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 an IndexedWidgetBuilder 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.

Sample code

This sample code shows how to use semanticIndexOffset to handle multiple delegates in a single scroll view.

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

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

Sample code

This sample code shows how to use semanticIndexCallback to handle annotating a subset of child nodes with a semantic index. There is a Spacer widget at odd indexes which should not have a semantic index.

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

See also:

Inheritance

Constructors

SliverChildBuilderDelegate(IndexedWidgetBuilder builder, { int childCount, bool addAutomaticKeepAlives: true, bool addRepaintBoundaries: true, bool addSemanticIndexes: true, SemanticIndexCallback semanticIndexCallback: _kDefaultSemanticIndexCallback, int semanticIndexOffset: 0 })
Creates a delegate that supplies children for slivers using the given builder callback. [...]
const

Properties

addAutomaticKeepAlives bool
Whether to wrap each child in an AutomaticKeepAlive. [...]
final
addRepaintBoundaries bool
Whether to wrap each child in a RepaintBoundary. [...]
final
addSemanticIndexes bool
Whether to wrap each child in an IndexedSemantics. [...]
final
builder IndexedWidgetBuilder
Called to build children for the sliver. [...]
final
childCount int
The total number of children this delegate can provide. [...]
final
estimatedChildCount int
Returns an estimate of the number of children this delegate will build. [...]
read-only, override
semanticIndexCallback SemanticIndexCallback
A SemanticIndexCallback which is used when addSemanticIndexes is true. [...]
final
semanticIndexOffset int
An initial offset to add to the semantic indexes generated by this widget. [...]
final
hashCode int
The hash code for this object. [...]
read-only, inherited
runtimeType Type
A representation of the runtime type of the object.
read-only, inherited

Methods

build(BuildContext context, int index) Widget
Returns the child with the given index. [...]
override
shouldRebuild(covariant SliverChildBuilderDelegate oldDelegate) bool
Called whenever a new instance of the child delegate class is provided to the sliver. [...]
override
debugFillDescription(List<String> description) → void
Add additional information to the given description for use by toString.
@mustCallSuper, @protected, inherited
didFinishLayout(int firstIndex, int lastIndex) → void
Called at the end of layout to indicate that layout is now complete. [...]
inherited
estimateMaxScrollOffset(int firstIndex, int lastIndex, double leadingScrollOffset, double trailingScrollOffset) double
Returns an estimate of the max scroll extent for all the children. [...]
inherited
noSuchMethod(Invocation invocation) → dynamic
Invoked when a non-existent method or property is accessed. [...]
inherited
toString() String
Returns a string representation of this object.
inherited

Operators

operator ==(dynamic other) bool
The equality operator. [...]
inherited