touched_detector.dart

class TouchedDetector extends StatefulWidget {
  final Function(FocusNode, bool) renderer;

  const TouchedDetector({Key key, this.renderer}) : super(key: key);

  @override
  _TouchedDetectorState createState() => _TouchedDetectorState();
}

class _TouchedDetectorState extends State<TouchedDetector> {
  final _focusNode = FocusNode();
  List<bool> _state = [];

  @override
  void initState() {
    super.initState();
    _focusNode.addListener(() {
      setState(() {
        _state.add(_focusNode.hasFocus);
      });
    });
  }

  @override
  Widget build(BuildContext context) {
    final _autoValidate = _state.length >= 2;
    final child = widget.renderer(_focusNode, _autoValidate);
    return Container(
      child: child,
    );
  }
}