Observer es un patrón de diseño de comportamiento que permite a un objeto notificar a otros objetos sobre cambios en su estado.
El patrón Observer proporciona una forma de suscribirse y cancelar la subscripción a estos eventos para cualquier objeto que implementa una interfaz suscriptora.
Ejemplos de uso: El patrón Observer es bastante habitual en el código Swift, sobre todo en los componentes GUI. Proporciona una forma de reaccionar a los eventos que suceden en otros objetos, sin acoplarse a sus clases.
Identificación: El patrón puede reconocerse por los métodos de subscripción, que almacenan objetos en una lista, y por las llamadas al método de actualización emitidas a todos los objetos de esa lista.
Este ejemplo ilustra la estructura del patrón de diseño Observer y se centra en las siguientes preguntas:
¿De qué clases se compone?
¿Qué papeles juegan esas clases?
¿De qué forma se relacionan los elementos del patrón?
Después de conocer la estructura del patrón, será más fácil comprender el siguiente ejemplo basado en un caso de uso real de Swift.
Example.swift: Ejemplo conceptual
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: Resultado de la ejecución
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...
Ejemplo del mundo real
Example.swift: Ejemplo del mundo real
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: Resultado de la ejecución
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
