设计模式之观察者模式

1 观察者模式

当一个对象被修改时，则会自动通知它的依赖对象。观察者模式属于行为型模式。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

2 类图

3 代码实现

1）Observer.java

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

2）AddObserver.java

public class AddObserver extends Observer{

   public AddObserver(Subject subject){
      this.subject = subject;
      this.subject.attach(this);
   }

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

3）SubObserver.java

public class SubObserver extends Observer{

   public SubObserver(Subject subject){
      this.subject = subject;
      this.subject.attach(this);
   }

   @Override
   public void update() {
      System.out.println( "sub value is: "
      + (subject.getState() - 5));
   }
}

4）Subject.java

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

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();
      }
   }
}

5）Test.java

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

      new AddObserver(subject);
      new SubObserver(subject);

      System.out.println("add value state to 5");
      subject.setState(5);
      System.out.println("add value state to 8");
      subject.setState(10);
   }
}

4 运行结果

add value state to 5
add value is: 10
sub value is: 0
add value state to 5
add value is: 16
sub value is: 3