@startuml
Subject -> Observer 
Subject <|-- ConcreteSubject
Observer <|-- ConcreteObserver
ConcreteSubject <- ConcreteObserver
ConcreteSubject --> State
ConcreteObserver --> State

class Subject {
  -observers : List<Observer*>
  +Attach(Observer*)
  +Detach(Observer*)
  +Notify()
}
class ConcreteSubject {
  -state : State
  +GetState()
  +SetState()
}
interface Observer {
  {abstract} +Update()
}
class ConcreteObserver {
  -state : State
  -subject : Subject*
  +Update()
}

note left of Subject
 Subject::Notify() {
   for (auto& o : observers)
     o->Update();
 }
end note

note right of ConcreteObserver
  ConcreteObserver::Update() {
    state = subject->GetState();
  }
end note

hide empty attributes
hide empty methods
@enduml