How to snap horizontal paging to multi-row collection view like App Store?

Question

I would like to replicate the paging in the multi-row App Store collection view:

So far I\'ve designed it as close as possible to the way it looks, including sh

Answer 1

There is no reason to subclass UICollectionViewFlowLayout just for this behavior.

UICollectionView is a subclass of UIScrollView, so its delegate protocol UICollectionViewDelegate is a subtype of UIScrollViewDelegate. This means you can implement any of UIScrollViewDelegate’s methods in your collection view’s delegate.

In your collection view’s delegate, implement scrollViewWillEndDragging(_:withVelocity:targetContentOffset:) to round the target content offset to the top left corner of the nearest column of cells.

Here's an example implementation:

override func scrollViewWillEndDragging(_ scrollView: UIScrollView, withVelocity velocity: CGPoint, targetContentOffset: UnsafeMutablePointer) {
    let layout = collectionViewLayout as! UICollectionViewFlowLayout
    let bounds = scrollView.bounds
    let xTarget = targetContentOffset.pointee.x

    // This is the max contentOffset.x to allow. With this as contentOffset.x, the right edge of the last column of cells is at the right edge of the collection view's frame.
    let xMax = scrollView.contentSize.width - scrollView.bounds.width

    if abs(velocity.x) <= snapToMostVisibleColumnVelocityThreshold {
        let xCenter = scrollView.bounds.midX
        let poses = layout.layoutAttributesForElements(in: bounds) ?? []
        // Find the column whose center is closest to the collection view's visible rect's center.
        let x = poses.min(by: { abs($0.center.x - xCenter) < abs($1.center.x - xCenter) })?.frame.origin.x ?? 0
        targetContentOffset.pointee.x = x
    } else if velocity.x > 0 {
        let poses = layout.layoutAttributesForElements(in: CGRect(x: xTarget, y: 0, width: bounds.size.width, height: bounds.size.height)) ?? []
        // Find the leftmost column beyond the current position.
        let xCurrent = scrollView.contentOffset.x
        let x = poses.filter({ $0.frame.origin.x > xCurrent}).min(by: { $0.center.x < $1.center.x })?.frame.origin.x ?? xMax
        targetContentOffset.pointee.x = min(x, xMax)
    } else {
        let poses = layout.layoutAttributesForElements(in: CGRect(x: xTarget - bounds.size.width, y: 0, width: bounds.size.width, height: bounds.size.height)) ?? []
        // Find the rightmost column.
        let x = poses.max(by: { $0.center.x < $1.center.x })?.frame.origin.x ?? 0
        targetContentOffset.pointee.x = max(x, 0)
    }
}

// Velocity is measured in points per millisecond.
private var snapToMostVisibleColumnVelocityThreshold: CGFloat { return 0.3 }

Result:

You can find the full source code for my test project here: https://github.com/mayoff/multiRowSnapper