Observateur en Ruby
L’Observateur est un patron de conception comportemental qui permet à certains objets d’envoyer des notifications concernant leur état à d’autres objets.
Ce patron fournit la possibilité aux objets qui implémentent une interface de souscription, de s’inscrire et de se désinscrire de ces événements.
Complexité :
Popularité :
Exemples d’utilisation : L’observateur est assez répandu en Ruby, surtout dans les composants GUI. Il fournit une manière de réagir aux événements qui se produisent chez d’autres objets sans se coupler à leurs classes.
Identification : Ce patron peut être reconnu dans les méthodes de souscription qui stockent des objets dans une liste et par les appels des objets de cette liste à la méthode update.
Exemple conceptuel
Dans cet exemple, nous allons voir la structure de l’Observateur. Nous allons répondre aux questions suivantes :
- Que contiennent les classes ?
- Quels rôles jouent-elles ?
- Comment les éléments du patron sont-ils reliés ?
main.rb: Exemple conceptuel
# The Subject interface declares a set of methods for managing subscribers.
class Subject
# Attach an observer to the subject.
def attach(observer)
raise NotImplementedError, "#{self.class} has not implemented method '#{__method__}'"
end
# Detach an observer from the subject.
def detach(observer)
raise NotImplementedError, "#{self.class} has not implemented method '#{__method__}'"
end
# Notify all observers about an event.
def notify
raise NotImplementedError, "#{self.class} has not implemented method '#{__method__}'"
end
end
# The Subject owns some important state and notifies observers when the state
# changes.
class ConcreteSubject < Subject
# For the sake of simplicity, the Subject's state, essential to all
# subscribers, is stored in this variable.
attr_accessor :state
# @!attribute observers
# @return [Array<Observer>] attr_accessor :observers private :observers
def initialize
@observers = []
end
# List of subscribers. In real life, the list of subscribers can be stored
# more comprehensively (categorized by event type, etc.).
# @param [Obserser] observer
def attach(observer)
puts 'Subject: Attached an observer.'
@observers << observer
end
# @param [Obserser] observer
def detach(observer)
@observers.delete(observer)
end
# The subscription management methods.
# Trigger an update in each subscriber.
def notify
puts 'Subject: Notifying observers...'
@observers.each { |observer| observer.update(self) }
end
# Usually, the subscription logic is only a fraction of what a Subject can
# really do. Subjects commonly hold some important business logic, that
# triggers a notification method whenever something important is about to
# happen (or after it).
def some_business_logic
puts "\nSubject: I'm doing something important."
@state = rand(0..10)
puts "Subject: My state has just changed to: #{@state}"
notify
end
end
# The Observer interface declares the update method, used by subjects.
class Observer
# Receive update from subject.
def update(_subject)
raise NotImplementedError, "#{self.class} has not implemented method '#{__method__}'"
end
end
# Concrete Observers react to the updates issued by the Subject they had been
# attached to.
class ConcreteObserverA < Observer
# @param [Subject] subject
def update(subject)
puts 'ConcreteObserverA: Reacted to the event' if subject.state < 3
end
end
class ConcreteObserverB < Observer
# @param [Subject] subject
def update(subject)
return unless subject.state.zero? || subject.state >= 2
puts 'ConcreteObserverB: Reacted to the event'
end
end
# The client code.
subject = ConcreteSubject.new
observer_a = ConcreteObserverA.new
subject.attach(observer_a)
observer_b = ConcreteObserverB.new
subject.attach(observer_b)
subject.some_business_logic
subject.some_business_logic
subject.detach(observer_a)
subject.some_business_logic
output.txt: Résultat de l’exécution
Subject: Attached an observer.
Subject: Attached an observer.
Subject: I'm doing something important.
Subject: My state has just changed to: 1
Subject: Notifying observers...
ConcreteObserverA: Reacted to the event
Subject: I'm doing something important.
Subject: My state has just changed to: 10
Subject: Notifying observers...
ConcreteObserverB: Reacted to the event
Subject: I'm doing something important.
Subject: My state has just changed to: 2
Subject: Notifying observers...
ConcreteObserverB: Reacted to the event
