Check whether cell at indexPath is visible on screen UICollectionView

Question

I have a CollectionView which displays images to the user. I download these in the background, and when the download is complete I call the following func to update the collectionViewCell and display the image.

func handlePhotoDownloadCompletion(notification : NSNotification) {
    let userInfo:Dictionary<String,String!> = notification.userInfo as! Dictionary<String,String!>
    let id = userInfo["id"]
    let index = users_cities.indexOf({$0.id == id})
    if index != nil {
        let indexPath = NSIndexPath(forRow: index!, inSection: 0)
        let cell = followedCollectionView.cellForItemAtIndexPath(indexPath) as! FeaturedCitiesCollectionViewCell
        if (users_cities[index!].image != nil) {
            cell.backgroundImageView.image = users_cities[index!].image!
        }
    }
}

This works great if the cell is currently visible on screen, however if it is not I get a fatal error: unexpectedly found nil while unwrapping an Optional value error on the following line :

 let cell = followedCollectionView.cellForItemAtIndexPath(indexPath) as! FeaturedCitiesCollectionViewCell

Now this function does not even need to be called if the collectionViewCell is not yet visible, because in this case the image will be set in the cellForItemAtIndexPath method anyway.

Hence my question, how can I alter this function to check whether the cell we are dealing with is currently visible or not. I know of the collectionView.visibleCells() however, I am not sure how to apply it here.

Answer 1

I needed to track cells that are visible so you can also do something like this to track your cells better.

private func configureVisibleIndexPath() {
        let visibleCells = collectionView.indexPathsForVisibleItems
        visibleCells.forEach { indexPath in
            if let cell = collectionView.cellForItem(at: indexPath), collectionView.bounds.contains(cell.frame) {
                print("visible row is \(indexPath.row)"
            }
        }
    }

Call this function when your collection view is configured properly and it also takes care on scrolling by calling it in scrollView delegates:

func scrollViewDidEndDragging(_ scrollView: UIScrollView, willDecelerate decelerate: Bool) {
    if !decelerate {
        configureVisibleIndexPath()
    }
}

func scrollViewDidEndDecelerating(_ scrollView: UIScrollView) {
    configureVisibleIndexPath()
}

Answer 2

Get current available cells

// get visible cells 
let visibleIndexPaths = followedCollectionView.indexPathsForVisibleItems

Then check if your indexPath is contained in the visibleIndexPaths array or not, before doing anything with cells.