使用Segue从先前的View Controller传递到另一个View Controller时，数据变为Nil

Question

I am about to pass data from a ViewController going to another ViewController using segue. When checking the data(event) from a variable thru breakpoint in the 1st View Controller the data(event) is not nil. But when I checked the 2nd View Controller the data(event) is nil . I am confuse whether if the reason is, I have error in my codes or because of the error appeared in my console that says Unable to insert COPY_SEND . Hope I can get some help from you. Thank you

Segue from First View Controller

override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
    if segue.identifier == "DashBoardViewController" {
        let dashBoardController = segue.destination as! DashBoardViewController
        dashBoardController.self.event = event
        dashBoardController.self.passcode = passcode
    }
}

Event and Passcode Turns Nil

override func viewDidLoad() {
    super.viewDidLoad()
    guard event != nil, passcode != nil else {
        _ = SCLAlertView(appearance: appearance).showError("No Event Details", subTitle: "There's no event details, please logout and try again")
        return
    }
    showEventDetails()
}

showEventDetails

func showEventDetails() {
    DispatchQueue.main.async{
        self.eventNameLabel.text =  "\(self.event.name.uppercased())"
        self.locationLabel.text =  "\(self.event.location.uppercased())"
        if let dateStringFromDate = getFormattedStringFromDate(date: (self.event.startDateTime), formatString: "MMMM dd, yyyy/ hh:mm a") {
            self.dateTimeLabel.text = dateStringFromDate
        } else {
            self.dateTimeLabel.text = "-"
        }
    }
}

Answer 1

I am assuming you linked the segue which goes to DashBoardViewController on your submitButton by Interface Builder, which means when you are tapping on the submit button, the @IBAction func submitButton(_ sender: UIButton) { } gets called, where you check if your passcode is good to go and if so you are calling validateEventPasscode() which calls an API endpoint (asynchronous) and only there you are populating the self.event = event (line 187 in ViewController.swift).

Now, what really happens is that when you link a segue from a button by IB (interface builder), there will be a perform segue internally which we have to stop by overriding the following method in ViewController.swift : source

func shouldPerformSegue(withIdentifier identifier: String, 
                 sender: Any?) -> Bool {
    return false
}

This way your call from line 190 - ViewController.swift:

self.performSegue(withIdentifier: "showEventDashboard", sender: self)

is the one that fires:

override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
      if segue.identifier == "DashBoardViewController" {
         let dashBoardController = segue.destination as!  DashBoardViewController
         dashBoardController.event = event
         dashBoardController.passcode = passcode
     }
   }

You can test my theory by placing three breakpoints in ViewController.swift :

1. line 134 at validateEventPasscode() from submitButton IBAction func;
2. line 190 at self.performSegue(withIdentifier: "showEventDashboard", sender: self) from validateEventPasscode() func;
3. line 108 at dashBoardController.event = event from prepare(for segue, sender) func;

Buggy order of execution: 1, 3, 2 - at the moment this I would expect if my theory is correct; Desired order of execution: 1, 2, 3.

Long story short, you populate your self.event after you perfomSegue and go to the next screen , that's why your event is nil in the next VC .

I used as reference the ViewController.swift file from your repo: ViewController.swift

Hope it helps, cheers!

Answer 2

Replace your prepareForSegue method in FirstViewController with this

override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
    if segue.identifier == "DashBoardViewController" {
        let dashBoardController = segue.destination as! DashBoardViewController
        dashBoardController.event = event
        dashBoardController.passcode = passcode
    }
}

you don't need to write

dashBoardController.self.event = event
dashBoardController.self.passcode = passcode

Just remove self from above two lines.