Design Pattern-Observer Pattern

Definition: defines a one-to-many dependency, allowing multiple observer objects to listen to a topic object at the same time. When the status of this topic object changes, it notifies all observer objects so that they can automatically update themselves.

Structure:

Sample Code:

Public interface observer {public void Update ();} public class subject {private list <observer> Observers = new arraylist <observer> (); Public void attach (Observer observer) {observers. add (observer);} public void detach (Observer observer) {observers. remove (observer);} public void Policy1 () {for (Observer observer: Observers) {observer. update () ;}} public class concretesubject extends subject {private string subjectstate; Public String getsubjectstate () {return subjectstate;} public void setsubjectstate (string subjectstate) {This. subjectstate = subjectstate;} public class concreteobserver implements observer {private string name; private string observerstate; private concretesubject subject; Public concreteobserver (concretesubject, string name) {This. subject = subject; this. name = Name ;}@ overridepublic void Update () {// todo auto-generated method stubobserverstate = subject. getsubjectstate (); system. out. println ("Observer" + name + "status is" + observerstate );}}

The client code is as follows:

public class Client {public static void main(String[] args) {ConcreteSubject s = new ConcreteSubject();s.attach(new ConcreteObserver(s, "x"));s.attach(new ConcreteObserver(s, "Y")); s.attach(new ConcreteObserver(s, "Y"));s.setSubjectState("ABC");s.notify1();}}

Running result:

The status of the observer X is ABC.

The y status of the observer is ABC.

The y status of the observer is ABC.

Design Pattern-Observer Pattern