createBoxPainter method
Returns a BoxPainter that will paint this decoration.
The 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).
Implementation
@override
BoxPainter createBoxPainter([VoidCallback? onChanged]) {
return _UnderlinePainter(this, onChanged);
}