If non-null, forces the children to have the given extent in the scroll direction.
Specifying an itemExtent is more efficient than letting the children determine their own extent because the scrolling machinery can make use of the foreknowledge of the children's extent to save work, for example when the scroll position changes drastically.
- SliverFixedExtentList, the sliver used internally when this property is provided. It constrains its box children to have a specific given extent along the main axis.
- The prototypeItem property, which allows forcing the children's extent to be the same as the given widget.
- The itemExtentBuilder property, which allows forcing the children's extent to be the value returned by the callback.
final double? itemExtent;