观察者模式

Java设计模式 - 观察者模式

观察者模式用于在一个对象被修改时通知其相关对象。

观察者模式是一种行为模式类别。

例子

import java.util.ArrayList;
import java.util.List;

class MyValue {
   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();
      }
   }   
}
abstract class Observer {
   protected MyValue subject;
   public abstract void update();
}
class PrinterObserver extends Observer{
   public PrinterObserver(MyValue subject){
      this.subject = subject;
      this.subject.attach(this);
   }

   @Override
   public void update() {
      System.out.println("Printer: " + subject.getState() ); 
   }
}
class EmailObserver extends Observer{

   public EmailObserver(MyValue subject){
      this.subject = subject;
      this.subject.attach(this);
   }

   @Override
   public void update() {
     System.out.println("Email: "+ subject.getState() ); 
   }
}
class FileObserver extends Observer{

   public FileObserver(MyValue subject){
      this.subject = subject;
      this.subject.attach(this);
   }

   @Override
   public void update() {
      System.out.println("File: " + subject.getState()); 
   }
}

public class Main {
   public static void main(String[] args) {
      MyValue subject = new MyValue();

      new FileObserver(subject);
      new EmailObserver(subject);
      new PrinterObserver(subject);

      subject.setState(15);

      subject.setState(10);
   }
}

上面的代码生成以下结果。

File:15
Email:15
Printer:15
File:10
Email:10
Printer:10