a simple performSegue throws and error in prepare for segue func

Question

Maybe something silly, but I am trying to simply move from a tableviewController to a viewController not passing any data just a simple move using

@IBAction func requestPanel(_ sender: Any) {

   performSegue(withIdentifier: "sendMail", sender: AnyObject)
}

And then I get an error in a prepare for segue func.

But what does that have to no with my other segue?

I understand that that segue will have trouble since there is no data.

As you see here segue is above prepare for segue.

@IBAction func requestPanel(_ sender: Any) {

    self.performSegue(withIdentifier: "sendMail", sender: AnyObject.self)
}

override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
    var indexPath: IndexPath = self.tableView.indexPathForSelectedRow!
    let desti = segue.destination as! DeviceDetailTableViewController

    let selectedRecord = onlineDevices[indexPath.row]

    let panelWidth = selectedRecord.object(forKey: "panelwidth") as? String
    let panelHight = selectedRecord.object(forKey: "panelhight") as? String
    let panelPitch = selectedRecord.object(forKey: "panelpitch") as? String
    let panelPower = selectedRecord.object(forKey: "panelpower") as? String
    let panelWeight = selectedRecord.object(forKey: "panelweight") as? String
    let panelMaker = selectedRecord.object(forKey: "maker") as? String
    let panelModel = selectedRecord.object(forKey: "model") as?      String

    desti.pawidth = panelWidth!
    desti.pahight = panelHight!
    desti.papitch = panelPitch!
    desti.papower = panelPower!
    desti.weight = panelWeight!
    desti.maker = panelMaker!
    desti.model = panelModel!
}



 override func numberOfSections(in tableView: UITableView) -> Int {
 return 1
 }
 override func tableView(_ tableView: UITableView,   numberOfRowsInSection section: Int) -> Int {
 return onlineDevices.count
 }
   override func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
    let cell = tableView.dequeueReusableCell(withIdentifier: "DeviceCell", for: indexPath)

    // Configure the cell...

    let noteRecord: CKRecord = onlineDevices[(indexPath as IndexPath).row]

    cell.textLabel?.text = noteRecord.value(forKey: "maker") as? String
    cell.detailTextLabel?.text = noteRecord.value(forKey: "model") as? String


    return cell
}


}

Answer 1

The error message is clear. You are saying tableView.indexPathForSelectedRow at a time when no row is selected. Therefore that value is nil , and you crash when you force-unwrap it.

You wrote your original prepareForSegue when your only segue was triggered by the user selecting a row of the table. But this segue is not triggered by the user selecting a row of the table; it is triggered by the user tapping a button. You have to account for that difference.

(This is a major flaw in the whole architecture: all segues from a given view controller flow into the same prepareForSegue , which becomes a bottleneck.)

Answer 2

All segues in a viewController will go through the same prepare(for:sender:) method. You need to use the segue.identifier to handle the different segues differently.

Your sendMail segue comes from a UIBarButton action instead of from a selected row, so check for that segue and handle it separately:

override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
    if segue.identifier == "sendMail" {
        // nothing extra to be done here
    } else {
        // do your normal path here
        if let indexPath = self.tableView.indexPathForSelectedRow {
            // you have a valid selected row so proceed
        }
    }
}