UICollectionView: how to detect when scrolling has stopped

Question

I'm using a UICollectionView to scroll through a set of thumbnails quickly. Once scrolling ends, I'd like to display a larger hi-res version of the current thumbnail.

How can I detect when the user has completed scrolling? I do implement didEndDisplayingCell, but that only tells me when a particular cell has scrolled off; it doesn't tell me when the scroll motion actually completes.

score 128 · Accepted Answer · answered Feb 15 '13 at 23:15

128

NS_CLASS_AVAILABLE_IOS(6_0) @interface UICollectionView : UIScrollView

UICollectionView is a subclass of UIScrollView. So if you have set the delegate and implemented UIScrollViewDelegate, you should be able to detect this the same way as UIScrollView.

For eg:-

- (void)scrollViewDidEndDecelerating:(UIScrollView *)scrollView;

As per documentation, the above method should tell when the scroll view has ended decelerating the scrolling movement.

answered Feb 15 '13 at 23:15

iDev

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Brilliant. You don't know how much nasty code this saved me. I was trying to monitor it using the shouldInvalidateLayoutForBoundsChange callback in my UICollectionViewFlowLayout. What a n00b. – Kyle Clegg May 11 '13 at 21:54
2

Please see D6mi's note below about scrollViewDidEndScrollingAnimation. scrollViewDidEndDecelerating does not get called for programmatic scrolling. – RajV Oct 24 '16 at 11:42
8

Indeed, I don't understand why the current answer is the accepted one, it does not work `scrollViewDidEndDecelerating ` is not called but like D6mi said, `scrollViewDidEndScrollingAnimation` is called – AkademiksQc Dec 16 '16 at 15:03
This answer is not complete. – Nathanael Jun 28 '17 at 22:54
Brilliant answer. @Nathanael, the answer is complete since he pointed out that UICollectionView is a subclass of UIScrollView and also quoted an eg:- delegate method. Rest of the answers just took this answer and added other delegate methods to it. Ideally rest should be just a comment on this answer. The keypart is collectionview is just a scrollview, rest can be figured out. – adev Jun 30 '17 at 07:04

score 60 · Answer 2 · answered Oct 24 '13 at 18:36

60

Just to cover your bases you should implement both these UIScrollViewDelegate methods. In some cases there may not be a deceleration (and scrollViewDidEndDecelerating would not be called), for e.g., the page is fully scrolled in place. In those case do your update right there in scrollViewDidEndDragging.

- (void)scrollViewDidEndDragging:(UIScrollView *)scrollView willDecelerate:(BOOL)decelerate
{
  if (!decelerate) {
    [self updateStuff];
  }
}

- (void)scrollViewDidEndDecelerating:(UIScrollView *)scrollView
{
  [self updateStuff];
}

answered Oct 24 '13 at 18:36

Abey M

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Thank you for sharing this! This should be the accepted answer b/c OP asked "scrolling has stopped" -- without both methods in place the user can easily scroll the collection view down and release finger without flicking and `scrollViewDidEndDecelerating` will NOT get called, `scrollViewDidEndDragging` does. Thank you for posting this answer. – John Erck Jul 18 '14 at 21:34

D6mi · Answer 3 · 2020-07-30T15:00:57.857

An important fact to note here.

This method gets called on User initiated scrolls (i.e a Pan gesture):

- (void)scrollViewDidEndDecelerating:(UIScrollView *)scrollView;

or in Swift:

func scrollViewDidEndDecelerating(_ scrollView: UIScrollView)

On the other hand, this one gets called on all manually (programatically) initiated scrolls (like scrollRectToVisible or scrollToItemAtIndexPath):

- (void)scrollViewDidEndScrollingAnimation:(UIScrollView *)scrollView

or in Swift:

func scrollViewDidEndScrollingAnimation(_ scrollView: UIScrollView)

score 9 · Answer 4 · answered Jun 20 '17 at 13:49

9

Swift 3 version:

func scrollViewDidEndDecelerating(_ scrollView: UIScrollView) {
    // Your code here
}

answered Jun 20 '17 at 13:49

Mr Stanev

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score 8 · Answer 5 · answered Sep 12 '17 at 14:20

Swift 3 version of Abey M and D6mi 's answers:

When scroll is caused by user action

public func scrollViewDidEndDragging(_ scrollView: UIScrollView, willDecelerate decelerate: Bool) {
    if (!decelerate) {
        //cause by user
        print("SCROLL scrollViewDidEndDragging")
    }
}

public func scrollViewDidEndDecelerating(_ scrollView: UIScrollView) {
    //caused by user
    print("SCROLL scrollViewDidEndDecelerating")
}

When scroll is caused by code action (programmatically): (like "scrollRectToVisible" or "scrollToItemAtIndexPath")

public func scrollViewDidEndScrollingAnimation(_ scrollView: UIScrollView) {
    //caused by code
    print("SCROLL scrollViewDidEndScrollingAnimation")
}

Notes:

Put these functions in your UIScrollViewDelegate or UICollectionViewDelegate delegate.
if you don't have a separate delegate, make your current class extend a UIScrollViewDelegate op top of your class file

.

open class MyClass: NSObject , UICollectionViewDelegate

and somewhere in your viewWillAppear make the class its own delegate

override open func viewWillAppear(_ animated: Bool) {
    super.viewWillAppear(animated)
    // ...
    self.myScrollView.delegate = self
    // ...
}

score 0 · Answer 6 · answered Aug 27 '17 at 21:57

if you want to use the visible indexpath:

- (void)scrollViewDidEndDecelerating:(UIScrollView *)scrollView
{
    [self scrollingFinish];
}
- (void)scrollingFinish {


    if([self.collectionView indexPathsForVisibleSupplementaryElementsOfKind:UICollectionElementKindSectionHeader]){
        NSIndexPath *firstVisibleIndexPath = [[self.collectionView indexPathsForVisibleSupplementaryElementsOfKind:UICollectionElementKindSectionHeader] firstObject];
        [self.collectionView scrollToItemAtIndexPath:firstVisibleIndexPath atScrollPosition:UICollectionViewScrollPositionTop animated:YES];
        [NSObject cancelPreviousPerformRequestsWithTarget:self];
    }
}

UICollectionView: how to detect when scrolling has stopped

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