Is it possible to achieve state management between two stateless widgets with no state manager?

Question

I think this is the right question. I have encountered the problem of wanting to keep my code super condensed, and simple for the sake of my demo project. And I am using two stateless widgets which I would like to have communicate, that is I want one widget to reload the other on user input.

Proviso Note: I'm getting acquainted to using GetX mainly for state management so I would like to try to always use stateless widgets. Please keep this in mind in your answer. I know I can solve this with setState() stateful widget. I also know I can solve this with a controller class extending GetXController() or even provider. However I am simply curious if what I'm trying to do is even possible without these obvious solutions...

So.. Question is:

Is it possible to have this widget's rangeStart & rangeEnd (when changed by user date selection) rebuild the text widget in the next widget below?

import 'package:flutter/material.dart';
import 'package:date_range_picker/date_range_picker.dart' as DateRangePicker;

// ignore: must_be_immutable
class DateRange extends StatelessWidget {
  DateTime rangeStart = DateTime.now().subtract(Duration(days: 7));
  DateTime rangeEnd = DateTime.now();

  @override
  Widget build(BuildContext context) {
    return IconButton(
        color: Colors.white,
        icon: Icon(Icons.calendar_today),
        onPressed: () async {
          final List<DateTime> picked = await DateRangePicker.showDatePicker(
              context: context,
              initialFirstDate: DateTime.now().subtract(Duration(days: 7)),
              initialLastDate: DateTime.now(),
              firstDate: DateTime(2015),
              lastDate: DateTime(DateTime.now().year + 2));
          if (picked != null && picked.length == 2) {
            rangeStart = picked.first;
            rangeEnd = picked.last;
          }
        });
  }
}

import 'package:flutter/material.dart';
import 'package:get/get.dart';
import 'package:linechart/add_data_button.dart';
import 'package:linechart/date_range.dart';
import 'package:syncfusion_flutter_charts/charts.dart';
import 'package:intl/intl.dart';

class LineChartScreen extends StatelessWidget {
  final DateRange dateRange = Get.put(DateRange());

  @override
  Widget build(BuildContext context) {
    String startDate = DateFormat.yMMMMd('en_US').format(dateRange.rangeStart);
    String endDate = DateFormat.yMMMMd('en_US').format(dateRange.rangeEnd);

    return Scaffold(
      appBar: AppBar(
        title: Text('GRAPH --- Range: $startDate - $endDate'),
        backgroundColor: Colors.lightBlue,
        actions: [
          AddDataButton(),
          DateRange(),
        ],
      ),
      body: Column(...),
    );
  }
}

Answer 1

This old post explains that:

A StatelessWidget will never rebuild by itself (but can from external events). A StatefulWidget can. That is the golden rule.

So you need some mechanism to cause a StatelessWidget to rebuild, and as you say there are many of these.

I like using flutter_hooks as a simple mechanism for managing such private state. Here is code for your example that uses it. In the code:

• The parent widget calls useState to define/access its state
• The child widget calls a callback to update the state, which causes the parent to rebuild

The code:

import 'package:flutter/material.dart';
import 'package:date_range_picker/date_range_picker.dart' as DateRangePicker;
import 'package:intl/intl.dart';
import 'package:flutter_hooks/flutter_hooks.dart';

// ignore: must_be_immutable
class DateRangeWidget extends StatelessWidget {
  Function _setDateRange;

  DateRangeWidget(this._setDateRange);

  DateTime rangeStart = DateTime.now().subtract(Duration(days: 7));
  DateTime rangeEnd = DateTime.now();

  @override
  Widget build(BuildContext context) {
    return IconButton(
        color: Colors.white,
        icon: Icon(Icons.calendar_today),
        onPressed: () async {
          final List<DateTime> picked = await DateRangePicker.showDatePicker(
              context: context,
              initialFirstDate: DateTime.now().subtract(Duration(days: 7)),
              initialLastDate: DateTime.now(),
              firstDate: DateTime(2015),
              lastDate: DateTime(DateTime.now().year + 2));
          if (picked != null && picked.length == 2) {
            rangeStart = picked.first;
            rangeEnd = picked.last;
            _setDateRange(rangeStart, rangeEnd);
          }
        });
  }
}

class DateRange {
  DateTime rangeStart, rangeEnd;
  DateRange(this.rangeStart, this.rangeEnd);
}

class LineChartScreen extends HookWidget {
//  final DateRange dateRange = Get.put(DateRange());
  @override
  Widget build(BuildContext context) {
    final dateRange = useState(
        DateRange(DateTime.now().subtract(Duration(days: 7)), DateTime.now()));

    void _setDateRange(rangeStart, rangeEnd) {
      dateRange.value = DateRange(rangeStart, rangeEnd);
    }

    String startDate =
        DateFormat.yMMMMd('en_US').format(dateRange.value.rangeStart);
    String endDate =
        DateFormat.yMMMMd('en_US').format(dateRange.value.rangeEnd);

    return Scaffold(
      appBar: AppBar(
        title: Text('GRAPH --- Range: $startDate - $endDate'),
        backgroundColor: Colors.lightBlue,
        actions: [
          // AddDataButton(),
          DateRangeWidget(_setDateRange),
        ],
      ),
      body: Column(
        children: [
          Text(startDate),
          Text(endDate),
        ],
      ),
    );
  }
}