UICollectionView: how to detect when scrolling has stopped - ios

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.

Answer 1

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.

Answer 2

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];
}

Answer 3

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)

Answer 4

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
// ...
}

Answer 5

Swift 3 version:
func scrollViewDidEndDecelerating(_ scrollView: UIScrollView) {
// Your code here
}

Answer 6

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];
}
}

Answer 7

Get your collection view index and Dont forget to import UISCrollViewDelegate in your class
public func scrollViewDidEndDecelerating(_ scrollView: UIScrollView) {
let xPoint = scrollView.contentOffset.x + scrollView.frame.width / 2
let yPoint = scrollView.frame.height / 2
let center = CGPoint(x: xPoint, y: yPoint)
if let ip = self.collectionView.indexPathForItem(at: center) {
pageIndex = ip.row
}
print(">>>>>>>>>\(pageIndex)")
}