UIScrollView recursion - ios

Question

In the UIScrollViewDelegate scrollViewDidScroll(UIScrollView) function, if you change the frame of the delegate's UIScrollView, then scrollViewDidScroll(UIScrollView) gets called again. A common example of this is when you have a header that shrinks as the user scrolls up and expands as the user scrolls down.
Aside from an implementation in which you unset and reset the scrollView's delegate property like below, are there any ways to prevent recursion? The UIScrollView object has isTracking and isDragging properties that overlap, and also remain true in the below example.
func scrollViewDidScroll(_ scrollView: UIScrollView) {
let d = scrollView.delegate // Prevent recursion by setting the
scrollView.delegate = nil
// Do stuff
if someCondition == true {
scrollView.contentOffset = somePoint
scrollView.frame = someRect
}
scrollView.delegate = d // Restore the scrollView's delegate
}
NOTE: I DO NOT NEED A TUTORIAL FOR MAKING A COLLAPSIBLE HEADER, NOR DO I NEED A SIMPLE IF CHECK. I'm only specifically asking what options are available for breaking out of scrolling events based on their origin. For example, you could put this at the start of scrollViewDidScroll:
if(!scrollView.isTracking && !scrollView.isDragging) {
// Break out if we got here through anything but a touch event
return
}

Answer 1

I just tried it, and setting .contentOffset does not cause recursion.
As you drag the content, the delegate receives continuous scrollViewDidScroll messages... when you call:
scrollView.contentOffset = CGPoint(x: 0, y: 0)
you are still dragging, and the content keeps getting reset to 0,0.
Change your function to this:
var i = 0
func scrollViewDidScroll(_ scrollView: UIScrollView) {
i += 1
print("did scroll", i)
scrollView.contentOffset = CGPoint.zero
}
and you will see it stops counting when you stop dragging - so it is not recursive.
Perhaps you really want to use scrollViewDidEndDragging or scrollViewDidEndDecelerating?
Edit:
Perhaps this will work better for you. Keep in mind, as long as you continue to drag inside the scroll view, scrollViewDidScroll() will continue to be called.
func scrollViewDidEndDragging(_ scrollView: UIScrollView, willDecelerate decelerate: Bool) {
print("did end dragging", scrollView.contentOffset)
}
func scrollViewDidScroll(_ scrollView: UIScrollView) {
print("did scroll", scrollView.contentOffset)
scrollView.setContentOffset(CGPoint.zero, animated: false)
}
If you look at the console output, you should never see more than 2 print()s after you "stop dragging".