Detect UITableViewCells that appear on the screen

Question

I need to make a request when the user saw the UITableViewCell, I used func tableView(_ tableView: UITableView, willDisplay cell: UITableViewCell, forRowAt indexPath: IndexPath) but this method call for each UITableViewCell, before it's displayed, but I need a delegate when UITableViewCell appeared in the screen.

Answer 1

What you need to do is detect when scrolling occurs by implementing the scrollViewDidScroll(_:) delegate method. When the delegate method is called you can determine which cells are visible by checking the tableView.indexPathsForVisibleRows property. Note that this property returns the IndexPath for all cells that are both fully visible and partially visible.

Assuming you that you are only concerned with cells that are fully visible, you can check the frame for each row to determine if it is in fact fully visible.

To determine which cells are newly visible, you need to keep track of cells that were previously visible and compare the two sets.

This code will let you know as soon as soon as a cell is displayed:

var previousVisiblePaths = Set<IndexPath>()

override func scrollViewDidScroll(_ scrollView: UIScrollView) {

    guard let visiblePaths = tableView.indexPathsForVisibleRows?.filter ({
        let rect = tableView.rectForRow(at: $0)
        return tableView.bounds.contains(rect)
    }).reduce(into: Set<IndexPath>(), { result, indexPath in
        result.insert(indexPath)
    }) else { return }

    let newVisiblePaths = visiblePaths.subtracting(previousVisiblePaths)

    if newVisiblePaths.count > 0 {
        print("Just displayed: \(newVisiblePaths)")
    }

    previousVisiblePaths = visiblePaths
}