SliverChildBuilderDelegate class

A delegate that supplies children for slivers using a builder callback.

Many slivers lazily construct their box children to avoid creating more children than are visible through the Viewport. This delegate provides children using a NullableIndexedWidgetBuilder callback, so that the children do not even have to be built until they are displayed.

The widgets returned from the builder callback are automatically wrapped in AutomaticKeepAlive widgets if addAutomaticKeepAlives is true (the default) and in RepaintBoundary widgets if addRepaintBoundaries is true (also the default).

Accessibility

The CustomScrollView requires that its semantic children are annotated using IndexedSemantics. This is done by default in the delegate with the addSemanticIndexes parameter set to true.

If multiple delegates are used in a single scroll view, then the indexes will not be correct by default. The semanticIndexOffset can be used to offset the semantic indexes of each delegate so that the indexes are monotonically increasing. For example, if a scroll view contains two delegates where the first has 10 children contributing semantics, then the second delegate should offset its children by 10.

CustomScrollView(
  semanticChildCount: 4,
  slivers: <Widget>[
    SliverGrid(
      gridDelegate: _gridDelegate,
      delegate: SliverChildBuilderDelegate(
        (BuildContext context, int index) {
           return const Text('...');
         },
         childCount: 2,
       ),
     ),
    SliverGrid(
      gridDelegate: _gridDelegate,
      delegate: SliverChildBuilderDelegate(
        (BuildContext context, int index) {
           return const Text('...');
         },
         childCount: 2,
         semanticIndexOffset: 2,
       ),
     ),
  ],
)

In certain cases, only a subset of child widgets should be annotated with a semantic index. For example, in ListView.separated() the separators do not have an index associated with them. This is done by providing a semanticIndexCallback which returns null for separators indexes and rounds the non-separator indexes down by half.

CustomScrollView(
  semanticChildCount: 5,
  slivers: <Widget>[
    SliverGrid(
      gridDelegate: _gridDelegate,
      delegate: SliverChildBuilderDelegate(
        (BuildContext context, int index) {
           if (index.isEven) {
             return const Text('...');
           }
           return const Spacer();
         },
         semanticIndexCallback: (Widget widget, int localIndex) {
           if (localIndex.isEven) {
             return localIndex ~/ 2;
           }
           return null;
         },
         childCount: 10,
       ),
     ),
  ],
)

See also:

• SliverChildListDelegate, which is a delegate that has an explicit list of children.
• IndexedSemantics, for an example of manually annotating child nodes with semantic indexes.