設計模式二：觀察者模式（推）

觀察者模式（本文是subject主動向observer推送消息且是自定義觀察者模式非java內置模式）定義了對象之間的一對多的依賴，這樣一來，當一個對象改變狀態時，它的全部依賴者都會收到通知並自動更新。

設計原則：爲了實現交互對象之間的鬆耦合設計。

Subject接口：

package com.sun;

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

Subject接口的實現類WeatherData:

package com.sun;

import java.util.ArrayList;

public class WeatherData implements Subject {
    private ArrayList observers;
    private float temperature;
    private float humidity;
    private float pressure;
    
    public WeatherData(){
    	observers = new ArrayList();
    }
	@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);
		}
	}

	@Override
	public void notifyObservers() {
		for (int i = 0; i < observers.size(); i++) {
			Observer oberver = (Observer)observers.get(i);
			oberver.update(temperature, humidity, pressure);
		}
	}
	
	//當從氣象站獲得更新觀測值時，咱們通知觀察者
    public void measurementsChanged(){
    	notifyObservers();
    }
    //利用下面這個方法測試佈告板
    public void setMeasurements(float temperature,float humidity,float pressure){
    	this.temperature = temperature;
    	this.humidity = humidity;
    	this.pressure = pressure;
    	measurementsChanged();
    }
    
}

Observer接口：

package com.sun;

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

Object接口的其中一個公佈板實現類：

package com.sun;

public class CurrentConditionsDisplay implements Observer,DisplayElement{
	private float temperature;
	private float humidity;
	private float pressure;
	private Subject weatherData;
	public CurrentConditionsDisplay(WeatherData weatherData) {
		this.weatherData = weatherData;
		weatherData.registerObserver(this);
	}
	
	@Override
	public void update(float temp, float humidity, float pressure) {
		this.temperature = temp;
		this.humidity = humidity;
		this.pressure = pressure;
		display();
	}
	
	public void display(){
		System.out.println("Current conditions:" + temperature + "F degrees and"+ humidity +" %humidity");
	}

}

DisplayElement公佈板顯示的接口：

package com.sun;

public interface DisplayElement {
   public void display();
}

測試類WeatherStation：

package com.sun;

public class WeatherStation {
	public static void main(String[] args) {
		WeatherData weatherData = new WeatherData();
		CurrentConditionsDisplay currentDisplay = new CurrentConditionsDisplay(weatherData);
		weatherData.setMeasurements(80, 65, 30.4f);
		weatherData.setMeasurements(82, 70, 29.2f);
	}
}