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Swift 单例模式讲解和代码示例
单例是一种创建型设计模式， 让你能够保证一个类只有一个实例， 并提供一个访问该实例的全局节点。
单例拥有与全局变量相同的优缺点。 尽管它们非常有用， 但却会破坏代码的模块化特性。
在某些其他上下文中， 你不能使用依赖于单例的类。 你也将必须使用单例类。 绝大多数情况下， 该限制会在创建单元测试时出现。
复杂度：
流行度：
使用示例： 许多开发者将单例模式视为一种反模式。 因此它在 Swift 代码中的使用频率正在逐步减少。
识别方法： 单例可以通过返回相同缓存对象的静态构建方法来识别。
以下示例可在 Swift Playgrounds 上使用。
感谢 Alejandro Mohamad 创建了Playground版本。
概念示例
本例说明了单例设计模式的结构并重点回答了下面的问题：
- 它由哪些类组成？
- 这些类扮演了哪些角色？
- 模式中的各个元素会以何种方式相互关联？
了解该模式的结构后， 你可以更容易地理解下面基于真实世界的 Swift 应用案例。
Example.swift: 概念示例
import XCTest /// The Singleton class defines the `shared` field that lets clients access the /// unique singleton instance. class Singleton { /// The static field that controls the access to the singleton instance. /// /// This implementation let you extend the Singleton class while keeping /// just one instance of each subclass around. static var shared: Singleton = { let instance = Singleton() // ... configure the instance // ... return instance }() /// The Singleton's initializer should always be private to prevent direct /// construction calls with the `new` operator. private init() {} /// Finally, any singleton should define some business logic, which can be /// executed on its instance. func someBusinessLogic() -> String { // ... return "Result of the 'someBusinessLogic' call" } } /// Singletons should not be cloneable. extension Singleton: NSCopying { func copy(with zone: NSZone? = nil) -> Any { return self } } /// The client code. class Client { // ... static func someClientCode() { let instance1 = Singleton.shared let instance2 = Singleton.shared if (instance1 === instance2) { print("Singleton works, both variables contain the same instance.") } else { print("Singleton failed, variables contain different instances.") } } // ... } /// Let's see how it all works together. class SingletonConceptual: XCTestCase { func testSingletonConceptual() { Client.someClientCode() } } Output.txt: 执行结果
Singleton works, both variables contain the same instance. 真实世界示例
Example.swift: 真实世界示例
import XCTest /// Singleton Design Pattern /// /// Intent: Ensure that class has a single instance, and provide a global point /// of access to it. class SingletonRealWorld: XCTestCase { func testSingletonRealWorld() { /// There are two view controllers. /// /// MessagesListVC displays a list of last messages from a user's chats. /// ChatVC displays a chat with a friend. /// /// FriendsChatService fetches messages from a server and provides all /// subscribers (view controllers in our example) with new and removed /// messages. /// /// FriendsChatService is used by both view controllers. It can be /// implemented as an instance of a class as well as a global variable. /// /// In this example, it is important to have only one instance that /// performs resource-intensive work. let listVC = MessagesListVC() let chatVC = ChatVC() listVC.startReceiveMessages() chatVC.startReceiveMessages() /// ... add view controllers to the navigation stack ... } } class BaseVC: UIViewController, MessageSubscriber { func accept(new messages: [Message]) { /// Handles new messages in the base class } func accept(removed messages: [Message]) { /// Hanldes removed messages in the base class } func startReceiveMessages() { /// The singleton can be injected as a dependency. However, from an /// informational perspective, this example calls FriendsChatService /// directly to illustrate the intent of the pattern, which is: "...to /// provide the global point of access to the instance..." FriendsChatService.shared.add(subscriber: self) } } class MessagesListVC: BaseVC { override func accept(new messages: [Message]) { print("MessagesListVC accepted 'new messages'") /// Handles new messages in the child class } override func accept(removed messages: [Message]) { print("MessagesListVC accepted 'removed messages'") /// Handles removed messages in the child class } override func startReceiveMessages() { print("MessagesListVC starts receive messages") super.startReceiveMessages() } } class ChatVC: BaseVC { override func accept(new messages: [Message]) { print("ChatVC accepted 'new messages'") /// Handles new messages in the child class } override func accept(removed messages: [Message]) { print("ChatVC accepted 'removed messages'") /// Handles removed messages in the child class } override func startReceiveMessages() { print("ChatVC starts receive messages") super.startReceiveMessages() } } /// Protocol for call-back events protocol MessageSubscriber { func accept(new messages: [Message]) func accept(removed messages: [Message]) } /// Protocol for communication with a message service protocol MessageService { func add(subscriber: MessageSubscriber) } /// Message domain model struct Message { let id: Int let text: String } class FriendsChatService: MessageService { static let shared = FriendsChatService() private var subscribers = [MessageSubscriber]() func add(subscriber: MessageSubscriber) { /// In this example, fetching starts again by adding a new subscriber subscribers.append(subscriber) /// Please note, the first subscriber will receive messages again when /// the second subscriber is added startFetching() } func startFetching() { /// Set up the network stack, establish a connection... /// ...and retrieve data from a server let newMessages = [Message(id: 0, text: "Text0"), Message(id: 5, text: "Text5"), Message(id: 10, text: "Text10")] let removedMessages = [Message(id: 1, text: "Text0")] /// Send updated data to subscribers receivedNew(messages: newMessages) receivedRemoved(messages: removedMessages) } } private extension FriendsChatService { func receivedNew(messages: [Message]) { subscribers.forEach { item in item.accept(new: messages) } } func receivedRemoved(messages: [Message]) { subscribers.forEach { item in item.accept(removed: messages) } } } Output.txt: 执行结果
MessagesListVC starts receive messages MessagesListVC accepted 'new messages' MessagesListVC accepted 'removed messages' ======== At this point, the second subscriber is added ====== ChatVC starts receive messages MessagesListVC accepted 'new messages' ChatVC accepted 'new messages' MessagesListVC accepted 'removed messages' ChatVC accepted 'removed messages' 
