Design Pattern-Observer Pattern

Observer mode

1. Definition and Type

• Definition: Defines one-to-many dependencies between objects, allowing multiple observer objects to monitor a certain subject object at the same time. When the subject object changes, all its dependencies will be notified and updated.
• Type: Behavioral

2. Applicable scenarios

• Associate behavior scenarios and establish a trigger mechanism

3. Advantages

• Establish an abstract coupling between the observer and the observed
• Observer mode supports broadcast communication

4. Disadvantages

• There is too much detail dependence between observers, which increases time consumption and program complexity
• Use it properly and avoid loop calls

5.Coding

• Create a problem class

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()+",收到一个来自"+question.getStudentName()+"提出的问题:"+question.getQuestionContent());
    }
}

• Create a course class

  • Inherit the Observable class, which means that it is an observable

• The method can be overridden. The setChange() flag tells the observer that there is a change; notifyObservers() informs the observer

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.getStudentName()+"在"+course.getCourseName()+"课程中，提出了问题:"+question.getQuestionContent());
        setChanged();
        notifyObservers(question);
    }
}

• Create teacher class

  • Implement the Observer interface, which means it is an observer
  • Override the update method to write business logic based on the parameters passed by the 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()+",收到一个来自"+question.getStudentName()+"提出的问题:"+question.getQuestionContent());
    }
}

• Test class

public class Test {
    public static void main(String[] args) {
        // 创建被观察者
        Course course = new Course("如何让富婆爱上我~");
        // 创建观察者
        Teacher teacher = new Teacher("HHHH");
        // 绑定观察者和被观察者
        course.addObserver(teacher);
        
        Question question = new Question();
        question.setStudentName("Root");
        question.setQuestionContent("有富婆的联系方式  吗？");
        
        course.produceQuestion(course,question);

    }
}

• Console output

6. Summary

• Two structures: Observer and Observed
• When the observed person changes, the observer can be notified
• The observer can accept the parameters passed in by the observed change and encode