

public void removeObserver(Observer o) {

int i = observers.indexOf(o);

if(i>=0){

observers.remove(i);

}

}

//这里是比较重要的，这是对观察者进行广播

@Override

public void notifyObserver() {

for(int i=0;i<observers.size();i++){

Observer obserrver = (Observer) observers.get(i);

//对观察者进行更新

obserrver.update(temperature, humidity, pressure);

}

}

public void measurementsChanged(){

notifyObserver();

}

public void setMeasurements(float temperature,float humidity,float pressure){

this.humidity = humidity;

this.pressure = pressure;

this.temperature = temperature;

measurementsChanged();

}

}

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);

}

@Override

public void display() {

System.out.println("Current conditions:" + temperature + " F degrees and " + humidity + "% humidity");

}

@Override

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

this.temperature = temp;

this.humidity = humidity;

display();

}

}

测试：

public class WeatherStation {

public static void main(String[] args) {

WeatherData weatherData = new WeatherData();

CurrentConditionsDisplay currentDisplay = new CurrentConditionsDisplay(weatherData);

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

}

}