Observer pattern

2023-10-11 in Design Patterns

Observer pattern에 대해 설명하는 페이지입니다.

Environment

• Programming Language: Java

Index

• Index
• Introduction
• How to Use
• Comments

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.

How to Use

• Observable (Publisher) side
  • To send notifications
    • Call SetChanged()
    • Call either notifyObservers() or notifyObservers(Object arg)

  import java.util.Observable;
  import java.util.Observer;
  
  public class MyData extends Observable {
      private int number;
  
      public MyData() {}
  
      public void measurementsChanged() {
          setChanged();
          notifyObservers();
      }
  
      public void setMeasurements(int number) {
          this.number = number;
          measurementsChanged();
      }
  
      public int getNumber() {
          return number;
      }
  }
  

• 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)

  import java.util.Observable;
  import java.util.Observer;
  
  public class MyObserver implements Observer {
      Observable observable;
      private int number;
  
      public MyObserver(Observable observable) {
          this.observable = observable;
          observable.addObserver(this);
      }
  
      public void update(Observable obs, Object arg) {
          if (obs instanceof MyData) {
              MyData myData = (MyData) obs;
              this.number = myData.getNumber();
  
              // Something to execute
          }
      }
  }
  

Comments

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