Salut, je viens de réduire le prix de tous les produits. Préparons nos compétences en programmation pour l'ère post-COVID. 
Observateur en Swift
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.
Utilisation du patron de conception en Swift
Complexité :
Popularité :
Exemples d’utilisation : L’observateur est assez répandu en Swift, 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 et répondre aux questions suivantes :
- Que contiennent les classes ?
- Quels rôles jouent-elles ?
- Comment les éléments du patron sont-ils reliés ?
Après avoir étudié la structure du patron, vous pourrez plus facilement comprendre l’exemple suivant qui est basé sur un cas d’utilisation réel en Swift.
Example.swift: Exemple conceptuel
import XCTest
/// The Subject owns some important state and notifies observers when the state
/// changes.
class Subject {
/// For the sake of simplicity, the Subject's state, essential to all
/// subscribers, is stored in this variable.
var state: Int = { return Int(arc4random_uniform(10)) }()
/// @var array List of subscribers. In real life, the list of subscribers
/// can be stored more comprehensively (categorized by event type, etc.).
private lazy var observers = [Observer]()
/// The subscription management methods.
func attach(_ observer: Observer) {
print("Subject: Attached an observer.\n")
observers.append(observer)
}
func detach(_ observer: Observer) {
if let idx = observers.firstIndex(where: { $0 === observer }) {
observers.remove(at: idx)
print("Subject: Detached an observer.\n")
}
}
/// Trigger an update in each subscriber.
func notify() {
print("Subject: Notifying observers...\n")
observers.forEach({ $0.update(subject: self)})
}
/// 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).
func someBusinessLogic() {
print("\nSubject: I'm doing something important.\n")
state = Int(arc4random_uniform(10))
print("Subject: My state has just changed to: \(state)\n")
notify()
}
}
/// The Observer protocol declares the update method, used by subjects.
protocol Observer: class {
func update(subject: Subject)
}
/// Concrete Observers react to the updates issued by the Subject they had been
/// attached to.
class ConcreteObserverA: Observer {
func update(subject: Subject) {
if subject.state < 3 {
print("ConcreteObserverA: Reacted to the event.\n")
}
}
}
class ConcreteObserverB: Observer {
func update(subject: Subject) {
if subject.state >= 3 {
print("ConcreteObserverB: Reacted to the event.\n")
}
}
}
/// Let's see how it all works together.
class ObserverConceptual: XCTestCase {
func testObserverConceptual() {
let subject = Subject()
let observer1 = ConcreteObserverA()
let observer2 = ConcreteObserverB()
subject.attach(observer1)
subject.attach(observer2)
subject.someBusinessLogic()
subject.someBusinessLogic()
subject.detach(observer2)
subject.someBusinessLogic()
}
}
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: 4
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...
ConcreteObserverA: Reacted to the event.
Subject: Detached an observer.
Subject: I'm doing something important.
Subject: My state has just changed to: 8
Subject: Notifying observers...
Analogie du monde réel
Example.swift: Analogie du monde réel
import XCTest
class ObserverRealWorld: XCTestCase {
func test() {
let cartManager = CartManager()
let navigationBar = UINavigationBar()
let cartVC = CartViewController()
cartManager.add(subscriber: navigationBar)
cartManager.add(subscriber: cartVC)
let apple = Food(id: 111, name: "Apple", price: 10, calories: 20)
cartManager.add(product: apple)
let tShirt = Clothes(id: 222, name: "T-shirt", price: 200, size: "L")
cartManager.add(product: tShirt)
cartManager.remove(product: apple)
}
}
protocol CartSubscriber: CustomStringConvertible {
func accept(changed cart: [Product])
}
protocol Product {
var id: Int { get }
var name: String { get }
var price: Double { get }
func isEqual(to product: Product) -> Bool
}
extension Product {
func isEqual(to product: Product) -> Bool {
return id == product.id
}
}
struct Food: Product {
var id: Int
var name: String
var price: Double
/// Food-specific properties
var calories: Int
}
struct Clothes: Product {
var id: Int
var name: String
var price: Double
/// Clothes-specific properties
var size: String
}
class CartManager {
private lazy var cart = [Product]()
private lazy var subscribers = [CartSubscriber]()
func add(subscriber: CartSubscriber) {
print("CartManager: I'am adding a new subscriber: \(subscriber.description)")
subscribers.append(subscriber)
}
func add(product: Product) {
print("\nCartManager: I'am adding a new product: \(product.name)")
cart.append(product)
notifySubscribers()
}
func remove(subscriber filter: (CartSubscriber) -> (Bool)) {
guard let index = subscribers.firstIndex(where: filter) else { return }
subscribers.remove(at: index)
}
func remove(product: Product) {
guard let index = cart.firstIndex(where: { $0.isEqual(to: product) }) else { return }
print("\nCartManager: Product '\(product.name)' is removed from a cart")
cart.remove(at: index)
notifySubscribers()
}
private func notifySubscribers() {
subscribers.forEach({ $0.accept(changed: cart) })
}
}
extension UINavigationBar: CartSubscriber {
func accept(changed cart: [Product]) {
print("UINavigationBar: Updating an appearance of navigation items")
}
open override var description: String { return "UINavigationBar" }
}
class CartViewController: UIViewController, CartSubscriber {
func accept(changed cart: [Product]) {
print("CartViewController: Updating an appearance of a list view with products")
}
open override var description: String { return "CartViewController" }
}
Output.txt: Résultat de l’exécution
CartManager: I'am adding a new subscriber: UINavigationBar
CartManager: I'am adding a new subscriber: CartViewController
CartManager: I'am adding a new product: Apple
UINavigationBar: Updating an appearance of navigation items
CartViewController: Updating an appearance of a list view with products
CartManager: I'am adding a new product: T-shirt
UINavigationBar: Updating an appearance of navigation items
CartViewController: Updating an appearance of a list view with products
CartManager: Product 'Apple' is removed from a cart
UINavigationBar: Updating an appearance of navigation items
CartViewController: Updating an appearance of a list view with products
