Flutter - Reuse same custom widget class for instances appear not at same time

In shorter word, my question is how to call child's initState() again on parent's setState().

I need to reuse a widget class for multiple instances. I need the widgets to appear in turn like below where I can use setState() to switch between them.

bool showthefirstone = true;

@override
Widget build(BuildContext context) {
  ...
  TheWidget it = showthefirstone ? TheWidget(...) : TheWidget(..);
  ...
}

The later one's initState() is not called. No problem showing them both together along at the same time. The issue only occurs upon setState().

For those who don't understand, this trigger the change

setState((){showthefirstone = !showthefirstone})