DifferenceTracker is a lightweight extension to Swift's CollectionDifference. It defines moves in addition to removals and insertions, critical when updating interfaces and managing reference types.

DifferenceTracker can be used to perform animated updates on NSTableViews and NSOutlineViews between beginUpdates() / endUpdates() calls. This does not currently support updating UITableView, UICollectionView or NSCollectionView within a performBatchUpdates() call, which requires subtly different ordering.

DifferenceTracker generates an array of steps that can be applied iteratively to an interface, or associated collection, to move, insert and remove elements. Apart from the first and last steps, all step indexes are transient and do not relate directly to the start or end collections.

outlineView.beginUpdates()
for step in collectionDifference.steps {
    switch step {
    case let .remove(_, index):
        outlineView.removeItems(at: [index], inParent: node, withAnimation: animation)
                    
    case let .insert(element, index):
        outlineView.insertItems(at: [index], inParent: node, withAnimation: animation)

    case let .move(element, from, to):
        outlineView.moveItem(at: from, inParent: node, to: to, inParent: node)
    }
}
outlineView.endUpdates()

DifferenceTracker development was aided by @paxos and his thorough test suite. Thank you.