观察者模式

以下是我参考《Head First设计模式》这本书里总结的

观察者模式定义：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。

设计原则：为了交互对象之间的松耦合设计而努力。

主题接口：Subject{registerObserver(); removeObserver(); notifyObservers();}//对象用此接口注册为观察者，或者把自己从观察者中删除。

public interface Subject {

public void registerObserver(Observer o);
public void removeObserver(Observer o);
public void notifyObservers();

}

观察者接口：Observer{update();}//所有潜在的观察者必须实现观察者接口，这个接口只有一个方法，当主题更新时它被调用。

public interface Observer {

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

}

具体主题：ConcreteSubject{registerObserver(){} removeObserver(){} notifyObservers(){} ...}//具体主题实现了上面的三个方法，其中的notifyObservers()方法用于在状态改变时更新所有当前观察者。

import java.util.ArrayList;

public class WeatherData implements Subject {

private ArrayList<Observer> observers;
private float temperature;
private float humidity;
private float pressure;

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

int i = observers.indexOf(o);
if (i >= 0) {
observers.remove(i);
}

}

@Override
public void notifyObservers() {
// TODO Auto-generated method stub

for (int i = 0; i < observers.size(); i++) {
Observer observer = observers.get(i);
observer.update(temperature, humidity, pressure);
}
}

public void measurementChanged() {
notifyObservers();
}

        //具体实例调用这个方法时，就可以改变观察者的值
public void setMessurements(float temperature, float humidity,
float pressure) {
this.temperature = temperature;
this.humidity = humidity;
this.pressure = pressure;
measurementChanged();
}

}

具体观察者：ConcreteObserver{update(){} ...}//观察者必须注册具体主题，以便接收更新。

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 update(float temp, float humidity, float pressure) {
// TODO Auto-generated method stub

this.temperature = temp;
this.humidity = humidity;
display();

}


@Override
public void display() {
// TODO Auto-generated method stub

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

测试程序：

public class WeatherStation {

public static void main(String[] args) {

WeatherData weatherData = new WeatherData();

CurrentConditionsDisplay currentDisplay = new CurrentConditionsDisplay(
weatherData);
weatherData.setMessurements(80, 65, 30.4f);
weatherData.setMessurements(82, 70, 29.2f);
weatherData.setMessurements(78, 90, 29.2f);

}
}