Why is properly implementing the shouldRepaint method on a CustomPainter important for performance, and what happens if it always incorrectly returns true?

The shouldRepaint method tells Flutter whether the custom painter actually needs to redraw itself again whenever its associated widget rebuilds, and correctly implementing it by comparing the old painter's relevant properties against the new painter's properties, only returning true when something that actually affects the visual output has genuinely changed, avoids unnecessary and potentially expensive repainting operations, whereas always returning true unconditionally means the potentially expensive paint method runs on every single rebuild regardless of whether anything visually relevant actually changed, which can noticeably hurt performance for a complex custom drawing.

@override
bool shouldRepaint(covariant MyChartPainter oldDelegate) {
  return oldDelegate.dataPoints != dataPoints;
}