您必须使用CustomPainter类来绘制您自己的自定义小部件

正如powerman23rus所说的，你应该使用CustomPainter，下面是一个基于你所提供的代码实现的例子：

class ImageWidget extends StatelessWidget {
  final Color color;
  final Size size;

  const ImageWidget({
    super.key,
    this.color = Colors.blue,
    this.size = const Size(512, 512),
  });

  @override
  Widget build(BuildContext context) {
    return CustomPaint(
      painter: ImagePainter(color: color),
      size: size,
    );
  }
}

class ImagePainter extends CustomPainter {
  final Color color;

  ImagePainter({required this.color});

  @override
  void paint(Canvas canvas, Size size) {
    final paint = Paint()..color = color;
    canvas.drawRect(Offset.zero & size, paint);
  }

  @override
  bool shouldRepaint(ImagePainter oldDelegate) => false;
}

您可以尝试使用full example on DartPad，自行检查性能。