Dart APIunself_componentUnSliverArtBarbackground background property Null safety Widget? background final Implementation final Widget? background; unself_component unself_component UnSliverArtBar background property UnSliverArtBar class Constructors UnSliverArtBar Properties action background description emoji hashCode isImmersive key leading name runtimeType trailing Methods createElement createState debugDescribeChildren debugFillProperties noSuchMethod toDiagnosticsNode toString toStringDeep toStringShallow toStringShort Operators operator ==