Is there a quicker way to detect UIScrollView's contentOffset/how does UIPickerView trigger feedback even at high velocities? - uiscrollview

Question

I have a horizontally-scrolling UICollectionView to mimic iOS's UIPickerView. Now, I want to trigger haptic feedback when selection changes just like UIPickerView does.
The problem is that when you scroll at high speeds, scrollViewDidScroll isn't called quickly enough.
Currently, I'm checking the x offset of the collection view and triggering haptic feedback when the x offset is a multiple of the cells' width (12pt), but when the x offset jumps by more than 12, feedback is skipped.

Answer 1

Instead of trying to provide feedback when the offset is an exact multiple of the cell's width, provide feedback when the "current cell" changes.
Keep a "current cell" (index) class-level variable:
var currentCellIndex: Int = 0
let cellWidth: CGFloat = 12.0 // or however you get it
on scrolling, get the "new" cell:
let newCellIndex: Int = Int(scrollView.contentOffset.x / cellWidth)
if newCellIndex != currentCellIndex {
// provide feedback
currentCellIndex = newCellIndex
}
So, at start, contentOffset.x will be equal to 0, and currentCellIndex will be 0. As the user scrolls (very, very slowly), you'll get:
Int( 1.0 / 12.0) // equals 0, no change
Int( 2.0 / 12.0) // equals 0, no change
Int( 3.0 / 12.0) // equals 0, no change
Int( 4.0 / 12.0) // equals 0, no change
etc ...
Int(12.0 / 12.0) // equals 1, so newIndex != curIndex... provide feedback and update curIndex
Int(13.0 / 12.0) // equals 1, no change
Int(14.0 / 12.0) // equals 1, no change
Int(15.0 / 12.0) // equals 1, no change
Int(16.0 / 12.0) // equals 1, no change
etc ...
Int(24.0 / 12.0) // equals 2, so newIndex != curIndex... provide feedback and update curIndex
When the user scrolls quickly:
Int( 9.0 / 12.0) // equals 0, no change
Int(16.0 / 12.0) // equals 1, so newIndex != curIndex... provide feedback and update curIndex
Int(31.0 / 12.0) // equals 2, so newIndex != curIndex... provide feedback and update curIndex
Int(64.0 / 12.0) // equals 5, so newIndex != curIndex... provide feedback and update curIndex
etc ...