SliverList.separated constructor

SliverList.separated({
  1. Key? key,
  2. required NullableIndexedWidgetBuilder itemBuilder,
  3. @Deprecated('Use findItemIndexCallback instead. ' 'findChildIndexCallback returns child indices (which include separators), ' 'while findItemIndexCallback returns item indices (which do not). ' 'If you were multiplying results by 2 to account for separators, ' 'you can remove that workaround when migrating to findItemIndexCallback. ' 'This feature was deprecated after v3.37.0-1.0.pre.') ChildIndexGetter? findChildIndexCallback,
  4. ChildIndexGetter? findItemIndexCallback,
  5. required NullableIndexedWidgetBuilder separatorBuilder,
  6. int? itemCount,
  7. bool addAutomaticKeepAlives = true,
  8. bool addRepaintBoundaries = true,
  9. bool addSemanticIndexes = true,
})

A sliver that places multiple box children, separated by box widgets, in a linear array along the main axis.

This constructor is appropriate for sliver lists 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 SliverList 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 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 findItemIndexCallback returns item indices (excluding separators), unlike the deprecated findChildIndexCallback which returns child indices (including both items and separators).

For example, in a list with 3 items and 2 separators:

  • Item indices: 0, 1, 2
  • Child indices: 0 (item), 1 (separator), 2 (item), 3 (separator), 4 (item)

This callback should be implemented if the order of items may change at a later time. If null, reordering items may result in state-loss as widgets may not map to their existing RenderObjects.

The separatorBuilder is similar to itemBuilder, except it is the widget that gets placed between itemBuilder(context, index) and itemBuilder(context, index + 1).

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 shows how to create a SliverList whose Container items are separated by Dividers. The SliverList would be provided in CustomScrollView.slivers.
link 
SliverList.separated(
  itemBuilder: (BuildContext context, int index) {
    return Container(
      alignment: Alignment.center,
      color: Colors.lightBlue[100 * (index % 9)],
      child: Text('list item $index'),
    );
  },
  separatorBuilder: (BuildContext context, int index) => const Divider(),
)

Implementation

SliverList.separated({
  super.key,
  required NullableIndexedWidgetBuilder itemBuilder,
  @Deprecated(
    'Use findItemIndexCallback instead. '
    'findChildIndexCallback returns child indices (which include separators), '
    'while findItemIndexCallback returns item indices (which do not). '
    'If you were multiplying results by 2 to account for separators, '
    'you can remove that workaround when migrating to findItemIndexCallback. '
    'This feature was deprecated after v3.37.0-1.0.pre.',
  )
  ChildIndexGetter? findChildIndexCallback,
  ChildIndexGetter? findItemIndexCallback,
  required NullableIndexedWidgetBuilder separatorBuilder,
  int? itemCount,
  bool addAutomaticKeepAlives = true,
  bool addRepaintBoundaries = true,
  bool addSemanticIndexes = true,
}) : assert(
       findItemIndexCallback == null || findChildIndexCallback == null,
       'Cannot provide both findItemIndexCallback and findChildIndexCallback. '
       'Use findItemIndexCallback as findChildIndexCallback is deprecated.',
     ),
     super(
       delegate: SliverChildBuilderDelegate(
         (BuildContext context, int index) {
           final int itemIndex = index ~/ 2;
           final Widget? widget;
           if (index.isEven) {
             widget = itemBuilder(context, itemIndex);
           } else {
             widget = separatorBuilder(context, itemIndex);
             assert(() {
               if (widget == null) {
                 throw FlutterError('separatorBuilder cannot return null.');
               }
               return true;
             }());
           }
           return widget;
         },
         findChildIndexCallback: findItemIndexCallback != null
             ? (Key key) {
                 final int? itemIndex = findItemIndexCallback(key);
                 return itemIndex == null ? null : itemIndex * 2;
               }
             : findChildIndexCallback,
         childCount: itemCount == null ? null : math.max(0, itemCount * 2 - 1),
         addAutomaticKeepAlives: addAutomaticKeepAlives,
         addRepaintBoundaries: addRepaintBoundaries,
         addSemanticIndexes: addSemanticIndexes,
         semanticIndexCallback: (Widget _, int index) {
           return index.isEven ? index ~/ 2 : null;
         },
       ),
     );