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 3.0 set your own UICollectionViewFlowLayout

let layout: UICollectionViewFlowLayout = UICollectionViewFlowLayout()
let width = UIScreen.main.bounds.width
layout.itemSize = CGSize(width: width, height: 154)
layout.sectionInset = UIEdgeInsets(top: 0, left: 0, bottom: 0, right: 0)
layout.minimumInteritemSpacing = 0
layout.minimumLineSpacing = 0
layout.scrollDirection = .horizontal
collectionView?.collectionViewLayout = layout

Answer 2

The code I just saw from Apple Official Guides and Sample Code:

AssetViewController.swift:

self.collectionView?.isPagingEnabled = true
self.collectionView?.frame = view.frame.insetBy(dx: -20.0, dy: 0.0)

Answer 3

Swift 3 solution based on @Santos's answer, for use if if you have a regular horizontally paging collection view without a page control like Paolo was using in his Swift 3 example.

I used this to solve an issue where a horizontally paging cell full screen cells with a custom UICollectionViewFlowLayout animator didn't finish rotating AND ended up offset so that the the edges of a full screen cell frame were increasingly horizontally off set from the collection view's bounds as you scrolled (like in the video OP shared).

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

    // Ensure the scrollview is the one on the collectionView we care are working with 
    if (scrollView == self.collectionView) {

        // Find cell closest to the frame centre with reference from the targetContentOffset.
        let frameCenter: CGPoint = self.collectionView.center
        var targetOffsetToCenter: CGPoint = CGPoint(x: targetContentOffset.pointee.x + frameCenter.x, y: targetContentOffset.pointee.y + frameCenter.y)
        var indexPath: IndexPath? = self.collectionView.indexPathForItem(at: targetOffsetToCenter)

        // Check for "edge case" where the target will land right between cells and then next neighbor to prevent scrolling to index {0,0}.
        while indexPath == nil {
            targetOffsetToCenter.x += 10
            indexPath = self.collectionView.indexPathForItem(at: targetOffsetToCenter)
        }
        // safe unwrap to make sure we found a valid index path
        if let index = indexPath { 
            // Find the centre of the target cell
            if let centerCellPoint: CGPoint = collectionView.layoutAttributesForItem(at: index)?.center {

                // Calculate the desired scrollview offset with reference to desired target cell centre.
                let desiredOffset: CGPoint = CGPoint(x: centerCellPoint.x - frameCenter.x, y: centerCellPoint.y - frameCenter.y)
                targetContentOffset.pointee = desiredOffset
            }
        }
    }
}

Answer 4

Remove spaces between items. For horizontal scrolling collection view set minimum line spacing to 0. You can do this with interface builder or with method of UICollectionViewDelegateFlowLayout protocol:

- (CGFloat)collectionView:(UICollectionView *)collectionView 
                   layout:(UICollectionViewLayout *)collectionViewLayout 
        minimumLineSpacingForSectionAtIndex:(NSInteger)section {
    return 0;    
}

Another way is making your cell's width less than collectionView's width for a value of horizontal space between items. Then add section insets with left and right insets that equal a half of horizontal space between items. For example, your minimum line spacing is 10:

- (CGFloat)collectionView:(UICollectionView *)collectionView
                   layout:(UICollectionViewLayout *)collectionViewLayout
        minimumLineSpacingForSectionAtIndex:(NSInteger)section {
    return 10;
}

- (CGSize)collectionView:(UICollectionView *)collectionView 
                  layout:(UICollectionViewLayout *)collectionViewLayout 
  sizeForItemAtIndexPath:(NSIndexPath *)indexPath {
    return CGSizeMake(collectionView.frame.size.width - 10, collectionView.frame.size.height);
}

- (UIEdgeInsets)collectionView:(UICollectionView *)collectionView 
                        layout:(UICollectionViewLayout *)collectionViewLayout 
        insetForSectionAtIndex:(NSInteger)section {
    return UIEdgeInsetsMake(0, 5, 0, 5);
}

And third way: manipulate collectionView scroll in scrollViewDidEndDecelerating: method:

- (void)scrollViewDidEndDecelerating:(UIScrollView *)scrollView {
    if (scrollView == self.collectionView) {
        CGPoint currentCellOffset = self.collectionView.contentOffset;
        currentCellOffset.x += self.collectionView.frame.size.width / 2;
        NSIndexPath *indexPath = [self.collectionView indexPathForItemAtPoint:currentCellOffset];
        [self.collectionView scrollToItemAtIndexPath:indexPath
                                    atScrollPosition:UICollectionViewScrollPositionCenteredHorizontally
                                            animated:YES];
    }
}

Answer 5

Swift 4 solution to remove line spacing to keep cells centered:

public func collectionView(_ collectionView: UICollectionView, layout 
collectionViewLayout: UICollectionViewLayout, 
minimumLineSpacingForSectionAt section: Int) -> CGFloat {
    return 0
}

Answer 6

Swift version of @vlad-che accepted answer:

extension GoodsViewController: UICollectionViewDelegateFlowLayout {

    func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, minimumLineSpacingForSectionAt section: Int) -> CGFloat {

        return 10
    }

    func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize {

        let frameSize = collectionView.frame.size
        return CGSize(width: frameSize.width - 10, height: frameSize.height)
    }

    func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, insetForSectionAt section: Int) -> UIEdgeInsets {

        return UIEdgeInsets(top: 0, left: 5, bottom: 0, right: 5)
    }
}