Some patterns for Singletons in Swift 2

Singletons.swift

// The easiest pattern. Just use a `static let`, which gets lazy loaded as a bonus.
class Singleton {
    static let sharedInstance = Singleton()
    
    func helloWorld() {
        print("Hello world")
    }
}

Singleton.sharedInstance.helloWorld() // prints "Hello world"


// What if you do want a Singleton that you don't pass around, but still want to change it when testing your code?
// With `setDefaultInstance`, you can replace what `sharedInstance` returns. 
// But, this makes it not strictly a singleton, and the pattern is dangerous if you start doing this in production code.
class ChangeableSingleton {
    private(set) static var sharedInstance = ChangeableSingleton()
    static func setDefaultInstance(instance: ChangeableSingleton) {
        sharedInstance = instance
    }

    var message: String
    
    required init(message: String = "Can I be shorter?") {
        self.message = message
    }
    
    func helloWorld() {
        print(message)
    }
}

ChangeableSingleton.sharedInstance.helloWorld() // prints "Can I be shorter?"
ChangeableSingleton.setDefaultInstance(ChangeableSingleton(message: "Shorter is better!"))
ChangeableSingleton.sharedInstance.helloWorld() // prints "Shorter is better!"

//~~

// So what if I have a ton of Singletons? I probably shouldn't, but I really want to!
// Let's make a Protocol with Associated Types (PAT)!
protocol Singletonable {
    associatedtype SingletonType
    static var sharedInstance: SingletonType { get }
}

class PATSingleton: Singletonable {
    private(set) static var sharedInstance = PATSingleton()
}

PATSingleton.sharedInstance

//~~

protocol Printable {
    var message: String { get }
    func helloWorld()
}

extension Printable {
    func helloWorld() {
        print(message)
    }
}

class PATSingletonWithParameters: Singletonable, Printable {
    private(set) static var sharedInstance = PATSingletonWithParameters()
    private(set) var message: String
    
    init(message: String = "Message") {
        self.message = message
    }
}

PATSingletonWithParameters.sharedInstance.helloWorld()