SliverPrototypeExtentList.builder constructor

SliverPrototypeExtentList.builder(
  1. {Key? key,
  2. required NullableIndexedWidgetBuilder itemBuilder,
  3. required Widget prototypeItem,
  4. ChildIndexGetter? findChildIndexCallback,
  5. int? itemCount,
  6. bool addAutomaticKeepAlives = true,
  7. bool addRepaintBoundaries = true,
  8. bool addSemanticIndexes = true}
)

A sliver that places its box children in a linear array and constrains them to have the same extent as a prototype item along the main axis.

This constructor is appropriate for sliver lists with a large (or infinite) number of children whose extent is already determined.

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

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

It is legal for itemBuilder to return null. If it does, the scroll view will stop calling itemBuilder, even if it has yet to reach itemCount. By returning null, the ScrollPosition.maxScrollExtent will not be accurate unless the user has reached the end of the ScrollView. This can also cause the Scrollbar to grow as the user scrolls.

For more accurate ScrollMetrics, consider specifying itemCount.

The prototypeItem argument is used to determine the extent of each item.

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.

This example, which would be inserted into a CustomScrollView.slivers list, shows an infinite number of items in varying shades of blue:
link 
SliverPrototypeExtentList.builder(
  prototypeItem: Container(
    alignment: Alignment.center,
    child: const Text('list item prototype'),
  ),
  itemBuilder: (BuildContext context, int index) {
    return Container(
      alignment: Alignment.center,
      color: Colors.lightBlue[100 * (index % 9)],
      child: Text('list item $index'),
    );
  },
)

Implementation

SliverPrototypeExtentList.builder({
  super.key,
  required NullableIndexedWidgetBuilder itemBuilder,
  required this.prototypeItem,
  ChildIndexGetter? findChildIndexCallback,
  int? itemCount,
  bool addAutomaticKeepAlives = true,
  bool addRepaintBoundaries = true,
  bool addSemanticIndexes = true,
}) : super(delegate: SliverChildBuilderDelegate(
       itemBuilder,
       findChildIndexCallback: findChildIndexCallback,
       childCount: itemCount,
       addAutomaticKeepAlives: addAutomaticKeepAlives,
       addRepaintBoundaries: addRepaintBoundaries,
       addSemanticIndexes: addSemanticIndexes,
     ));