觀察者模式

1.       觀察者模式定義了一種一對多的依賴關係，讓多個觀察者對象同時監聽某一個主題對象。這個主題對象在狀態發生變化時，會通知所有觀察者對象，使他們能夠自動更新自己。另：當一個對象的改變需要同時改變其他對象的時候，而且它不知道具體有多少對象有待改變時，應該考慮使用觀察者模式。

2.       結構圖

3.       舉例：

A．抽象通知者

public interface Publisher {

    public String state = null;

    public void register(Observer o);

    public void unregister(Observer o);

}

B． 抽象觀察者

public abstract class Observer {

        public abstract void update();

}

C． 具體通知者

public class ConcretPublisher1 implements Publisher{

private List<Observer> observerList = new ArrayList<Observer>();



    public void register(Observer observer) {

       observerList.add(observer);

    }



    public void unregister(Observer observer) {

       observerList.remove(observer);

    }



    public void newNotify() {

       for(Observer observer : observerList) {

           observer.update();

       }

    }

}



public class ConcretPublisher2 implements Publisher{



    private List<Observer> observerList = new ArrayList<Observer>();



    public void register(Observer observer) {

       observerList.add(observer);

    }



    public void unregister(Observer observer) {

       observerList.remove(observer);

    }



    public void newNotify() {

       for(Observer observer : observerList) {

           observer.update();

       }

    }

}

D．具體觀察者

public class ConcreteObserver1 extends Observer{



    private Publisher publisher;

    private String name;



    public ConcreteObserver1(Publisher publisher,String name) {

       this.publisher = publisher;

       this.name = name;

    }

    @Override

    public void update() {

       // TODO Auto-generated method stub

       System.out.println("對"+name+"說:訂單支付成功，請求供貨方發貨");

    }

    public Publisher getPublisher() {

       return publisher;

    }

    public void setPublisher(Publisher publisher) {

       this.publisher = publisher;

    }

    public String getName() {

       return name;

    }

    public void setName(String name) {

       this.name = name;

    }



}



public class ConcreteObserver2 extends Observer{



    private Publisher publisher;

    private String name;



    public ConcreteObserver2(Publisher publisher,String name) {

       this.publisher = publisher;

       this.name = name;

    }

    @Override

    public void update() {

       // TODO Auto-generated method stub

       System.out.println("對"+name+"說：供貨方發貨成功，修改狀態吧！");

    }

    public Publisher getPublisher() {

       return publisher;

    }

    public void setPublisher(Publisher publisher) {

       this.publisher = publisher;

    }

    public String getName() {

       return name;

    }

    public void setName(String name) {

       this.name = name;

    }

}

4.       其他：

JDK提供對觀察者模式的內置支持。Observable類與Observer接口。下面是Observer模式的實現過程：

創建一個被觀察者，繼續java.util.Observable

創建一個觀察者，實現java.util.Observer接口

註冊觀察者，調用addObserver(Observer observer)

在被觀察者改變對象內部狀態的地方，調用setChanged()方法，然後調用 notifyObservers(Object)方法，通知被觀察者

在觀察者的update（Object）方法中，對改變做出響應。

 

代碼舉例：

public class Watched extends Observable{

    private String data = "";

    public String retrieveData() {

       return data;

    }

    public void changeData(String data) {

       if(!this.data.equals(data)) {

           this.data = data;

           this.setChanged();

       }

       this.notifyObservers();

    }

}



public class Watcher implements Observer{



    public Watcher(Watched w) {

       w.addObserver(this);

    }



    @Override

    public void update(Observable ob, Object arg) {

       // TODO Auto-generated method stub

       System.out.println("Data has bean changed to :" + ((Watched)ob).retrieveData() + "");

    }



}



public class Tester {



    static private Watched watched;

    static private Observer watcher;



    public static void main(String[] args) {

       watched = new Watched();

       watcher = new Watcher(watched);



       watched.changeData("In c,we create bugs.");

       watched.changeData("In java,we inherit bugs.");

       watched.changeData("In java,we inherit bugs.");

       watched.changeData("In visual Basic,We visualize bugs.");

    }



}

5.       應用場景：

A.     當訂單有退款的時候，需要做一系統的操作，商品自動下架或者合作方自動下架等。