PageView.builder constructor Null safety

PageView.builder(
  1. {Key? key,
  2. Axis scrollDirection = Axis.horizontal,
  3. bool reverse = false,
  4. PageController? controller,
  5. ScrollPhysics? physics,
  6. bool pageSnapping = true,
  7. ValueChanged<int>? onPageChanged,
  8. required IndexedWidgetBuilder itemBuilder,
  9. ChildIndexGetter? findChildIndexCallback,
  10. int? itemCount,
  11. DragStartBehavior dragStartBehavior = DragStartBehavior.start,
  12. bool allowImplicitScrolling = false,
  13. String? restorationId,
  14. Clip clipBehavior = Clip.hardEdge,
  15. ScrollBehavior? scrollBehavior,
  16. bool padEnds = true}
)

Creates a scrollable list that works page by page using widgets that are created on demand.

This constructor is appropriate for page 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 lets the PageView compute the maximum scroll extent.

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

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 allowImplicitScrolling parameter must not be null. If true, the PageView will participate in accessibility scrolling more like a ListView, where implicit scroll actions will move to the next page rather than into the contents of the PageView.

Implementation

PageView.builder({
  Key? key,
  this.scrollDirection = Axis.horizontal,
  this.reverse = false,
  PageController? controller,
  this.physics,
  this.pageSnapping = true,
  this.onPageChanged,
  required IndexedWidgetBuilder itemBuilder,
  ChildIndexGetter? findChildIndexCallback,
  int? itemCount,
  this.dragStartBehavior = DragStartBehavior.start,
  this.allowImplicitScrolling = false,
  this.restorationId,
  this.clipBehavior = Clip.hardEdge,
  this.scrollBehavior,
  this.padEnds = true,
}) : assert(allowImplicitScrolling != null),
     assert(clipBehavior != null),
     controller = controller ?? _defaultPageController,
     childrenDelegate = SliverChildBuilderDelegate(
       itemBuilder,
       findChildIndexCallback: findChildIndexCallback,
       childCount: itemCount,
     ),
     super(key: key);