How to export and use global variable - flutter

Question

I am creating a Flutter App and I need a global http client with some setup.
Flutter: 1.5.4
Dart: 2.3.2
// utils/http.dart
import 'dart:io';
import 'package:cookie_jar/cookie_jar.dart';
import 'package:dio/dio.dart';
import 'package:path_provider/path_provider.dart';
Dio http = Dio();
void main() async {
// Save cookies to cookie jar.
Directory appDir = await getApplicationDocumentsDirectory();
http.interceptors.add(CookieManager(PersistCookieJar(dir: appDir.path)));
}
// main.dart
import 'package:flutter/material.dart';
import 'app.dart';
import 'utils/http.dart';
void main() {
print(http.interceptors); // returns []
runApp(App());
}
I expect that main function in http should be automatically executed.

Answer 1

A common solution to this is to wrap it in a Widget and place that widget in the Widget Tree. High up in the tree, so that it's sort of global. It's called "lifing state up".
The out of box solution of Flutter for such things is InheritedWidget. It's worth looking at and understand because most 3rd-party solutions rely on it.
To make life a tad easier, however, I use pacakge:provider. Code would look like so:
Creating
Provider<Dio>(
builder: (_) => Dio(),
child: MaterialApp(/*...*/)
);
Consuming
later in the widget tree:
Consumer<Dio>(
builder: (ctx, dio, _) {
// widget builder method
debugPrint('Dio instance: ${dio}');
return Container();
}
);