1. override
_BoxDecorationPainter createBoxPainter([VoidCallback onChanged ])

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 BackgroundImage).

Source 

@override
_BoxDecorationPainter createBoxPainter([VoidCallback onChanged]) {
  assert(onChanged != null || backgroundImage == null);
  return new _BoxDecorationPainter(this, onChanged);
}