Flutter TextFormField lost focus when showDialog

I have a TextFormField that only allows 20 characters input. If entering more than 20 characters, there will be a prompt using the showDialog and AlertDialog. It's an ephemeral prompt and disappear automatically. Therefore, I don't want the textFormField lost the focus.

The fact is when dialog shows up the focus on TextFormField is lost. Is there a way to keep the focus ?

You should use OverlayEntry or SnackBar instead of showDialog.

void showOverlay(BuildContext context) {
  final overlay = Overlay.of(context);
  final overlayEntry = OverlayEntry(
    builder: (context) => Positioned(
      top: 100,
      left: 50,
      child: Material(
        color: Colors.transparent,
        child: Container(
          padding: EdgeInsets.all(10),
          color: Colors.black,
          child: Text("Maximum 20 characters!", style: TextStyle(color: Colors.white)),
        ),
      ),
    ),
  );

  overlay.insert(overlayEntry);

  Future.delayed(Duration(seconds: 2), () {
    overlayEntry.remove();
  });
}

or

ScaffoldMessenger.of(context).showSnackBar(
  SnackBar(content: Text("Maximum 20 characters allowed!"))
);