출처 :

https://blog.gskinner.com/archives/2020/09/flutter-tricks-widget-size-position.html

위젯의 크기 구하기

RenderBox rb = context.findRenderObject() as RenderBox;
return rb?.size ?? Size.zero;

위젯의 크기 구하기 2 (크기가 변하고 난 1프레임 후)

Future.microtask(() {
  RenderBox rb = key.currentContext.findRenderObject() as RenderBox;
  if (rb?.size != null) {
    callback(size);
  }
});

위젯의 위치

RenderBox rb = context.findRenderObject() as RenderBox;
return rb?.localToGlobal(offset ?? Offset.zero);

RenderBox rb = context.findRenderObject() as RenderBox;
if(rb != null && size rb.size != Size.zero){
  Offset centerPtInWidgetSpace = Offset(size.width/2, size.height/2);
  Offset centerPtInScreenSpace = rb.localToGlobal(centerPtInLocal); 
  print("Our global center is: $centerPtInScreenSpace);
}