观察者模式

 

1 观察操作

public interface Observer
...{
    void update();
}

 

2 具体观察者操作

public class ConcreteObserver implements Observer
...{
    public void update()
    ...{
        System.out.println("I am notified");
    }
}

 

3 主题抽象

import java.util.Vector;
import java.util.Enumeration;

abstract public class Subject 
...{

    /** *//**
     * @associates <{Observer}>
     * @link aggregation
     * @supplierCardinality 0..*
     */
    private Vector observersVector = new java.util.Vector();

    public void attach(Observer observer)
    ...{
        observersVector.addElement(observer);
        System.out.println("Attached an observer.");
    }

    public void detach(Observer observer)
    ...{
        observersVector.removeElement(observer);
    }

    public void notifyObservers()
    ...{
        java.util.Enumeration enumeration = observers();
        while (enumeration.hasMoreElements())
        ...{
            System.out.println("Before notifying");
            ((Observer)enumeration.nextElement()).update();
    }
    }

    public Enumeration observers()
    ...{
        return ((java.util.Vector) observersVector.clone()).elements();
    }

}

 

4 具体主题操作

public class ConcreteSubject extends Subject
...{
    private String state;

    public void change(String newState)
    ...{
        state = newState;

        this.notifyObservers();
    }

 

5 客户端实现，主题改变通知后各具体观察者进行更新。

public class Client
...{
    private static ConcreteSubject subject;
    private static Observer observer;
    private static Observer ob;
    
    public static void main(String[] args)
    ...{
    subject = new ConcreteSubject();

        observer = new ConcreteObserver();
        ob = new ConcreteObserver();

        subject.attach(observer);
        subject.attach(ob);

        subject.change("new state");
    }
}

运行结果：

init:
deps-jar:
Compiling 1 source file to D:/test/desPatten/build/classes
compile-single:
run-single:
Attached an observer.
Attached an observer.
Before notifying
I am notified
Before notifying
I am notified
生成成功（总时间：1 秒）