1. override
void debugPaint(PaintingContext context, Offset offset)

Override this method to paint debugging information.

Source 

@override
void debugPaint(PaintingContext context, Offset offset) {
  assert(() {
    if (debugPaintSizeEnabled)
      debugPaintSize(context, offset);
    if (debugPaintBaselinesEnabled)
      debugPaintBaselines(context, offset);
    if (debugPaintPointersEnabled)
      debugPaintPointers(context, offset);
    return true;
  });
}