Observer Pattern

The Observer Pattern §

C++ Project Example §

Link: The Weather Station

Theory §

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.

The relationship between subject and observers is one-to-many. Whenever the state of the subject changes, all the observers are notified.

This pattern is also known as,

• Subject-Observer
• Publisher-Subscriber

The general class diagram is given below.

• There are 2 interfaces, one for Subject and one for Observer
• The class that will implement the Subject interface need to define the functions for registering observer, removing observer and notifying observers
• There are 2 ways to get updated state in the observers
  • Either the subject pushes the state to observers
  • Or the observers pull the data from the subject state
  • pull is considered more correct as it allows observers to only fetch the data that they need.
• Meanwhile, the classes implementing observer interface need to define the update method
• In this way, the two objects are loosely coupled, they can interact, but have very little knowledge of each other.
• Changes to either the subject or an observer will not affect the other

classDiagram

class Subject {
    <<interface>>
    +registerObserver()
    +removeObserver()
    +notify()
}

class Observer {
    <<interface>>
    +update()
}
Observer "*" <-- "1" Subject

class ConcreteSubject {
    +registerObserver()
    +removeObserver()
    +notify()
}
Subject <|.. "implements" ConcreteSubject

class ConcreteObserver {
    +update()
}
Observer <|.. "implements" ConcreteObserver
ConcreteObserver "subscribe" --|> ConcreteSubject

Some key points to remember §

• Strive for loosely coupled designs between objects that interact
• Loosely coupled designs allow us to build flexible OO systems that can handle change because they minimize the interdependency between objects
• Publishers + Subscribers = Observer Pattern