Setting contentOffset programmatically triggers scrollViewDidScroll

Question

I've got a a few UIScrollView on a page. You can scroll them independently or lock them together and scroll them as one. The problem occurs when they are locked.

I use UIScrollViewDelegate and scrollViewDidScroll: to track movement. I query the contentOffset of the UIScrollView which changed and then reflect change to other scroll views by setting their contentOffset property to match.

Great.... except I noticed a lot of extra calls. Programmatically changing the contentOffset of my scroll views triggers the delegate method scrollViewDidScroll: to be called. I've tried using setContentOffset:animated: instead, but I'm still getting the trigger on the delegate.

How can I modify my contentOffsets programmatically to not trigger scrollViewDidScroll:?

Implementation notes.... Each UIScrollView is part of a custom UIView which uses delegate pattern to call back to the presenting UIViewController subclass that handles coordinating the various contentOffset values.

Answer 1

It is possible to change the content offset of a UIScrollView without triggering the delegate callback scrollViewDidScroll:, by setting the bounds of the UIScrollView with the origin set to the desired content offset.

CGRect scrollBounds = scrollView.bounds;
scrollBounds.origin = desiredContentOffset;
scrollView.bounds = scrollBounds;

Answer 2

Try

id scrollDelegate = scrollView.delegate;
scrollView.delegate = nil;
scrollView.contentOffset = point;
scrollView.delegate = scrollDelegate;

Worked for me.

Answer 3

What about using existing properties of UIScrollView?

func scrollViewDidScroll(_ scrollView: UIScrollView) {
    if (scrollView.isTracking || scrollView.isDragging || scrollView.isDecelerating) {
        /// The content offset was changed programmatically.
        /// Your code goes here.
    }
}

Answer 4

Another approach is to add some logic in your scrollViewDidScroll delegate to determine whether or not the change in content offset was triggered programatically or by the user's touch.

• Add an 'isManualScroll' boolean variable to your class.
• Set its initial value to false.
• In scrollViewWillBeginDragging set it to true.
• In your scrollViewDidScroll check to see that is it true and only respond if it is.
• In scrollViewDidEndDecelerating set it to false.
• In scrollViewWillEndDragging add logic to set it to false if the velocity is 0 (as scrollViewDidEndDecelerating won't be called in this case).