SwiftListTreeDataSource delete to return indices

SwiftListTreeDataSourceExtension.swift

    /// Deletes the array of`items`.
    /// - Parameter items: The array of items to delete.
    public func delete(_ items: [ItemIdentifierType]) -> [Int] {
        let deleteItemSet = Set(items)
        let filterPredicate: (TreeItem<ItemIdentifierType>) -> Bool = { !deleteItemSet.contains($0.value) }

        /// Depth first traversal to get include all related deleted expanded children
        let deletedItemsWithRelatedExpandedChildren = Set(
        items
            .compactMap { lookup($0) }
            .flatMap { itemToDelete in
            depthFirstFlattened(items: [itemToDelete], itemChildren: { $0.isExpanded ? $0.subitems : [] })
        })
        /// Traverse `shownFlatItems` and add idx if it's in deletion set.
        var deletedIndices = [Int]()
        for idx in shownFlatItems.indices {
            if deletedExpandedItemSet.contains(shownFlatItems[idx]) {
                deletedIndices.append(idx)
            }
        }
        
        // filter backing store and all subitems
        backingStore = backingStore.filter(filterPredicate)
        let theDepthFirstFlattened = depthFirstFlattened(items: backingStore)
        for item in theDepthFirstFlattened {
            item.subitems = item.subitems.filter(filterPredicate)
        }
        
        // delete from lookupTable
        deleteItemSet.forEach { lookupTable[$0] = nil }
        return deletedIndices
    }