ArcPainter, rendering background colored circle.

ArcPainter3.dart

class ArcPainter extends CustomPainter {
  // ...
  
  @override
  void paint(Canvas canvas, Size size) {
    if (backgroundColor != null) {
      final Paint backgroundPaint = Paint()
        ..color = backgroundColor
        ..strokeWidth = strokeWidth
        ..style = PaintingStyle.stroke;
      
      canvas.drawArc(
          Offset(-strokeWidth / 2, -strokeWidth / 2) &
              Size(size.width + strokeWidth, size.height + strokeWidth),
          0,
          2 * pi, // complete circumference
          false,
          backgroundPaint);
    }
  }
}