How do the height and width of user interface elements change when scrolling on iOS?

In a standard implementation, the height and width of UIView elements remain unchanged when scrolling inside a UIScrollView or its descendants (UITableView, UICollectionView).

The position of elements (the frame.origin property) changes relative to the bounds of the scroll view. The bounds property of the scroll view shifts during scrolling, but its size (bounds.size) remains constant (equal to the size of frame.size).

The size of the scrollable content is determined by the contentSize property of the scroll view. Elements within the contentSize will be visible when scrolling.

Changing the size of elements during scrolling (parallax effect or compression/expansion) does not happen automatically and requires additional logic:

1. Implementation in UIScrollViewDelegate: Tracking changes in contentOffset in the scrollViewDidScroll(_:) method and manually changing the frame or bounds of the desired element.

   // Example of changing the height of an element during scrolling
   func scrollViewDidScroll(_ scrollView: UIScrollView) {
       let yOffset = scrollView.contentOffset.y
       let initialHeight: CGFloat = 200
       let maxHeight: CGFloat = 300
   
       // Compression when scrolling down, expansion when scrolling up
       let newHeight = max(0, min(maxHeight, initialHeight - yOffset))
       
       someView.frame.size.height = newHeight
       // Handle position if the anchor is not at the top
   }
   

2. Using UIVisualEffectView: For blur or vibrancy effects that can change during scrolling depending on the content beneath.

3. Custom layouts: In UICollectionView, you can create a custom UICollectionViewLayout to control cell transformations during scrolling.

4. Animations: Using UIViewPropertyAnimator or other animation mechanisms triggered by scroll events.

Thus, the basic behavior is a constant size. Any size changes are the result of specific implementations.