How to pass callback functions in Swift

Question

I have a simple class which init method takes an Int and a callback function.

class Timer {   var timer = NSTimer()   var handler: (Int) -> Void    init(duration: Int, handler: (Int) -> Void) {       self.duration = duration       self.handler = handler       self.start()   }   @objc func someMethod() {               self.handler(10)   } } 

Then in the ViewController I have this:

var timer = Timer(duration: 5, handler: displayTimeRemaining) func displayTimeRemaining(counter: Int) -> Void {     println(counter) } 

This doesn't work, I get the following:

'Int' is not a subtype of 'SecondViewController'

Edit 1: Adding full code.

Timer.swift

import UIKit  class Timer {     lazy var timer = NSTimer()     var handler: (Int) -> Void      let duration: Int     var elapsedTime: Int = 0      init(duration: Int, handler: (Int) -> Void) {         self.duration = duration         self.handler = handler         self.start()     }      func start() {         self.timer = NSTimer.scheduledTimerWithTimeInterval(1.0,             target: self,             selector: Selector("tick"),             userInfo: nil,             repeats: true)     }      func stop() {         timer.invalidate()     }      func tick() {         self.elapsedTime++          self.handler(10)          if self.elapsedTime == self.duration {             self.stop()         }     }      deinit {         self.timer.invalidate()     } } 

SecondViewController.swift

import UIKit  class SecondViewController: UIViewController {      @IBOutlet var cycleCounter: UILabel!     var number = 0      var timer = Timer(duration: 5, handler: displayTimeRemaining)      override func viewDidLoad() {         super.viewDidLoad()         // Do any additional setup after loading the view, typically from a nib.     }      override func didReceiveMemoryWarning() {         super.didReceiveMemoryWarning()         // Dispose of any resources that can be recreated.     }      @IBAction func btnIncrementCycle_Click(sender: UIButton){         cycleCounter.text = String(++number)         println(number)     }      func displayTimeRemaining(counter: Int) -> Void {         println(counter)     } } 

I just started with Swift so I'm very green. How are you supposed to pass callbacks? I've looked at examples and this should be working I think. Is my class defined incorrectly for the way I'm passing the callback?

Thanks

Answer 1

Ok, now with the full code I was able to replicate your issue. I'm not 100% sure what the cause is but I believe it has something to do with referencing a class method (displayTimeRemaining) before the class was instantiated. Here are a couple of ways around this:

Option 1: Declare the handler method outside of the SecondViewController class:

func displayTimeRemaining(counter: Int) -> Void {   println(counter) }   class SecondViewController: UIViewController {        // ...   var timer = Timer(duration: 5, handler: displayTimeRemaining) 

Option 2: Make displayTimeRemaining into a type method by adding the class keyword to function declaration.

class SecondViewController: UIViewController {    var timer: Timer = Timer(duration: 5, handler: SecondViewController.displayTimeRemaining)    class func displayTimeRemaining(counter: Int) -> Void {     println(counter)   } 

Option 3: I believe this will be the most inline with Swift's way of thinking - use a closure:

class SecondViewController: UIViewController {  var timer: Timer = Timer(duration: 5) {         println($0) //using Swift's anonymous method params     }