Why will didDeselectItemAt of UICollectionView throw an error of unexpected found nil and crash the app Swift? [duplicate]

Answer 1

So the crash is happening because when selecting an item, didDeselectItemAt can be called for an item that's not visible anymore. The UICollectionView reuses UICollectionViewCells, this means that the collectionView.cellForItem(at: indexPath) method will return nil for cells that are not visible. To fix this you can use the following code:

func collectionView(_ collectionView: UICollectionView, didDeselectItemAt indexPath: IndexPath) {
    guard let cell = collectionView.cellForItem(at: indexPath) as! ImageCollectionViewCell else {
        return //the cell is not visible
    }

    UIView.animate(withDuration: 0.3, animations: {
        cell.imageView.alpha = 0.6
    })
}

Some other suggestions I have for improving your code:

You have a lot of places where you're force unwrapping values.

Consider taking the following approach:

guard let url = self.collection?.imageUrls?[indexPath.row] else {
    fatalError("url was nil")
}

self.selectedImageUrl = url

Alamofire.request(url).responseImage(completionHandler: {
    (response) in
    if response.result.value != nil {
        self.selectedImage.image = response.result.value
    }
})

By using the guard statement you force the app to crash with an appropriate error message, which will help you when debugging. It's a better practice compared to force unwrapping.

Also, when dequeing cells, you could go for something like this:

let cellIdentifier = "ImageCell"
guard let cell = collectionView.dequeueReusableCell(withReuseIdentifier: cellIdentifier, for: indexPath) as? ImageCollectionViewCell else  {
    fatalError("Wrong cell type")
}

This can happen when the cell type is different