设计模式之观察者设计模式

▶ 事件源：就是被监听的对象，
▶ 事件类：里面封装了事件源对象，
▶ 监听器：里面绑定了事件类，在监听器里面拿到事件类对象，就可以监听事件源中的行为（调用了什么方法）

案例一

    //监听器
    public interface StudentListener {
        void preStudy(StudentEvent e);
        void preSleep(StudentEvent e);
    }

public class Student {
    private String name;
    //绑定监听器
    private StudentListener listener;
    public Student(String name){
        this.name = name;
    }
    public String getName(){
        return name;
    }
    public void study(){
        if(listener!=null){
            listener.preStudy(new StudentEvent(this));
        }
        System.out.println(name + "在学习");
    }

    public void sleep(){
        if(listener!=null){
            listener.preSleep(new StudentEvent(this));
        }
        System.out.println(name + "要睡觉");
    }

    public void addStudentListener(StudentListener listener){
        this.listener = listener;
    }
}

   //封装事件源对象：student事件
    public class StudentEvent {
        private Object source;
        public StudentEvent(Object source){
            this.source = source;
        }

        public Object getSource(){
            return source;
        }
    }

   //demo
    public class StudentListenerDemo {
    public static void main(String[] args) {
        Student s = new Student("小明明");
        //添加监听器
        s.addStudentListener(new StudentListener() {

            @Override
            public void preStudy(StudentEvent e) {
                Student s = (Student) e.getSource();
                String name = s.getName();
                System.out.println(name + "喝杯牛奶吧");
            }

            @Override
            public void preSleep(StudentEvent e) {
                Student s = (Student) e.getSource();
                String name = s.getName();
                System.out.println(name + "睡个好觉哦");
            }
        });

        s.sleep();/*打印：小明明睡个好觉小明明要睡觉*/
        s.study();/*打印小明明喝杯牛奶吧小明明在学习*/
    }

}

案例二

    //典型的窗体事件：
    public class FrameDemo {
    public static void main(String[] args) {
        //事件源：出发事件的对象
        Frame f = new Frame("我的窗户");
        f.setSize(1000, 1000);
        f.setVisible(true);
        f.addWindowListener(new MyWindowListener());
      }
   }

    class MyWindowListener extends WindowAdapter{

        @Override
        public void windowClosed(WindowEvent e) {//WindowEvent封装了事件源对象，就是哪个对象发生了改变
            Frame f = (Frame) e.getSource();//拿到源对象
            f.dispose();//执行方法
        }

        @Override
        public void windowClosing(WindowEvent e) {
            Frame f = (Frame) e.getSource();
            f.dispose();
        }

    }

案例三

 //Spring的启动时监听，就是使用的监听，它监听的是ServletContextEvent
public class ContextLoaderListener extends ContextLoader implements ServletContextListener {
    public ContextLoaderListener() {
    }
    public ContextLoaderListener(WebApplicationContext context) {
        super(context);
    }   
    @Override
    public void contextInitialized(ServletContextEvent event) {
        initWebApplicationContext(event.getServletContext());
    }   
    @Override
    public void contextDestroyed(ServletContextEvent event) {
        closeWebApplicationContext(event.getServletContext());
        ContextCleanupListener.cleanupAttributes(event.getServletContext());
    }

}