@startuml top to bottom direction abstract Subject { -list<Observer> $observers +attach(Observer $observer): void +detach(Observer $observer): void +notify(): void } note left of Subject::notify foreach ($this->observers as $observer) { $observer->update($this) } end note interface Observer { +update(Subject $changedSubject): void +equals(mixed $other): bool } class ConcreteSubject extends Subject { -$state +getState() +setState($state) } note left of ConcreteSubject::getState return $this->state; end note note left of ConcreteSubject::setState $this->state = $state; $this->notify(); end note class ConcreteObserver implements Observer { -$state -ConcreteSubject $subject +update(Subject $changedSubject): void +equals(mixed $other): bool } note right of ConcreteObserver::update $this->state = $this->subject->getState(); end note Subject "1" o-right-> "*" Observer ConcreteObserver "1" o-left-> "1" ConcreteSubject @enduml Submit amiga aws-orange black-knight bluegray blueprint cerulean-outline cerulean crt-amber crt-green cyborg-outline cyborg hacker lightgray mars materia-outline materia metal mimeograph minty plain reddress-darkblue reddress-darkgreen reddress-darkorange reddress-darkred reddress-lightblue reddress-lightgreen reddress-lightorange reddress-lightred sandstone silver sketchy-outline sketchy spacelab spacelab-white superhero-outline superhero toy united vibrant       🎉 Discover the future PlantUML Web Editor! 🚀  PNG  SVG  ASCII Art