Observer pattern

Observer pattern에 대해 설명하는 페이지입니다.

Posted Oct 11, 2023

By HyunJinNo

Tags

Design Pattern, Java

1. Introduction

• a.k.a
  • Publish/Subscibe model
• Purpose
  • Lets one or more objects be notified of state changes in other objects within the system
• Use When
  • Loose coupling is needed for communications
  • State changes in one or more objects should trigger behavior in other objects
  • Broadcasting capabilities are required.
  • An understanding exists that objects will be blind to the expense of notification
• Definition
  • It defines a one-to-many dependency between objects so that when one object changes state, all of its dependents are notified and updated automatically.

2. How to Use

• Observable (Publisher) side

  • To send notifications
    • Call SetChanged()
    • Call either notifyObservers() or notifyObservers(Object arg)
  java

  1import java.util.Observable;
  2import java.util.Observer;
  3
  4public class MyData extends Observable {
  5    private int number;
  6
  7    public MyData() {}
  8
  9    public void measurementsChanged() {
  10        setChanged();
  11        notifyObservers();
  12    }
  13
  14    public void setMeasurements(int number) {
  15        this.number = number;
  16        measurementsChanged();
  17    }
  18
  19    public int getNumber() {
  20        return number;
  21    }
  22}

• Observer (Subsciber) side

  • To become an observer
    • Implement java.util.Observer interface
  • To subscribe
    • Call addObserver() on any Observable object
  • To unsubscribe
    • Call deleteObserver()
  • To get updated
    • Implement update(Observable o, Object arg)
  java

  1import java.util.Observable;
  2import java.util.Observer;
  3
  4public class MyObserver implements Observer {
  5    Observable observable;
  6    private int number;
  7
  8    public MyObserver(Observable observable) {
  9        this.observable = observable;
  10        observable.addObserver(this);
  11    }
  12
  13    public void update(Observable obs, Object arg) {
  14        if (obs instanceof MyData) {
  15            MyData myData = (MyData) obs;
  16            this.number = myData.getNumber();
  17
  18            // Something to execute
  19        }
  20    }
  21}