Controlled Stateful Widget

main.dart

/*
 * Controlled Stateful Widget
 * https://gist.github.com/PlugFox/629202b0a7bcde8de3a7503bd33308bc
 * https://dartpad.dev?id=629202b0a7bcde8de3a7503bd33308bc
 * Mike Matiunin <plugfox@gmail.com>, 05 July 2024
 */

import 'dart:async';

import 'package:flutter/material.dart';

void main() {
  final controller = Controller();
  runApp(
    MaterialApp(
      home: Scaffold(
        body: SafeArea(
          child: Center(
            child: Controlled(
              controller: controller,
            ),
          ),
        ),
      ),
    ),
  );
  Timer.periodic(const Duration(seconds: 5), (_) => controller.showSnackBar());
}

final class Controller with ChangeNotifier {
  Controller({int? value}) : _value = value ?? 0;

  /// Current value
  int get value => _value;

  int _value = 0;

  /// Increment value
  void increment() {
    _value++;
    notifyListeners();
  }

  /// Show snack bar with current value
  void showSnackBar() {
    if (_showSnackBar == null) throw Exception('Is not set to widget');
    _showSnackBar?.call();
  }

  void Function()? _showSnackBar;
}

class Controlled extends StatefulWidget {
  const Controlled({
    this.controller,
    super.key,
  });

  final Controller? controller;

  static Controller? maybeOf(BuildContext context) =>
      context.findAncestorStateOfType<_ControlledState>()?._controller;

  @override
  State<Controlled> createState() => _ControlledState();
}

class _ControlledState extends State<Controlled> {
  Controller? _controller;

  @override
  void initState() {
    super.initState();
    _controller = (widget.controller ?? Controller())
      ..addListener(_onChanged)
      .._showSnackBar = _showSnackBar;
  }

  @override
  void didUpdateWidget(covariant Controlled oldWidget) {
    super.didUpdateWidget(oldWidget);
    if (!identical(widget.controller, oldWidget.controller)) {
      _controller
        ?.._showSnackBar = null
        ..removeListener(_onChanged);
      _controller = (widget.controller ?? Controller(value: _controller?.value))
        .._showSnackBar = _showSnackBar
        ..addListener(_onChanged);
    }
  }

  @override
  void dispose() {
    _controller
      ?.._showSnackBar = null
      ..removeListener(_onChanged);
    super.dispose();
  }

  void _onChanged() {
    if (!mounted) return;
    setState(() {});
  }

  void _showSnackBar() {
    if (!mounted) return;
    ScaffoldMessenger.maybeOf(context)
      ?..clearSnackBars()
      ..showSnackBar(
        SnackBar(
          content: Text('Value: ${_controller?.value ?? 0}'),
          duration: const Duration(seconds: 2),
        ),
      );
  }

  @override
  Widget build(BuildContext context) => ElevatedButton.icon(
        icon: const Icon(Icons.add),
        label: Text('${_controller?.value ?? 0}'),
        onPressed: () => _controller?.increment(),
      );
}