ios - Can't cast value of type UIViewController to PatternDetailViewController

Question

Am trying to downcast a view controller to a detail view controller but can't. Am using Core Data (for the first time). The error is in the prepareForSegue method and reads: "Could not cast value of type 'UIViewController' (0x1b81cdc) to 'Patternz.PatternDetailViewController' (0x32488). (lldb) "

Would appreciate an explanation of why it doesn't work. Here are the files.

ViewController.swift

import UIKit
import CoreData

class ViewController: UIViewController, UITableViewDataSource, UITableViewDelegate {

    @IBOutlet weak var tableView: UITableView!

    var patterns : [Pattern] = []

    var selectedPattern : Pattern? = nil


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

        createTestPatterns()

        var context = (UIApplication.sharedApplication().delegate as! AppDelegate).managedObjectContext!

        var request = NSFetchRequest(entityName: "Pattern")

        var results = context.executeFetchRequest(request, error: nil)

        if results != nil {
            self.patterns = results! as! [Pattern]
        }
    }

    func createTestPatterns() {
        var context = (UIApplication.sharedApplication().delegate as! AppDelegate).managedObjectContext!

        var pattern = NSEntityDescription.insertNewObjectForEntityForName("Pattern", inManagedObjectContext: context) as! Pattern
        pattern.name = "Dress Shirt"
        pattern.frontimage = UIImageJPEGRepresentation(UIImage(named: "examplePattern.jpg"), 1)
        context.save(nil)
    }


    func tableView(tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
        return self.patterns.count
    }

    func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell {
        var cell = UITableViewCell()
        var pattern = self.patterns[indexPath.row]
        cell.textLabel!.text = pattern.name
        cell.imageView!.image = UIImage(data: pattern.frontimage)
        return cell
    }

    func tableView(tableView: UITableView, didSelectRowAtIndexPath indexPath: NSIndexPath) {
        self.selectedPattern = self.patterns[indexPath.row]
        self.performSegueWithIdentifier("patternDetailSegue", sender: self)
    }

    override func prepareForSegue(segue: UIStoryboardSegue, sender: AnyObject?) {
        if segue.identifier == "patternDetailSegue" {
            var detailViewController = segue.destinationViewController as! PatternDetailViewController // Could not cast value of type 'UIViewController' to 'Patternz.PatternDetailViewController'
            detailViewController.pattern = self.selectedPattern
        }
    }



}

PatternDetailViewController.swift

import UIKit

class PatternDetailViewController: UIViewController {

    var pattern : Pattern? = nil

    override func viewDidLoad() {
        super.viewDidLoad()

        // Do any additional setup after loading the view.

        self.navigationItem.title = self.pattern!.name
    }



}

Answer 1

The problem, as you have said, is in these lines:

if segue.identifier == "patternDetailSegue" {
    var detailViewController = segue.destinationViewController as! PatternDetailViewController 
    // Could not cast value of type 'UIViewController' to 'Patternz.PatternDetailViewController'

The error message tells you that the destinationViewController of this segue is not, in fact, a PatternDetailViewController. You may think it is, but it isn't. You need to examine this segue in the storyboard and see what's really at the destination end of it.

The fact that the error message describes it as a UIViewController makes me suspect that you forgot to enter any view controller type in this view controller's Identity inspector in the storyboard: