Swift: Strange downcasting failure

Question

I have a rather strange case of downcasting failure that I cannot understand.

I have almost identical code in two UITableViewControllers. ProjectTableViewController displays list of Project, and its datasource is [Project]. NewsfeedsTableViewController displays list of Newsfeed, but Newsfeed can contain different types of source data, including Project.

Depending on the type of the source data, each cell of NewsfeedsTableViewController is downcasted to appropriate subclass of UITableViewCell.

ProjectTableViewCell a subclass of UITableViewCell, and is used in both ProjectTableViewController and NewsfeedsTableViewController.

Now the interesting thing is, the code works without issue in ProjectTableViewController but crashes in NewsfeedsTableViewController, giving following error message:

Could not cast value of type 'UITableViewCell' (0x102e68128) to 'sample_app.ProjectTableViewCell' (0x1010ce6d0).

I have following codes in each class:

ProjectTableViewController.swift

override func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell {
    let project = projects[indexPath.row]
    let cell = tableView.dequeueReusableCellWithIdentifier("ProjectTableViewCell") as! ProjectTableViewCell

    cell.projectTitle.text = project.title
    cell.projectKeywords.text = project.keywords

    return cell
}


NewsfeedsTableViewController.swift

override func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell {
    let newsfeed = self.newsfeeds[indexPath.row]
    switch newsfeed.newsfeedableType {
    case "Project":
        let cell = tableView.dequeueReusableCellWithIdentifier("NewsfeedTableViewCell") as! ProjectTableViewCell
        let source = newsfeed.newsfeedable as! Project
        cell.projectTitle.text = source.title
        cell.projectKeywords.text = source.keywords
        return cell

    default:
        let cell = tableView.dequeueReusableCellWithIdentifier("NewsfeedTableViewCell")!
        cell.textLabel!.text = newsfeed.newsfeedableType + String(newsfeed.id)
        return cell
    }
}

I would love to understand what's causing this issue.

http://stackoverflow.com/questions/30429269/cast-value-of-type-uitableviewcell-to-custom-cell or http://stackoverflow.com/questions/29812168/could-not-cast-value-of-type-uitableviewcell-to-appname-customcellname would help you bit — Viral Savaj, Mar 08 '16 at 14:06
@ViralSavaj thanks! those links got me to look into storyboard again... and the issue was there! — Harfangk, Mar 08 '16 at 14:28

score 0 · Answer 1 · answered Mar 08 '16 at 14:32

The issue was solved by adding another prototype cell of ProjectTableViewCell to the NewsfeedsTableViewController, and using it to display Newsfeed with Project as source.

Which means I will have to add prototype cells for all the source data types, and then configure them on the storyboard. I'm not looking forward to doing that.

I thought I could duck-type through this all, but I guess Swift does not work like that.

Swift: Strange downcasting failure

1 Answers1