_dev.dart

import 'dart:async';

import '../config_loader.dart';
import '../env_config.dart';

class Config implements EnvConfig {
  Config._();

  static Future<EnvConfig> create() async {
    // read secrets
    await ConfigLoader.initialize();
    // return the new env Config
    return Config._();
  }

  @override
  String get name => 'DEV';

  @override
  String get hostURL => ConfigLoader.hostURL;
}

_prod.dart

import 'dart:async';

import '../config_loader.dart';
import '../env_config.dart';

class Config implements EnvConfig {
  Config._();

  static Future<EnvConfig> create() async {
    // read secrets
    await ConfigLoader.initialize(isDev: false);
    // return the new env Config
    return Config._();
  }

  @override
  String get name => 'PROD';

  @override
  String get hostURL => ConfigLoader.hostURL;
}

app_config.json

{
  "devHost": "Development Host",
  "prodHost": "Production Host"
}

config_loader.dart

import 'dart:convert';

import 'package:flutter/services.dart';

class ConfigLoader {
  static Map<String, dynamic> _config;
  static bool _isDev;

  static Future<void> initialize({bool isDev = true}) async {
    try {
      //Set Environment
      _isDev = isDev;

      //load rawConfig String
      final rawConfig = await rootBundle.loadString('config/app_config.json');
      _config = json.decode(rawConfig) as Map<String, dynamic>;
    } catch (e) {
      print(e.toString());
    }
  }

  static String get hostURL =>
      _config[_isDev ? 'devHost' : 'prodHost'] as String;
}

env.dart

import '../env_config.dart';
import '_dev.dart' as _dev;
import '_prod.dart' as _prod;

abstract class Environment {
  static Future<EnvConfig> get dev => _dev.Config.create();
  static Future<EnvConfig> get prod => _prod.Config.create();
}

env_config.dart

abstract class EnvConfig {
  String get name;
  String get hostURL;
}

main_common.dart

import 'package:flutter/foundation.dart';
import 'package:flutter/material.dart';
import 'package:socket_chat/config/env_config.dart';

import 'config/config_loader.dart';

void mainCommon(EnvConfig envConfig) async {
  print(ConfigLoader.hostURL);
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Socket Chat',
      home: Home(),
      debugShowCheckedModeBanner: false,
    );
  }
}

main_dev.dart

Future<void> main() async {
  WidgetsFlutterBinding.ensureInitialized();

  final envConfig = await Environment.dev;

  mainCommon(envConfig);
}

main_prod.dart

Future<void> main() async {
  WidgetsFlutterBinding.ensureInitialized();

  final envConfig = await Environment.prod;

  mainCommon(envConfig);
}