Singleton in Swift
Singleton is a creational design pattern, which ensures that only one object of its kind exists and provides a single point of access to it for any other code.
Singleton has almost the same pros and cons as global variables. Although they’re super-handy, they break the modularity of your code.
You can’t just use a class that depends on a Singleton in some other context, without carrying over the Singleton to the other context. Most of the time, this limitation comes up during the creation of unit tests.
Complexity:
Popularity:
Usage examples: A lot of developers consider the Singleton pattern an antipattern. That’s why its usage is on the decline in Swift code.
Identification: Singleton can be recognized by a static creation method, which returns the same cached object.
Conceptual Example
This example illustrates the structure of the Singleton design pattern and focuses on the following questions:
- What classes does it consist of?
- What roles do these classes play?
- In what way the elements of the pattern are related?
After learning about the pattern’s structure it’ll be easier for you to grasp the following example, based on a real-world Swift use case.
Example.swift: Conceptual example
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: Execution result
Singleton works, both variables contain the same instance.
Real World Example
Example.swift: Real world example
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]) {
/// handle new messages in the base class
}
func accept(removed messages: [Message]) {
/// handle 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'")
/// handle new messages in the child class
}
override func accept(removed messages: [Message]) {
print("MessagesListVC accepted 'removed messages'")
/// handle 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'")
/// handle new messages in the child class
}
override func accept(removed messages: [Message]) {
print("ChatVC accepted 'removed messages'")
/// handle 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: Execution result
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'
