createBoxPainter method Null safety
- [VoidCallback onChanged]
Returns a BoxPainter that will paint this decoration.
onChanged argument configures BoxPainter.onChanged. It can be
omitted if there is no chance that the painter will change (for example,
if it is a BoxDecoration with definitely no DecorationImage).
@factory BoxPainter createBoxPainter([ VoidCallback onChanged ]);