How to detect when a TextField is selected in Flutter?

I suppose you are looking for FocusNode.

To listen to focus change, you can add a listner to the FocusNode and specify the focusNode to TextField.

Example:

class TextFieldFocus extends StatefulWidget {
  @override
  _TextFieldFocusState createState() => _TextFieldFocusState();
}

class _TextFieldFocusState extends State<TextFieldFocus> {
  FocusNode _focus = FocusNode();

  TextEditingController _controller = TextEditingController();

  @override
  void initState() {
    super.initState();
    _focus.addListener(_onFocusChange);
  }

  @override
  void dispose() {
    super.dispose();
    _focus.removeListener(_onFocusChange);
    _focus.dispose();
  }

  void _onFocusChange() {
    debugPrint("Focus: ${_focus.hasFocus.toString()}");
  }
  
  @override
  Widget build(BuildContext context) {
    return new Container(
      color: Colors.white,
      child: new TextField(
        focusNode: _focus,
      ),
    );
  }
}

This gist represents how to ensure a focused node to be visible on the ui.