Flutter: Coding so that the logic waits until class members are populated before rendering app on screen

A flutter newbie, I’m trying to get my app to take values from a local json file and render them on-screen.

The logic isn’t waiting for the class constructor to populate the relevant string variable before rendering the app on screen.

Here’s the code that illustrates my problem.

First, my main.dart file:

import 'dart:io';

import 'package:flutter/foundation.dart';
import 'package:flutter/material.dart';
import 'sampleDataClass.dart';
import 'package:provider/provider.dart';

String assetFilePath = 'assets/basicData.json';

void main() async {
  WidgetsFlutterBinding.ensureInitialized();
  FlutterError.onError = (details) {
    FlutterError.presentError(details);
    if (kReleaseMode) exit(1);
  };

  runApp(const MyApp());
}

class MyApp extends StatefulWidget {

  const MyApp({super.key});

  @override
  State<MyApp> createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {
  @override
  Widget build(BuildContext context) {
    return ChangeNotifierProvider(
      create: (context) => MyAppState(),
      child: const MaterialApp(
        title: "Sample screen",
        home: MyHomePage(),
      )
    );
  }
}

class MyAppState extends ChangeNotifier {
  SampleDataClass current = SampleDataClass(assetFilePath);
}

class MyHomePage extends StatelessWidget {
  const MyHomePage({super.key});


  @override
  Widget build(BuildContext context) {
    var myAppState = context.watch<MyAppState>();

    var myAppBasicData = myAppState.current;

    return Scaffold(
        appBar: AppBar(
          backgroundColor: Colors.indigo,
          foregroundColor: Colors.amberAccent,
          title: const Text("This is the App Bar"),
          elevation: 10,
        ),
        body: Column(
            crossAxisAlignment: CrossAxisAlignment.start,
            children: [
              Row(children: [
                Expanded(
                    child: Container(
                        color: Colors.blueGrey,
                        padding: const EdgeInsets.all(10),
                        child: (
                            Text(myAppBasicData.language,
                                style: const TextStyle(
                                  color: Colors.white,
                                ))))),
              ]),
            ]
        ),
    );
  }
}

Here is my SampleDataClass.dart file:

import 'dart:core';
import 'dart:convert';
import 'package:flutter/services.dart' show rootBundle;

class SampleDataClass {
  String classFilePath = "";

  String language = "not populated";
  String batteryName = "not populated";

  SampleDataClass(filePath) {
    rootBundle.loadString(filePath).then((jsonDataString) {
      Map classDataMap = jsonDecode(jsonDataString);
      language = classDataMap['language'];
      print(language);
      batteryName = classDataMap['batteryName'];
      print(batteryName);
    });
  }

  Map<String, dynamic> toJson() => {
    'language': language,
    'batteryName': batteryName,
  };
}

And here’s my pubspec.yaml file:

name: samples
description: A new Flutter project.

environment:
  sdk: '>=2.18.6 <3.0.0'

  flutter:
    sdk: flutter
  cupertino_icons: ^1.0.2
  provider: ^4.1.1

dev_dependencies:
  flutter_test:
    sdk: flutter
  flutter_lints: ^2.0.0

flutter:
  uses-material-design: true
  assets:
    - assets/basicData.json

Finally, my basicData.json file looks like this:

{
  "language": "English",
  "batteryName": "Exercise 32"
}

The print statements in the sampleDataClass class work fine but as you can see if you run the code (from the command line using "flutter run –no-sound-null-safety"), the app renders the initial values of the variables before the class constructor assigns the json values to them.

What I expected was the class constructor to complete writing the relevant json values to the class members before handing back the class instance to the build widget in main.dart to render the app on-screen.

I realise that this is a very elementary question, but it is a key pending learning task for me!

Thank you all very much in advance!