reorderList<T> method

List<T> reorderList<T>({
  1. required List<T> items,
  2. required List<ReorderUpdateEntity> reorderUpdateEntities,
})

Reorders list of items that depends on reorderUpdateEntities.

This function optimizes the reordering process of items. This is very helpful when having big lists. The function only orders the items sublist that is affected of the reordered items.

E. g. if you have the list = 0, 1, 2, 3, 4 and changes the position between "2" and "3", then it is more performant when only updating the items on position 2 and 3 by creating a sublist containing 2, 3 and changing them to 3, 2 and inserting this sublist afterwards to the list and getting 0, 1, 3, 2, 4.

Implementation

List<T> reorderList<T>({
  required List<T> items,
  required List<ReorderUpdateEntity> reorderUpdateEntities,
}) {
  final updatedItems = items.toList();

  for (final reorder in reorderUpdateEntities) {
    final reorderLeftToRight = reorder.oldIndex < reorder.newIndex;
    final start = reorderLeftToRight ? reorder.oldIndex : reorder.newIndex;
    final end = reorderLeftToRight ? reorder.newIndex : reorder.oldIndex;
    final sublist = updatedItems.sublist(start, end + 1);
    final child =
        reorderLeftToRight ? sublist.removeAt(0) : sublist.removeLast();
    sublist.insert(reorderLeftToRight ? sublist.length : 0, child);
    updatedItems.replaceRange(start, end + 1, sublist);
  }

  return updatedItems;
}