flexibleSpace property
final
This widget is stacked behind the toolbar and the tab bar. It's height will be the same as the app bar's overall height.
When using SliverAppBar.flexibleSpace, the SliverAppBar.expandedHeight must be large enough to accommodate the SliverAppBar.flexibleSpace widget.
Typically a FlexibleSpaceBar. See FlexibleSpaceBar for details.
Implementation
final Widget flexibleSpace