2018-03-08观察者模式

观察者模式又被称作发布/订阅模式，观察者模式定义了一种一对多的依赖关系，让多个观察者对象同时监听某一个主题对象。这个主题对象在状态发生变化时，会通知所有观察者对象，使它们能够自动更新自己。

Java库中的观察者和被观察者.png image.png

JDK库代码：

libcore/ojluni/src/main/java/java/util/Observer.java
public interface Observer {
    /**
     * This method is called whenever the observed object is changed. An
     * application calls an <tt>Observable</tt> object's
     * <code>notifyObservers</code> method to have all the object's
     * observers notified of the change.
     *
     * @param   o     the observable object.
     * @param   arg   an argument passed to the <code>notifyObservers</code>
     *                 method.
     */
    void update(Observable o, Object arg);
}

libcore/ojluni/src/main/java/java/util/Observable.java
public class Observable {
private final ArrayList<Observer> observers;
public synchronized void addObserver(Observer o){...}
public synchronized void deleteObserver(Observer o){...}
public void notifyObservers() {
         notifyObservers(null);
     }
public void notifyObservers(Object arg) {

        Observer[] arrLocal; 
         synchronized (this) {
...
             arrLocal = observers.toArray(new Observer[observers.size()]);
             clearChanged();
         }
 
        for (int i = arrLocal.length-1; i>=0; i--)
             arrLocal[i].update(this, arg);// 这里逐个调用观察者的update方法
     }

简单的代码实现：

public class User implements Observer {
    public String name;

    public User(String name) {
        this.name = name;
    }

    @Override
    public void update(Observable o, Object arg) {
        System.out.println("Hi "+name +",公众号更新了内容："+arg);
    }
}

public class Gamedaily extends Observable {
    public void postNewArticle(String content){
        //内容发生改变
        setChanged();
        //通知所有订阅者改变的内容
        notifyObservers(content);
    }
}

public class Client {
    public static void main(String[] args) {
        Gamedaily gamedaily = new Gamedaily();
        User user1 = new User("user1");
        User user2 = new User("user2");
        User user3 = new User("user3");
        //将观察者注册到可观察者的通知列表中。
        gamedaily.addObserver(user1);
        gamedaily.addObserver(user2);
        gamedaily.addObserver(user3);

        gamedaily.postNewArticle("新文章来了");
    }
}