定義：定義了對象之間一對多的依賴暇务，這樣一來泼掠，當一個對象改變時，它所有的依賴者都會收到通知并自動更新
優(yōu)點:
觀察者模式解除了主題和具體觀察者的耦合垦细，讓耦合的雙方都依賴于抽象择镇，而不是依賴具體。
從而使得各自的變化都不會影響另一邊的變化括改。
缺點:
依賴關系并未完全解除腻豌，抽象通知者依舊依賴抽象的觀察者。
適用場景:
當一個對象的改變需要給變其它對象時嘱能，而且它不知道具體有多少個對象有待改變時吝梅。
一個抽象某型有兩個方面，當其中一個方面依賴于另一個方面惹骂，這時用觀察者模式可以將
這兩者封裝在獨立的對象中使它們各自獨立地改變和復用苏携。、

/**
 * @author: sjx
 * @date: 2018/7/28 13:22
 * @description:  圖書訂閱者
 */
public class BookSubject implements Subject {
    List<Observer> observerList = new ArrayList<>();
    String bookName;

    @Override
    public void registerObserver(Observer observer) {
        if (!observerList.contains(observer)) {
            observerList.add(observer);
        }
    }

    @Override
    public void removeObserver(Observer observer) {
        if (observerList.contains(observer)) {
            observerList.remove(observer);
        }
    }

    @Override
    public void notifyObserver() {
        for (Observer observer : observerList) {
            observer.update(bookName);
        }
    }

    public void setBookName(String bookName) {
        this.bookName = bookName;
    }
}
/**
 * @Auther: sjx
 * @Date: 2018/7/28 13:14
 * @Description:
 */
public interface Observer {
    void update(String name);
    void disConnect();
}


/**
 * @author: sjx
 * @date: 2018/7/28 13:30
 * @description: 被觀察者
 */
public class ReaderBook implements Observer {
    private Subject subject;

    public ReaderBook(Subject subject) {
        this.subject = subject;
    }

    public void register(){
        subject.registerObserver(this);
    }

    @Override
    public void update(String name) {
        System.out.println("新書來了对粪，名稱是" + name);
    }

    @Override
    public void disConnect() {
        subject.removeObserver(this);
    }
}

public class Main {

    public static void main(String[] args) {
        Subject subject = new BookSubject();
        Observer observer = new ReaderBook(subject);
        ((ReaderBook) observer).register();
        ((BookSubject) subject).setBookName("java");
        subject.notifyObserver();
    }
}

//java  jdk對觀察者模式的支持

/**
 * @author: sjx
 * @date: 2018/7/28 13:46
 * @description: 觀察者
 */
public class Teacher extends Observable {

    String info = "";

    public void setHomeWork(String work) {
        info = work;
        System.out.println("老師布置的作業(yè)是" + info);
        //設置通知時間生效
        setChanged();
        notifyObservers();
    }
    public String getInfo() {
        return info;
    }
}

/**
 * @author: sjx
 * @date: 2018/7/28 13:47
 * @description: 被觀察者
 */
public class Student implements Observer {
    private String name;

    public Student(String name, Observable observable) {
        this.name = name;
        observable.addObserver(this);
    }

    @Override
    public void update(Observable o, Object arg) {
        Teacher teacher = (Teacher) o;
        System.out.println(name + "收到作業(yè)-" + teacher.getInfo());
    }
}

/**
 * @author: sjx
 * @date: 2018/7/28 13:44
 * @description: java jdk 內(nèi)置觀察者
 */
public class JdkMain {
    public static void main(String[] asrgs) {
        Teacher observable = new Teacher();
        Student student1 = new Student("張三",observable);
        Student student2 = new Student("李四",observable);
        Student student3 = new Student("王五",observable);
        observable.setHomeWork("第二頁第六題");
        observable.setHomeWork("第三頁第七題");
        observable.setHomeWork("第五頁第八題");
    }
}