How to do View Controller callback in Swift

Question

I am trying to do a callback from a child view controller back to its parent. The value passed back is printed successfully but I then get an EXC_BAD_INSTRUCTION.

In the parent view controller I construct the child controller like so:

let storyboard : UIStoryboard = UIStoryboard(name: "Main", bundle: nil)
if let vc = storyboard.instantiateViewControllerWithIdentifier("dataEntryView") as? DataEntryViewController {
    dataEntryVC = vc
    vc.callback = calculateHeartRate
    self.presentViewController(dataEntryVC!, animated: true, completion: nil)
}

I call back from the child controller like this:

@IBAction func done(sender : AnyObject) {
    dismissModalViewControllerAnimated(true)
    if let cb = callback {
        cb(beatCount)
    }
}

To this function in the parent controller:

func calculateHeartRate(beats : Int?) {
    println("Beats = \(beats)")

    if beats {
        let heartrate = 60/secondsCount * beats!

        println("Heart rate \(heartrate)")
    }
}

The println correctly outputs to the console and then I get an exception on the "if beats {" line.

Is there something inherently wrong with doing a callback like this?

Answer 1

I finally figured this out. It had nothing to do with the Optionals syntax I was using or with using a closure vs a function.

The root cause is that @IBOutlets are weak references.

When I changed my "done" method to immediate cache the value entered in a variable everything started working :)

---

Initialize

class DataEntryViewController: UIViewController {
    @IBOutlet var beatsText : UITextField
    var callback : ((Int?) -> Void)?

    var beatCount : Int?

    @IBAction func done(sender : AnyObject) {
        beatCount = beatsText.text.toInt()

        dismissModalViewControllerAnimated(true)
        if let cb = callback {
            cb(beatCount)
        }
    }

---

Usage

let dataEntryViewController = DataEntryViewController()
dataEntryViewController.callback = {
    (beatCount) in
    print(beatCount)
}

Answer 2

You should unwrap the optional like

   if let beatCount = beats {
        let heartrate = 60/secondsCount * beatCount

        println("Heart rate \(heartrate)")
    }

But i would suggest you to use Closures instead of functions. Even though functions are special kind of closure, use of closure make sense just like completion blocks in ObjC