观察者模式

1.定义：定义了对象之间的一对多依赖，让多个观察者对象同时监听某一个主题对象，
　　　   当主题对象发生变化时，它的所有依赖者(观察者)都会收到通知并更新。

2.类型：行为型

3.适用场景：关联行为场景，建立一套触发机制。

4.优点：观察者和被观察者之间建立一个抽象的耦合；观察者模式支持广播通信。

5.缺点：观察者之间有过多的细节依赖、提高时间消耗及程序复杂度；
　　　   使用要得当，要避免循环调用。

6.实例目录package

7.实例UML类图

8.代码

package com.geely.design.pattern.behavioral.observer;

import java.util.Observable;

public class Course extends Observable {
    private String courseName;

    public Course(String courseName) {
        this.courseName = courseName;
    }

    public String getCourseName() {
        return courseName;
    }

    public void produceQuestion(Course course,Question question){
        System.out.println(question.getUserName() + "在" + course.courseName +"提交了一个问题");
        setChanged();
        notifyObservers(question);
    }
}

package com.geely.design.pattern.behavioral.observer;

public class Question {
    private String userName;
    private String questionContent;

    public String getUserName() {
        return userName;
    }

    public void setUserName(String userName) {
        this.userName = userName;
    }

    public String getQuestionContent() {
        return questionContent;
    }

    public void setQuestionContent(String questionContent) {
        this.questionContent = questionContent;
    }
}

package com.geely.design.pattern.behavioral.observer;

import java.util.Observable;
import java.util.Observer;

public class Teacher implements Observer {
    private String teacherName;

    public Teacher(String teacherName) {
        this.teacherName = teacherName;
    }

    @Override
    public void update(Observable o, Object arg) {
        Course course = (Course) o;
        Question question = (Question) arg;
        System.out.println(teacherName + "老师的" + course.getCourseName()+ "课程接收到一个" 
　　　　　　　　+ question.getUserName()+ "提交的问答：" + question.getQuestionContent());
    }
}

package com.geely.design.pattern.behavioral.observer;

public class Test {
    public static void main(String[] args) {
        Course course = new Course("Java设计模式精讲");
        Teacher teacher1 = new Teacher("Alpha");
        Teacher teacher2 = new Teacher("Beta");
        course.addObserver(teacher1);
        course.addObserver(teacher2);
        //业务逻辑代码
        Question question = new Question();
        question.setUserName("Geely");
        question.setQuestionContent("Java的主函数如何编写");
        course.produceQuestion(course,question);
    }
}

package com.geely.design.pattern.behavioral.observer.guavatest;

import com.google.common.eventbus.Subscribe;

public class GuavaEvent {
    @Subscribe
    public void subscribe(String str){
        System.out.println("执行subscribe方法，传入的参数是：" + str);
    }
}

package com.geely.design.pattern.behavioral.observer.guavatest;

import com.google.common.eventbus.EventBus;

public class GuavaEventTest {
    public static void main(String[] args) {
        EventBus eventBus = new EventBus();
        GuavaEvent guavaEvent = new GuavaEvent();
        eventBus.register(guavaEvent);
        eventBus.post("post的内容");
    }
}