Example of using dispatch_once() in Swift

dispatch_once.swift

import Foundation

var token: dispatch_once_t = 0
func test() {
    dispatch_once(&token) {
        println("This is printed only on the first call to test()")
    }
    println("This is printed for each call to test()")
}

for _ in 0..<4 {
    test()
}

/* Output:

This is printed only on the first call to test()
This is printed for each call to test()
This is printed for each call to test()
This is printed for each call to test()
This is printed for each call to test()

*/

BTW, if we take a good look at the documentation https://developer.apple.com/library/ios/documentation/Performance/Reference/GCD_libdispatch_Ref/
Apple says that variables of this dispatch_once_t must have global or static scope. The result of using this type with automatic or dynamic allocation is undefined.

I don't understand why that code makes sense! The token is a variable, and nothing guarantees it won't be reset:

var token: dispatch_once_t = 0
func test() {
    dispatch_once(&token) {
        print("This is printed only on the first call to test()")
    }
    print("This is printed for each call to test()")
}

test()

token = 0

test()


/* Output:
This is printed only on the first call to test()
This is printed for each call to test()
This is printed only on the first call to test()
This is printed for each call to test()
*/

The Swift 2.x version of this should look like the following:

import Foundation

struct Static {
    static var dispatchOnceToken: dispatch_once_t = 0
}

func test() {
    dispatch_once(&Static.dispatchOnceToken) {
        print("This is printed only on the first call to test()")
    }
    print("This is printed for each call to test()")
}

for _ in 0..<4 {
    test()
}

How about Swift 3.0? compiler nags about dispatch_once . Says should use lazily initialized globals instead.

Swift 3 extension:

import Foundation

public extension DispatchQueue {

    private static var _onceTracker = [String]()

    public class func once(token: String, block:(Void)->Void) {
        objc_sync_enter(self); defer { objc_sync_exit(self) }

        if _onceTracker.contains(token) {
            return
        }

        _onceTracker.append(token)
        block()
    }
}

Usage:

DispatchQueue.once(token: "your-unique-token")  {
    // code to run once
}

In Swift 3 something like this:

class Test {
  static let once: Void = {
    print("only first time")
  }()

  func hi() {
    Test.once
    print("every time")
  }
}

let test = Test()

for i in 0..<3 {
  test.hi()
}