ListView.builder constructor Null safety

ListView.builder(
  1. {Key? key,
  2. Axis scrollDirection = Axis.vertical,
  3. bool reverse = false,
  4. ScrollController? controller,
  5. bool? primary,
  6. ScrollPhysics? physics,
  7. bool shrinkWrap = false,
  8. EdgeInsetsGeometry? padding,
  9. double? itemExtent,
  10. Widget? prototypeItem,
  11. required IndexedWidgetBuilder itemBuilder,
  12. ChildIndexGetter? findChildIndexCallback,
  13. int? itemCount,
  14. bool addAutomaticKeepAlives = true,
  15. bool addRepaintBoundaries = true,
  16. bool addSemanticIndexes = true,
  17. double? cacheExtent,
  18. int? semanticChildCount,
  19. DragStartBehavior dragStartBehavior = DragStartBehavior.start,
  20. ScrollViewKeyboardDismissBehavior keyboardDismissBehavior = ScrollViewKeyboardDismissBehavior.manual,
  21. String? restorationId,
  22. Clip clipBehavior = Clip.hardEdge}
)

Creates a scrollable, linear array of widgets that are created on demand.

This constructor is appropriate for list views with a large (or infinite) number of children because the builder is called only for those children that are actually visible.

Providing a non-null itemCount improves the ability of the ListView to estimate the maximum scroll extent.

The itemBuilder callback will be called only with indices greater than or equal to zero and less than itemCount.

The itemBuilder should always return a non-null widget, and actually create the widget instances when called. Avoid using a builder that returns a previously-constructed widget; if the list view's children are created in advance, or all at once when the ListView itself is created, it is more efficient to use the ListView constructor. Even more efficient, however, is to create the instances on demand using this constructor's itemBuilder callback.

The findChildIndexCallback corresponds to the SliverChildBuilderDelegate.findChildIndexCallback property. If null, a child widget may not map to its existing RenderObject when the order of children returned from the children builder changes. This may result in state-loss. This callback needs to be implemented if the order of the children may change at a later time.

The addAutomaticKeepAlives argument corresponds to the SliverChildBuilderDelegate.addAutomaticKeepAlives property. The addRepaintBoundaries argument corresponds to the SliverChildBuilderDelegate.addRepaintBoundaries property. The addSemanticIndexes argument corresponds to the SliverChildBuilderDelegate.addSemanticIndexes property. None may be null.

Implementation

ListView.builder({
  Key? key,
  Axis scrollDirection = Axis.vertical,
  bool reverse = false,
  ScrollController? controller,
  bool? primary,
  ScrollPhysics? physics,
  bool shrinkWrap = false,
  EdgeInsetsGeometry? padding,
  this.itemExtent,
  this.prototypeItem,
  required IndexedWidgetBuilder itemBuilder,
  ChildIndexGetter? findChildIndexCallback,
  int? itemCount,
  bool addAutomaticKeepAlives = true,
  bool addRepaintBoundaries = true,
  bool addSemanticIndexes = true,
  double? cacheExtent,
  int? semanticChildCount,
  DragStartBehavior dragStartBehavior = DragStartBehavior.start,
  ScrollViewKeyboardDismissBehavior keyboardDismissBehavior = ScrollViewKeyboardDismissBehavior.manual,
  String? restorationId,
  Clip clipBehavior = Clip.hardEdge,
}) : assert(itemCount == null || itemCount >= 0),
     assert(semanticChildCount == null || semanticChildCount <= itemCount!),
     assert(
       itemExtent == null || prototypeItem == null,
       'You can only pass itemExtent or prototypeItem, not both.',
     ),
     childrenDelegate = SliverChildBuilderDelegate(
       itemBuilder,
       findChildIndexCallback: findChildIndexCallback,
       childCount: itemCount,
       addAutomaticKeepAlives: addAutomaticKeepAlives,
       addRepaintBoundaries: addRepaintBoundaries,
       addSemanticIndexes: addSemanticIndexes,
     ),
     super(
       key: key,
       scrollDirection: scrollDirection,
       reverse: reverse,
       controller: controller,
       primary: primary,
       physics: physics,
       shrinkWrap: shrinkWrap,
       padding: padding,
       cacheExtent: cacheExtent,
       semanticChildCount: semanticChildCount ?? itemCount,
       dragStartBehavior: dragStartBehavior,
       keyboardDismissBehavior: keyboardDismissBehavior,
       restorationId: restorationId,
       clipBehavior: clipBehavior,
     );