Swift 3 - Prepare Segue

Question

I have 3 scenes in my storyboard. My initial View Controller is a Navigation Controller, then there is a relationship root view controller to a UI ViewController (view controller a) and then I have a push segue from a button in the ViewController to the third ViewController (view controller b) in the scene. I have given the push segue an identifier. Now I am trying to prepare my segue in the 2nd view controller (view controller a) like so:

override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
        if segue.identifier == "HistorySegue" {
            if let viewController = segue.destination as? HistoryController {
                viewController.detailItem = barcodeInt as AnyObject
            }
        }
    }

However when I run this code and push the button in controller a I get the following error:

fatal error: attempt to bridge an implicitly unwrapped optional containing nil

What am I doing wrong?

Answer 1

I had the same issue. The logic is that one first prepares the segue (loads the UIViewController referenced by the container view), assigns it to a variable, and then uses it in viewDidLoad(). This code should work:

Swift 4.2

// usually an IBoutlet
var viewController: HistoryController!

override func viewDidLoad() {
    super.viewDidLoad()

    viewController.detailItem = barcodeInt as AnyObject
}

override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
    if segue.identifier == "HistorySegue"
    { viewController = segue.destination as! HistoryController }
}

detailItem could possibly be defined as an IBoutlet in HistoryController, it depends on the OP code. In my case, where I had two simple container views with a label each inside, this has been the final working code for the main view controller class:

import UIKit

class ViewController: UIViewController {

    @IBOutlet var firstView: ReusableViewController!
    @IBOutlet var secondView: ReusableViewController!

    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view, typically from a nib.

        firstView.myLabel.text = "My 1st reuse!!!"
        secondView.myLabel.text = "And that's the 2nd!"
    }

    override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
        if (segue.identifier == "segueFirstView")
        { firstView = (segue.destination as! ReusableViewController) }
        if (segue.identifier == "segueSecondView")
        { secondView = (segue.destination as! ReusableViewController) }
    }
}

With that I could finally change the text of the two different UILabel directly from the main controller!

For a detailed explanation of how to use the container views one may check this S.O. answer.

Answer 2

Replace your code with the following, it will not crash at least.

override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
        if segue.identifier == "HistorySegue" {
            if let viewController = segue.destination as? HistoryController {
              if(barcodeInt != nil){
                viewController.detailItem = barcodeInt as AnyObject
               }
            }
        }
    }

Answer 3

It must be that barcodeInt is defined as an implicitly unwrapped optional, like:

var barcodeInt:Int!

In that case, if it is nil when assigning it to detailItem, because of the !, swift takes your word for it that there is a non-nil value in there and dereferences it. That's a runtime error. Your best bet is to avoid ! in code you write (it's ok to leave the Apple generated code for IBOutlets, for example) if at all possible and learn more about optionals before going back to implicitly unwrapped optionals. And then, still use them sparingly.

Safer code for your situation:

if let viewController = segue.destination as? HistoryController, 
   let barcodeInt = barcodeInt as? AnyObject {

   viewController.detailItem = barcodeInt
} else {
   NSLog("Error: expected barcodeInt to be set") 
}