How to Find All Fully Visible Cells in Table and Collection View

Imagine you are given a task to find out which table view cells that are currently fully visible on screen. So you fire up Xcode, access the UITableView‘s indexPathsForVisibleRows and expect that to get the job done.

Very soon, you will notice that something is not right. The result you get is a bit off. Cells that are partially visible on screen are also included in the result, and this is not what you want. How should you go about this?

The solution is actually pretty simple, all you need to do is think out of the box. Let me show you how.

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Getting the Index Paths for UITableView

In order to find index paths for all fully visible cells, we must first have a way to determine if a cell is completely visible based on its index path.

So what is another way of saying “a cell is fully visible”?

A cell where its content view completely intersects with the table view is considered a fully visible cell.

With that in mind, we translate the above statement into code like so:

let cellFrame = tableView.rectForRow(at: indexPath)
let isCellFullyVisible = tableView.bounds.contains(cellFrame)

Once we are able to determine if a cell is completely visible based on its index path, getting index paths for all fully visible cells has become fairly straightforward.

let visibleIndexPaths = tableView.indexPathsForVisibleRows!
let fullyVisibleIndexPaths = visibleIndexPaths.filter { indexPath in
    // Filter out all the partially visible cells   
    let cellFrame = tableView.rectForRow(at: indexPath)
    let isCellFullyVisible = tableView.bounds.contains(cellFrame)
    return isCellFullyVisible
}

That’s it for table view. Next up, collection view!

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Getting the Index Paths for UICollectionView

In terms of the collection view, the general approach is basically the same. However, the collection view works a bit differently from the table view, we can’t get the collection view cells’ frame directly. We must access the cell’s frame via its layout attribute like so:

let layoutAttribute = collectionView.layoutAttributesForItem(at: indexPath)!
let cellFrame = layoutAttribute.frame
let isCellFullyVisible = collectionView.bounds.contains(cellFrame)

The filtering part is basically the same as the table view, just make sure to use indexPathsForVisibleItems instead of  indexPathsForVisibleRows.

let visibleIndexPaths = collectionView.indexPathsForVisibleItems
let fullyVisibleIndexPaths = visibleIndexPaths.filter { indexPath in
    // Filter out all the partially visible cells
    let layoutAttribute = collectionView.layoutAttributesForItem(at: indexPath)!
    let cellFrame = layoutAttribute.frame
    let isCellFullyVisible = collectionView.bounds.contains(cellFrame)
    return isCellFullyVisible
}

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Taking One Step Further

As a good developer, we should always strive for writing clean and reusable code. Thus, let’s make the code I just show you an extension of the table view and collection view functionality.

extension UITableView {

    func isCellAtIndexPathFullyVisible(_ indexPath: IndexPath) -> Bool {

        let cellFrame = rectForRow(at: indexPath)
        return bounds.contains(cellFrame)
    }

    func indexPathsForFullyVisibleRows() -> [IndexPath] {

        let visibleIndexPaths = indexPathsForVisibleRows ?? []

        return visibleIndexPaths.filter { indexPath in
            return isCellAtIndexPathFullyVisible(indexPath)
        }
    }
}

extension UICollectionView {

    func isCellAtIndexPathFullyVisible(_ indexPath: IndexPath) -> Bool {

        guard let layoutAttribute = layoutAttributesForItem(at: indexPath) else {
            return false
        }

        let cellFrame = layoutAttribute.frame
        return self.bounds.contains(cellFrame)
    }

    func indexPathsForFullyVisibleItems() -> [IndexPath] {

        let visibleIndexPaths = indexPathsForVisibleItems

        return visibleIndexPaths.filter { indexPath in
            return isCellAtIndexPathFullyVisible(indexPath)
        }
    }
}

With that in place, getting index paths for all fully visible cells has become as simple as:

let fullyVisibleIndexPaths = tableView.indexPathsForFullyVisibleRows()
let fullyVisibleIndexPaths = collectionView.indexPathsForFullyVisibleItems()

Pretty clean isn’t it?

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Thanks for reading. 👨🏻‍💻

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