Java设计模式 -- 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。

当两个对象之间松耦合，他们依然可以交互，但是不太清楚彼此的细节。观察者模式提供了一种对象设计让主题和观察者之间松耦合。

我们来看看一种push方式的 观察者模式。

首先是观察者 主题 等的接口。

    //主题  
    public interface Subject {  
        public void registerObserver(Observer o);  
        public void removeObserver(Observer o);  
        public void notifyObservers();  
    }  
      
    //观察者  
    public interface Observer {  
         
       public void update(float temp, float humidity, float pressure);  
   }  
   //other 操作  
   public interface DisplayElement {  
         
       public void display();  
   } 

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

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

//other 操作 
public interface DisplayElement 
{ 
 public void display(); 
} 

实现这些接口。

	 //实现主题  
   public class WeatherData implements Subject{  
       private ArrayList observers;  
       private float temperature;  
       private float humidity;  
       private float pressure;  
         
       public WeatherData()  
       {  
           observers = new ArrayList();  
       }  
         
       public void notifyObservers() {  
           // TODO Auto-generated method stub  
           for(int i=0, len=observers.size(); i < len; i++)  
           {  
               Observer observer = (Observer) observers.get(i);  
               observer.update(temperature, humidity, pressure );  
           }  
       }  
       public void registerObserver(Observer o) {  
           // TODO Auto-generated method stub  
           observers.add(o);  
       }  
       public void removeObserver(Observer o) {  
           // TODO Auto-generated method stub  
           int i = observers.indexOf(o);  
           if(i >= 0)  
           {  
               observers.remove(i);  
           }  
       }  
         
       public void measurementsChange()  
       {  
           notifyObservers();  
       }  
         
       public void setMeasurements(float temperature, float humidity, float pressure)  
       {  
           this.temperature = temperature;  
           this.humidity = humidity;  
           this.pressure = pressure;  
           measurementsChange();  
       }  
   }  
     
   //实现观察者  
   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 temp, float humidity, float pressure) {  
           // TODO Auto-generated method stub  
           this.temperature = temp;  
           this.humidity = humidity;  
           display();  
       }  
       public void display() {  
           // TODO Auto-generated method stub  
           System.out.println("Current conditions: "+ temperature + "F degrees and " + humidity + "% humidity");  
       }  
   }  

//实现主题 
public class WeatherData implements Subject
{ 

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

	public WeatherData() 
	{ 
		observers = new ArrayList(); 
	}
	 
	public void notifyObservers() 
	{ 
	for(int i=0, len=observers.size(); i < len; i++) 
	{ 
		Observer observer = (Observer) observers.get(i); 
		observer.update(temperature, humidity, pressure ); } 
	} 
	
	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 measurementsChange() 
	{ 
		notifyObservers(); 
	} 

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

//实现观察者 
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 temp, float humidity, float pressure) 
	{ 
		this.temperature = temp; 
		this.humidity = humidity; 
		display();
	} 
	 
	public void display() 
	{ 
		System.out.println("Current conditions: "+ temperature + "F degrees and " + humidity + "% humidity");
	}
}

实现完了这些接口，测试观察者模式


public class WeatherStation {  
	public static void main(String[] args)  
	{  
		WeatherData weatherData = new WeatherData();  
		           
		CurrentConditionsDisplay display = new CurrentConditionsDisplay(weatherData);  
		           
		weatherData.setMeasurements(80, 65, 30.4f);  
		weatherData.setMeasurements(82, 76, 30.2f);  
		weatherData.setMeasurements(78, 90, 29.4f);  
	}  
}