Swift 3: dispatch_once workarounds overview

GlobalOnceClass.swift

import Foundation

private let justAOneTimeThing: () = {
    print("\(GlobalOnceClass.self): Do This once")
}()

public final class GlobalOnceClass {
    
    public init() {
    }
    
    public func someAction() {
        justAOneTimeThing
    }
}

InitializeOnceClass.swift

import Foundation

public final class InitializeOnceClass: NSObject {
    
    private static var __once: () = {
        print("\(InitializeOnceClass.self): Do This once")
    }()
    
    public override class func initialize() {
        _ = InitializeOnceClass.__once
    }
    
    public func someAction() {
        _ = InitializeOnceClass.__once
    }
}

LazyOnceClass.swift

import Foundation

public final class LazyOnceClass {
    
    private lazy var foo: Void = {
        print("\(LazyOnceClass.self): Do this once")
    }()
    
    public init() {
    }
    
    public func someAction() {
        _ = foo
    }
}

Test_Once_Workaround.xcplaygroundpage

//: [Previous](@previous)

import Foundation

print("> Test: LazyOnceClass")
let a = LazyOnceClass()
a.someAction()
a.someAction()
a.someAction()
let b = LazyOnceClass()
b.someAction()
b.someAction()
b.someAction()

// prints:
// "LazyOnceClass: Do this once"
// "LazyOnceClass: Do this once"

print("> Test: GlobalOnceClass")
let c = GlobalOnceClass()
c.someAction()
c.someAction()
c.someAction()
let d = GlobalOnceClass()
d.someAction()
d.someAction()
d.someAction()

// prints:
// "GlobalOnceClass: Do This once"

print("> Test: InitializeOnceClass")
let e = InitializeOnceClass()
e.someAction()
e.someAction()
e.someAction()
let f = InitializeOnceClass()
f.someAction()
f.someAction()
f.someAction()

// prints:
// "InitializeOnceClass: Do This once"

//: [Next](@next)