观察者模式

Posted 2021-03-11 dearnotes

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定义

定义对象间的一种一对多的依赖关系，当一个对象的状态发生改变时，所有依赖于它的对象都得到通知并被自动更新。

例：我们以新闻机构接收到新闻而后通知各个频道为例。那么新闻机构就是被观察对象，各个频道就是观察者。

实现方式

1、一般模式

① 创建Subject类

public class Subject {
   
   private List<Observer> observers 
      = new ArrayList<Observer>();
   private int state;
 
   public int getState() {
      return state;
   }
 
   public void setState(int state) {
      this.state = state;
      notifyAllObservers();
   }
 
   public void attach(Observer observer){
      observers.add(observer);      
   }
 
   public void notifyAllObservers(){
      for (Observer observer : observers) {
         observer.update();
      }
   }  
}

② 创建Observer类

public abstract class Observer {
   protected Subject subject;
   public abstract void update();
}

③ 创建实体观察者类

public class BinaryObserver extends Observer{
 
   public BinaryObserver(Subject subject){
      this.subject = subject;
      this.subject.attach(this);
   }
 
   @Override
   public void update() {
      System.out.println( "Binary String: " 
      + Integer.toBinaryString( subject.getState() ) ); 
   }
}

public class OctalObserver extends Observer{
 
   public OctalObserver(Subject subject){
      this.subject = subject;
      this.subject.attach(this);
   }
 
   @Override
   public void update() {
     System.out.println( "Octal String: " 
     + Integer.toOctalString( subject.getState() ) ); 
   }
}

④ 主方法

public class ObserverPatternDemo {
   public static void main(String[] args) {
      Subject subject = new Subject();
 
      new OctalObserver(subject);
      new BinaryObserver(subject);
 
      System.out.println("First state change: 15");   
      subject.setState(15);
      System.out.println("Second state change: 10");  
      subject.setState(10);
   }
}

⑤ 运行结果

First state change: 15
Octal String: 17
Binary String: 1111
Second state change: 10
Octal String: 12
Binary String: 1010

这个例子是参考菜鸟教程，总结得很详细。

2、实现PropertyChangeListener接口

① 创建被观察者

public class Subject {
     private int state;
     private String message="0";
     private PropertyChangeSupport support;
     
     public Subject() {
        // TODO Auto-generated constructor stub
         support=new PropertyChangeSupport(this);
    }
     public int getState() {
          return state;
       }
     
       public void setState(int state) {
          this.state = state;  
         
          notifyTo(String.valueOf(state));
       }
       
       public void addPropertyChangeListener(PropertyChangeListener pcl) {
           support.addPropertyChangeListener(pcl);
       }
       
       public void removePropertyChangeListener(PropertyChangeListener pcl) {
           support.removePropertyChangeListener(pcl);
       }
       
       public void notifyTo(String value) {
           support.firePropertyChange("",message,value);
           
           
       }
     
     
}

② 创建观察者

public class BinaryObserver implements PropertyChangeListener {
    
    @Override
    public void propertyChange(PropertyChangeEvent evt) {
        // TODO Auto-generated method stub
         System.out.println( "值更新为 "+evt.getNewValue()); 
        
    }
}

③ 主方法

public class MyObserver {
    public static void main(String[] args) {
        Subject subject=new Subject();
        BinaryObserver binaryObserver=new BinaryObserver();
        
        subject.addPropertyChangeListener(binaryObserver);
        subject.setState(10);
        subject.setState(15);

    }

}