viewWillAppear for subviews

Question

I have UIScrollView with multiple UIVIew subviews. I would like to update the data that is displayed by each UIView when they appear in the visible portion of the UIScrollView.

What is the callback that gets triggered? I tried viewWillAppear, but it does not seem to get called.

Thanks. :)

Answer 1

You have to do the calculation yourself. Implement scrollViewDidScroll: in your scroll view delegate and calculate manually which views are visible (e.g. by checking if CGRectIntersectsRect(scrollView.bounds, subview.frame) returns true.

Answer 2

Swift 3 solution

func scrollViewDidScroll(_ scrollView: UIScrollView) {     let viewFrame = greenView.frame      let container = CGRect(x: scrollView.contentOffset.x, y: scrollView.contentOffset.y, width: scrollView.frame.size.width, height: scrollView.frame.size.height)       // We may have received messages while this tableview is offscreen     if (viewFrame.intersects(container)) {         // Do work here         print("view is visible")     }     else{         print("nope view is not on the screen")     } }