todos-contra-el-fuego-mobile/lib/widgets/material_app_with_intro.dart

import 'package:flutter/material.dart';
import 'package:meta/meta.dart';
import 'package:shared_preferences/shared_preferences.dart';
import 'dart:async';

@immutable
abstract class MaterialAppWithIntro extends StatelessWidget {
  const MaterialAppWithIntro(this.name, this.theme, this.routes,
      this.introWidget, this.continueWidget, this.prefsKey);

  final String name;
  final ThemeData theme;
  final Map<String, WidgetBuilder> routes;
  final WidgetBuilder introWidget;
  final WidgetBuilder continueWidget;
  final String prefsKey;

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: MaterialAppWithIntroHome(introWidget, continueWidget, prefsKey),
      title: name,
      theme: theme,
      routes: routes,
    );
  }
}

class MaterialAppWithIntroHome extends StatefulWidget {
  const MaterialAppWithIntroHome(
      this.introWidget, this.continueWidget, this.prefsKey);

  final WidgetBuilder introWidget;
  final WidgetBuilder continueWidget;
  final String prefsKey;

  @override
  _MaterialAppWithIntroState createState() =>
      _MaterialAppWithIntroState(introWidget, continueWidget, prefsKey);
}

class _MaterialAppWithIntroState extends State<MaterialAppWithIntroHome> {
  _MaterialAppWithIntroState(
      this.introWidget, this.continueWidget, this.prefsKey);

  final WidgetBuilder introWidget;
  final WidgetBuilder continueWidget;
  final String prefsKey;

  @override
  void initState() {
    super.initState();
    Timer(const Duration(milliseconds: 1000), () {
      checkFirstStart();
    });
  }

  // https://stackoverflow.com/questions/50654195/flutter-one-time-intro-screen
  Future<void> checkFirstStart() async {
    final String initialWizardKey = prefsKey;
    final SharedPreferences prefs = await SharedPreferences.getInstance();
    final bool showInitialWizard = (prefs.getBool(initialWizardKey) ?? true);

    if (showInitialWizard) {
      await prefs.setBool(initialWizardKey, false);
      if (mounted) {
        await Navigator.of(context)
            .pushReplacement(MaterialPageRoute<void>(builder: introWidget));
      }
    } else {
      if (mounted) {
        await Navigator.of(context)
            .pushReplacement(MaterialPageRoute<void>(builder: continueWidget));
      }
    }
  }

  @override
  Widget build(BuildContext context) {
    return const Scaffold(
      body: Center(
        child: CircularProgressIndicator(),
      ),
    );
  }
}