UICollectionView Horizontal Paging not centered

Question

I have a horizontal scrolling collectionView with each cell the size of the view. When I page through the collectionView it doesn't page by cell. The cells aren't in the center of the screen. I've tried a bunch of things to try to fix it and haven't had any luck. Here's a video of the problem: https://www.youtube.com/watch?v=tXsxWelk16w Any ideas?

Answer 1

Swift 3

func scrollViewDidEndDecelerating(_ scrollView: UIScrollView) {
        if scrollView == self.collectionView {
            var currentCellOffset = self.collectionView.contentOffset
            currentCellOffset.x += self.collectionView.frame.width / 2
            if let indexPath = self.collectionView.indexPathForItem(at: currentCellOffset) {
              self.collectionView.scrollToItem(at: indexPath, at: .centeredHorizontally, animated: true)
            }
        }
    }

Answer 2

After having a similar issue, I fixed mine by realizing that when using horizontal scrolling the height is now the width and the width is now the height because the default is set for vertical scrolling. Try switching the values and see if that helps. https://developer.apple.com/library/content/documentation/WindowsViews/Conceptual/CollectionViewPGforIOS/UsingtheFlowLayout/UsingtheFlowLayout.html

Answer 3

Being able to have cells that are smaller the collectionView frame with space between the cells allows for hinting to the user that there other cells either side to scroll to which is a big win for UX. But for the centering of the pages doesn't work as expected with each cell progressively becoming more offset as the user scrolls. I've found the following to work well. The centering/snapping animation on each cell is almost invisible to user since it is only tweaking where the collectionView scrolling would end naturally rather than jerking the collectionView to quickly scroll to another indexPath. It's still important to to have the sectionInset property set large enough to allow cell not to stick to the containing frame edges. Also since there are spaces between the cells the target could land on an indexPath of nil which would cause the collectionView to scroll back to the start. I've fixed this offsetting a little and then trying again but different approaches could be taken here.

- (void)scrollViewWillEndDragging:(UIScrollView *)scrollView
                 withVelocity:(CGPoint)velocity
          targetContentOffset:(inout CGPoint *)targetContentOffset
{
    //Ensure the scrollview is the collectionview we care about
    if (scrollView == self.collectionView) {

        // Find cell closest to the frame centre with reference from the targetContentOffset.
        CGPoint frameCentre = self.collectionView.center;
        CGPoint targetOffsetToCentre = CGPointMake((* targetContentOffset).x + frameCentre.x, (* targetContentOffset).y + frameCentre.y);

        NSIndexPath *indexPath = [self.collectionView indexPathForItemAtPoint:targetOffsetToCentre];

            //Check for "edgecase" that the target will land between cells and then find a close neighbour to prevent scrolling to index {0,0}.
        while (!indexPath) {
            targetOffsetToCentre.x += ((UICollectionViewFlowLayout *)self.collectionView.collectionViewLayout).minimumInteritemSpacing;
            indexPath = [self.collectionView indexPathForItemAtPoint:targetOffsetToCentre];
        }

        // Find the centre of the target cell
        CGPoint centreCellPoint = [self.collectionView layoutAttributesForItemAtIndexPath:indexPath].center;

        // Calculate the desired scrollview offset with reference to desired target cell centre.
        CGPoint desiredOffset = CGPointMake(centreCellPoint.x - frameCentre.x, centreCellPoint.y - frameCentre.y);
        *targetContentOffset = desiredOffset;
    }
}

Answer 4

Demo here in Swift 3: https://github.com/damienromito/CollectionViewCustom

func scrollViewWillEndDragging(_ scrollView: UIScrollView, withVelocity velocity: CGPoint, targetContentOffset: UnsafeMutablePointer<CGPoint>) {

    let pageWidth = Float(itemWidth + itemSpacing)
    let targetXContentOffset = Float(targetContentOffset.pointee.x)
    let contentWidth = Float(collectionView!.contentSize.width  )
    var newPage = Float(self.pageControl.currentPage)

    if velocity.x == 0 {
        newPage = floor( (targetXContentOffset - Float(pageWidth) / 2) / Float(pageWidth)) + 1.0
    } else {
        newPage = Float(velocity.x > 0 ? self.pageControl.currentPage + 1 : self.pageControl.currentPage - 1)
        if newPage < 0 {
            newPage = 0
        }
        if (newPage > contentWidth / pageWidth) {
            newPage = ceil(contentWidth / pageWidth) - 1.0
        }
    }
    self.pageControl.currentPage = Int(newPage)
    let point = CGPoint (x: CGFloat(newPage * pageWidth), y: targetContentOffset.pointee.y)
    targetContentOffset.pointee = point
}

Swift 4:

func scrollViewWillEndDragging(_ scrollView: UIScrollView, withVelocity velocity: CGPoint, targetContentOffset: UnsafeMutablePointer<CGPoint>) {

    let pageWidth = Float(itemWidth + itemSpacing)
    let targetXContentOffset = Float(targetContentOffset.pointee.x)
    let contentWidth = Float(collectionView!.contentSize.width  )
    var newPage = Float(self.pageControl.currentPage)

    if velocity.x == 0 {
        newPage = floor( (targetXContentOffset - Float(pageWidth) / 2) / Float(pageWidth)) + 1.0
    } else {
        newPage = Float(velocity.x > 0 ? self.pageControl.currentPage + 1 : self.pageControl.currentPage - 1)
        if newPage < 0 {
            newPage = 0
        }
        if (newPage > contentWidth / pageWidth) {
            newPage = ceil(contentWidth / pageWidth) - 1.0
        }
    }

    self.pageControl.currentPage = Int(newPage)
    let point = CGPoint (x: CGFloat(newPage * pageWidth), y: targetContentOffset.pointee.y)
    targetContentOffset.pointee = point
}