flutter_custom_painter.dart

class MyPainter extends CustomPainter {
  ...
  
  @override
  void paint(Canvas canvas, Size size) {
      final line = Paint()
          ..color = lineColor
          ..strokeCap = StrokeCap.round
          ..style = PaintingStyle.stroke
          ..strokeWidth = width;

      final center  = Offset(size.width/2, size.height/2);
      final radius  = min(size.width/2, size.height/2);

      canvas.drawCircle(
          center,
          radius,
          line
      );
  )
)