Animating UICollectionView contentOffset does not display non-visible cells

Question

I\'m working on some ticker-like functionality and am using a UICollectionView. It was originally a scrollView, but we figure a collectionView will make it easier

Answer 1

If you need to start animation before user start dragging UICollectionView (e.g. from one page to another page), you can use this workaround to preload side cells:

func scroll(to index: Int, progress: CGFloat = 0) {
    let isInsideAnimation = UIView.inheritedAnimationDuration > 0

    if isInsideAnimation {
        // workaround
        // preload left & right cells
        // without this, some cells will be immediately removed before animation starts
        preloadSideCells()
    }

    collectionView.contentOffset.x = (CGFloat(index) + progress) * collectionView.bounds.width

    if isInsideAnimation {
        // workaround
        // sometimes invisible cells not removed (because of side cells preloading)
        // without this, some invisible cells will persists on superview after animation ends
        removeInvisibleCells()

        UIView.performWithoutAnimation {
            self.collectionView.layoutIfNeeded()
        }
    }
}

private func preloadSideCells() {
    collectionView.contentOffset.x -= 0.5
    collectionView.layoutIfNeeded()
    collectionView.contentOffset.x += 1
    collectionView.layoutIfNeeded()
}

private func removeInvisibleCells() {
    let visibleCells = collectionView.visibleCells

    let visibleRect = CGRect(
        x: max(0, collectionView.contentOffset.x - collectionView.bounds.width),
        y: collectionView.contentOffset.y,
        width: collectionView.bounds.width * 3,
        height: collectionView.bounds.height
    )

    for cell in visibleCells {
        if !visibleRect.intersects(cell.frame) {
            cell.removeFromSuperview()
        }
    }
}

Without this workaround, UICollectionView will remove cells, that not intersects target bounds, before the animation starts.

P.S. This working only if you need animate to next or previous page.