定義

當(dāng)一個對象變化時弹渔，其它依賴該對象的對象都會收到通知媒熊，并且隨著變化！對象之間是一種一對多的關(guān)系谜酒。觀察者模式又叫做發(fā)布-訂閱（Publish/Subscribe）模式叹俏、模型-視圖（Model/View）模式、源-監(jiān)聽器（Source/Listener）模式或從屬者（Dependents）模式僻族。

角色

• Subject: 目標(biāo)
• ConcreteSubject: 具體目標(biāo)
• Observer: 觀察者
• ConcreteObserver: 具體觀察者

實例

創(chuàng)建observer

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

創(chuàng)建ConcreteObserver類

public class ConcreteObserver implements Observer {
    private String name;

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

    @Override
    public void update(String massage) {
        System.out.println(massage+" "+name);
    }
}

創(chuàng)建subject

public interface Subject {
    /**
     * @author Royle.Huang
     * @date 2019/1/1 17:10
     * @description: 增加訂閱
     */
    void attach(Observer observer);
    /**
     * @author Royle.Huang
     * @date 2019/1/1 17:11
     * @description: 刪除訂閱
     */
    void dettach(Observer observer);
    /**
     * @author Royle.Huang
     * @date 2019/1/1 17:11
     * @description: 通知訂閱者
     */
    void notify(String massage);
}

創(chuàng)建ConcreteSubject類

public class ConcreteSubject implements Subject {

    List<Observer> list = new ArrayList<>();

    @Override
    public void attach(Observer observer) {
        list.add(observer);
    }

    @Override
    public void dettach(Observer observer) {
        list.remove(observer);
    }

    @Override
    public void notify(String massage) {
        for (Observer observer : list) {
            observer.update(massage);
        }
    }
}

測試代碼

public class Test {
    public static void main(String[] args) {
        Observer observer = new ConcreteObserver("goudan");
        Observer observer2 = new ConcreteObserver("luffy");
        Observer observer3 = new ConcreteObserver("java");

        Subject subject = new ConcreteSubject();
        subject.attach(observer);
        subject.attach(observer2);
        subject.attach(observer3);

        subject.notify("a message for");
    }
}

運行結(jié)果

a message for goudan
a message for luffy
a message for java