Dart APIunself_componentUnselfExtendedSliverNavigationBarmiddle middle property Null safety Widget? middle final Implementation final Widget? middle; unself_component unself_component UnselfExtendedSliverNavigationBar middle property UnselfExtendedSliverNavigationBar class Constructors UnselfExtendedSliverNavigationBar Properties automaticallyImplyLeading automaticallyImplyTitle backgroundColor border brightness hashCode heroTag key largeTitle leading middle opaque padding previousPageTitle runtimeType stretch trailing transitionBetweenRoutes Methods createElement createState debugDescribeChildren debugFillProperties noSuchMethod toDiagnosticsNode toString toStringDeep toStringShallow toStringShort Operators operator ==