UI is not being updated on the main thread when performing segue

Question

I'm trying to add an image to another imageView.image which is located in another view controller by performing/preparing for segue, however when I try to do the variables in the bg thread and then hop to the main thread to update it, it doesn't work. But if I remove the dispatch thing, then it works. Here's the code:

 override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
    SwiftSpinner.show(progress: 0.2, title: "Loading full image, please wait...")
    let vc = segue.destination as! PhotoViewController
    let cell = sender as! UICollectionViewCell
    DispatchQueue.global(qos: .background).async {
        if let indexPath = self.collectionView?.indexPath(for: cell) {
            let url = URL(string: self.images[indexPath.row].rawURL)
            let imageData = NSData(contentsOf: url!)
            let image = UIImage(data: imageData as! Data)
            DispatchQueue.main.async {
                vc.image = image!
                vc.mainImage = image!
                vc.photographerName = self.images[(indexPath.row)].photographerName
                SwiftSpinner.hide()
            }
        }
    }
}

Answer 1

I would recommend KingFisher for things like this. It takes all that junk and turns it into:

 . . .
let cell = sender as! UICollectionViewCell
let url = URL(string: self.images[indexPath.row].rawURL)
vc.imageView.kf.setImage(with: url)
 . . .

But yeah. @dlbuckley is right. Don't do that stuff in prepareForSegue if you don't do something like KingFisher.