Widget Architecture for Flutter App

Flutter's design means that it doesn't matter that you reload the green Widgets; the way it works makes that really inexpensive. So, drag your state up to a StatefulWidget containing all your text/columns/etc (which can now be stateless).

This example shows one way of pushing down a fragment of state (into the constructor of DurationPartWidget). I've shown that you could do the date mathematics in the build method. Equally, you could do it in setState and make years, months, etc instance variables of _AnniversaryWidgetState.

class AnniversaryWidget extends StatefulWidget {
  final DateTime firstDate;

  AnniversaryWidget(this.firstDate);

  @override
  State createState() {
    return new _AnniversaryWidgetState();
  }
}

class _AnniversaryWidgetState extends State<AnniversaryWidget> {
  Timer _timer;

  @override
  void initState() {
    super.initState();
    _timer = new Timer.periodic(new Duration(seconds: 1), (Timer t) {
      setState(() {});
    });
  }

  @override
  void dispose() {
    _timer.cancel();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    DateTime now = new DateTime.now();
    // todo - calculate these from now minus firstDate
    int years = 0;
    int months = 4;
    int days = 13;
    int hours = 21;
    int minutes = now.difference(widget.firstDate).inMinutes % 60;
    return new Center(
      child: new Column(
        mainAxisAlignment: MainAxisAlignment.center,
        children: <Widget>[
          new _DurationPartWidget('years', years),
          new _DurationPartWidget('months', months),
          new _DurationPartWidget('days', days),
          new _DurationPartWidget('hours', hours),
          new _DurationPartWidget('minutes', minutes),
        ],
      ),
    );
  }
}

class _DurationPartWidget extends StatelessWidget {
  final int _numberPart;
  final String _label;

  _DurationPartWidget(this._label, this._numberPart);

  @override
  Widget build(BuildContext context) {
    return new Row(
      children: <Widget>[
        new Text(_numberPart.toString().padLeft(2, '0')),
        new Text(_label),
      ],
    );
  }
}

If, later, you want to bring you state even higher up your Widget tree, you could use an InheritedWidget. (I sometimes use one above MaterialApp.) Brian Egan gave an awesome talk at DartConf 2018 on this whole topic. https://www.youtube.com/watch?v=zKXz3pUkw9A&list=PLOU2XLYxmsIIJr3vjxggY7yGcGO7i9BK5&index=10