Addressing the Challenge of Prepending Items in a UICollectionView

Encountering a shift in content offset while using UICollectionViewDiffableDataSource to prepend items or sections to a collection view can be a disconcerting experience. This unexpected behavior disrupts the user’s current position within the view and can be frustrating to debug. Fortunately, there’s an elegant way to handle this issue, as we’ll explore below.

Our solution

Capture Initial State: Before applying any changes to your data snapshot, store the current contentOffset and contentSize of the collection view’s underlying UIScrollView into temporary variables.
Apply Snapshot: Use apply(snapshot, animatingDifferences:) to introduce your data changes into the collection view.
Refresh Layout: Invalidate the existing collection view layout and force it to redraw itself.
Calculate and Set New Offset: Determine the new contentOffset based on the difference between the old and new content sizes. Then, update the contentOffset accordingly.

    // Store the current state
    let previousContentOffset = myCollectionView.contentOffset
    let previousContentHeight = myCollectionView.contentSize.height

    // Apply your diffable data source snapshot
    diffableDataSource.apply(snapshot, animatingDifferences: false)

    // Invalidate the current layout and trigger a layout pass
    myCollectionView.collectionViewLayout.invalidateLayout()
    myCollectionView.layoutIfNeeded()

    // Calculate the new content height and the total height of newly added items
    let newContentHeight = myCollectionView.contentSize.height
    let heightOfNewItems = newContentHeight - previousContentHeight

    // Compute the new content offset
    let updatedContentOffset = CGPoint(x: 0.0, y: previousContentOffset.y + heightOfNewItems)

    // Set the new content offset
    myCollectionView.contentOffset = updatedContentOffset

In this case, we’re assuming the use of a UICollectionViewFlowLayout set to vertical configuration as the layout for the collection view, but the same principle would apply in a horizontal configuration.

By incorporating these steps into your UICollectionView management, you ensure a smooth user experience when dealing with dynamic data changes, effectively preserving the current scroll position even when items or sections are prepended.