CollectionWidget<T extends Widget> constructor Null safety

CollectionWidget<T extends Widget>(
  1. {Key? key,
  2. required int itemCount(
      ),
    1. required T itemForIndex(
      1. int index
      ),
    2. void itemVisibility(
      1. Widget widget,
      2. bool visibility,
      3. List<Widget> widgets
      )?,
    3. Axis direction = Axis.vertical,
    4. bool reverse = false,
    5. bool shrinkWrap = false,
    6. double visibilityFraction = 0,
    7. double startFraction = 0.25,
    8. double endFraction = 0.25,
    9. VoidCallback? onStart,
    10. VoidCallback? onEnd}
    )

    Invoke itemCount and itemForIndex for collection.