Observer

處理物件一對多的關係，當被觀察者的狀態改變，觀察者們會收到通知並更新。也稱發布訂閱模式(Publish/Subscribe)

from: Observer Pattern Tutorial with Java Examples

Example

訂閱報紙

Subject

pubilc interface Subject {
    public void registerObserver(Observer o);
    public void removeObserver(Observer o);
    public void notifyObservers();
}

public class NewspaperOffice implements Subject() {

    // 使用 List 來記錄所有訂閱者
    List mObservers;

    // 報紙內容
    String mContent;

    public NewspaperOffice()
    {
        mObservers = new ArrayList();
        mContent = "";
    }

    public void sendNewspaper(String content)
    {
        mContent = content;
        notifyObservers();
    }

    @Override
    public void registerObserver(Observer o)
    {
        mObservers.add(o);
    }

    @Override
    public void removeObserver(Observer o)
    {
        int i = mObservers.indexOf(0);
        if(i > 0)
            mObservers.remove(i);
    }

    @Override
    public void notifyObservers()
    {
        for(int i = 0; i < mObservers.size(); i++)
        {
            Observer o = mObservers.get(i);
            o.update(mContent);
        }
    }
}

Observer

public interface Observer {
    public void update(String content);
}

public class Customer implements Observer {

    // 訂閱者名稱
    String mName;

    public Customer(String name)
    {
        mName = name;
    }

    @Override
    public void update(String content)
    {
        System.out.println(name + "get newspaper: "
            + content);
    }
}

Usage

public class Test {
    public static void main(String[] args)
    {
        // 建立報社
        NewspaperOffice office = new NewspaperOffice();

        // 訂閱者 Tom
        Customer Tom = new Customer("Tom");

        // 訂閱者 Peter
        Customer Peter = new Customer("Peter");

        // 訂閱者訂報紙
        office.registerObserver(Tom);
        office.registerObserver(Peter);

        // 報社送報紙囉
        office.sendNewspaper("Newspaper 1...");

        // Tom 不想訂報紙了
        office.removeObserver(Tom);
        office.sendNewspaper("Newspaper ２...");
    }
}

from: 觀察者模式 (Observer Pattern)

其他

• Java有內建java.util.Observable 和 java.util.Observer