Observer を Swift で
Observer は、 振る舞いに関するデザインパターンの一つで、 オブジェクトが別のオブジェクトに状態の変化を通知できるようにします。
Observer パターンは、 サブスクライバー・インターフェースを実装するいかなるオブジェクトのイベント通知の申し込みと停止をする方法を提供します。
複雑度：
人気度：
使用例： Observer パターンは、 Swift コードではよく見かけます。 特に、 GUI コンポーネントで。 他のオブジェクトで起きるイベントに、 そのクラスに結合することなく、 反応する方法を提供します。
見つけ方： このパターンは、 オブジェクトをリストに保存するサブスクリプション・メソッドと、 そのリスト中のオブジェクトの更新メソッドの呼び出しにより、 識別できます。
以下の例は
Swift Playgroundsで利用できます。
Playgroundバージョンを作成してくれた
Alejandro Mohamadに感謝します。
概念的な例
この例は、 Observer デザインパターンの構造を説明するためのものです。 以下の質問に答えることを目的としています：
- どういうクラスからできているか？
- それぞれのクラスの役割は？
- パターンの要素同士はどう関係しているのか？
ここでパターンの構造を学んだ後だと、 これに続く、 現実世界の Swift でのユースケースが理解しやすくなります。
Example.swift: 概念的な例
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: 実行結果
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...
現実的な例
Example.swift: 現実的な例
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: 実行結果
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
