ListView.separated constructor
Creates a fixed-length scrollable linear array of list "items" separated by list item "separators".
This constructor is appropriate for list views with a large number of item and separator children because the builders are only called for the children that are actually visible.
The itemBuilder
callback will be called with indices greater than
or equal to zero and less than itemCount
.
Separators only appear between list items: separator 0 appears after item 0 and the last separator appears before the last item.
The separatorBuilder
callback will be called with indices greater than
or equal to zero and less than itemCount - 1
.
The itemBuilder
and separatorBuilder
callbacks should actually create
widget instances when called. Avoid using a builder that returns a
previously-constructed widget; if the list view's children are created in
advance, or all at once when the ListView itself is created, it is more
efficient to use new ListView.
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. None may be
null.
Implementation
ListView.separated({
Key key,
Axis scrollDirection = Axis.vertical,
bool reverse = false,
ScrollController controller,
bool primary,
ScrollPhysics physics,
bool shrinkWrap = false,
EdgeInsetsGeometry padding,
@required IndexedWidgetBuilder itemBuilder,
@required IndexedWidgetBuilder separatorBuilder,
@required int itemCount,
bool addAutomaticKeepAlives = true,
bool addRepaintBoundaries = true,
bool addSemanticIndexes = true,
double cacheExtent,
}) : assert(itemBuilder != null),
assert(separatorBuilder != null),
assert(itemCount != null && itemCount >= 0),
itemExtent = null,
childrenDelegate = SliverChildBuilderDelegate(
(BuildContext context, int index) {
final int itemIndex = index ~/ 2;
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;
},
childCount: _computeSemanticChildCount(itemCount),
addAutomaticKeepAlives: addAutomaticKeepAlives,
addRepaintBoundaries: addRepaintBoundaries,
addSemanticIndexes: addSemanticIndexes,
semanticIndexCallback: (Widget _, int index) {
return index.isEven ? index ~/ 2 : null;
}
), super(
key: key,
scrollDirection: scrollDirection,
reverse: reverse,
controller: controller,
primary: primary,
physics: physics,
shrinkWrap: shrinkWrap,
padding: padding,
cacheExtent: cacheExtent,
semanticChildCount: _computeSemanticChildCount(itemCount),
);