Detect if UIScrollView is scrolling

Question

I'm looking to detect if (not when) a UIScrollView is scrolling.

i.e.

BOOL isScrolling = myscrollview.scrolling;

How would I go about implementing this? .dragging and .decelerating do not apply when setContentOffset:animated: is used.

Answer 1

EDIT: This method seems to have stopped working since iOS 14. I'll try and see if I can find an alternative method, and I'll update this answer if I do.

---

This may be a slightly dodgy way of doing it, but I've used it in my code before and it hasn't failed me yet. :)

Swift

let isScrolling = scrollView.layer.animation(forKey: "bounds") != nil

Objective-C

BOOL isScrolling = [scrollView.layer animationForKey:@"bounds"] != nil;

In addition to self.contentOffset the offset of a UIScrollView is stored in the bounds.origin property of the UIView as well (I guess since it's a bit more convenient in lower-level code). When you call [setContentOffset:animated:], it's passing that off to the UIScrollView as a CABasicAnimation, manipulating the bounds property.

Granted, this check will also return true if the shape of the scroll view itself is animating. In which case you may need to actually pull the bounds object out of the animation ([CAAnimation toValue]) and do a manual compare to see if origin itself is changing at all.

It's not the most elegant solution, but I like being able to avoid having to make use of the scroll view delegate whenever possible. Especially if the scroll view in question might be a subclass explicitly vending its delegate as part of its public API. :)

Answer 2

Not a very clean solution .. but should be the most reliable:

var isScrolling = false

func scrollViewWillBeginDragging(_ scrollView: UIScrollView) {
    isScrolling = true
}

func scrollViewDidEndDragging(_ scrollView: UIScrollView, willDecelerate decelerate: Bool) {
    if !decelerate { scrollViewDidEndScrolling(scrollView) }
}

func scrollViewDidEndDecelerating(_ scrollView: UIScrollView) {
    scrollViewDidEndScrolling(scrollView)
}

func scrollViewDidEndScrollingAnimation(_ scrollView: UIScrollView) {
    scrollViewDidEndScrolling(scrollView)
}

func scrollViewDidEndScrolling(_ scrollView: UIScrollView) {
    isScrolling = false
}

Answer 3

I usually do this by setting a boolean variable in the scrollViewWillBeginDragging: and scrollViewDidEndDragging:willDecelerate: events.

UIScrollView Docs