Called, as needed, to build children widgets.
Children are only built when they're scrolled into view.
The AnimatedItemBuilder index parameter indicates the item's position in the scroll view. The value of the index parameter will be between 0 and initialItemCount plus the total number of items that have been inserted with AnimatedListState.insertItem or AnimatedGridState.insertItem and less the total number of items that have been removed with AnimatedListState.removeItem or AnimatedGridState.removeItem.
Implementations of this callback should assume that
removeItem removes an item immediately.
final AnimatedItemBuilder itemBuilder;