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.

Solution 1

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.

Solution 2

Nested UICollectionViews need oft to not scroll at all, so that no contentOffset is ever provided and thus iOS understands all cells as being always visible. In that case it is possible to take the screen boundaries as reference:

    let cellRect = cell.contentView.convert(cell.contentView.bounds, to: UIScreen.main.coordinateSpace)
    if UIScreen.main.bounds.intersects(cellRect) {
        print("cell is visible")
    }

Solution 3

You can get current index by

// Called before the cell is displayed    
func collectionView(_ collectionView: UICollectionView, willDisplay cell: UICollectionViewCell, forItemAt indexPath: IndexPath) {
    print(indexPath.row)
}

// Called when the cell is displayed
func collectionView(_ collectionView: UICollectionView, didEndDisplaying cell: UICollectionViewCell, forItemAt indexPath: IndexPath) {
    print(indexPath.row)
}

Solution 4

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()
}

Solution 5

You can simply use if collectionView.cellForItem(at: indexPath) == nil { }. The collectionView will only return a cell if it is visible.

Or in your case specifically change:

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

to:

if let cell = followedCollectionView.cellForItemAtIndexPath(indexPath) as? FeaturedCitiesCollectionViewCell { }

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Comments

[Marcus]

This method is really good for nested UICollectionViews. Most of the time because we disable scrolling for nested UICollectionViews, visibleIndexPaths will always return all the cells even they are not visible. And this solution is the best I have found thus far online. Great one.

[famfamfam]

indexPathsForVisibleItems return wrong value, in my cast is must display 3,4,5,6 but it display 0,1,2,3