Observer.md

Observer

Type: Behavioral Pattern

Structure:

In my Codes(Role - Classes):

• Subject: Subject
• ConcreteSubject: ClockTimer
• Observer: Observer
• ConcreteObserver: AnalogClock, DigitalClock

Key Points

• Subject:
  • It maintains the observer objects, that is there can be multiple observers observing one subject
  • When the states of the subject changed, it should notify the observers which it contains
• Observer:
  • It maintains a reference to the subject
  • It saves the states of the subject
  • It should contain and implement the update interface that make the states in the observer consistent with the referenced subject
• If the update operations are complicated, we can use a ChangeManager to do this, which uses the mediator pattern
• ChangeManager:
  • It offers an interface to maintain the mapping of subjects and observers (Register & Unregister)
  • It can define a specified strategy to update
• Order to use Observer Mode:
  1. Create a subject
  2. Create observers with a specified subject
  3. Execute subject.notify() to update the observers
  4. *Observers show the updated states to us

Collaboration Diagram

MVC Structure

• Model: Subject
• View: Observer
• Control: *ChangeManager