How to test a saved file in flutter - flutter

Question

This flutter web page reading-writing-files
provides an example of saving a file, in this case containing a counter. I have pasted this into an example app. How can I unit test the file saving part of it?
My failed test attempt is below the app code.
There are 2 issues:
How to write the test itself - what is the testable return from the
t = storage.writeCounter(counter);
how can I shut the app down and prove that the file persists and is later readable
main.dart
import 'package:flutter/foundation.dart';
import 'package:flutter/material.dart';
import 'package:path_provider/path_provider.dart';
void main() {
runApp(
MaterialApp(
title: 'Reading and Writing Files',
home: FlutterDemo(storage: CounterStorage()),
),
);
}
class CounterStorage {
Future<String> get _localPath async {
final directory = await getApplicationDocumentsDirectory();
return directory.path;
}
Future<File> get _localFile async {
final path = await _localPath;
return File('$path/counter.txt');
}
Future<int> readCounter() async {
try {
final file = await _localFile;
// Read the file
final contents = await file.readAsString();
return int.parse(contents);
} catch (e) {
// If encountering an error, return 0
return 0;
}
}
Future<File> writeCounter(int counter) async {
final file = await _localFile;
// Write the file
return file.writeAsString('$counter');
}
}
class FlutterDemo extends StatefulWidget {
const FlutterDemo({Key? key, required this.storage}) : super(key: key);
final CounterStorage storage;
#override
_FlutterDemoState createState() => _FlutterDemoState();
}
class _FlutterDemoState extends State<FlutterDemo> {
int _counter = 0;
#override
void initState() {
super.initState();
widget.storage.readCounter().then((int value) {
setState(() {
_counter = value;
});
});
}
Future<File> _incrementCounter() {
setState(() {
_counter++;
});
// Write the variable as a string to the file.
return widget.storage.writeCounter(_counter);
}
#override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: const Text('Reading and Writing Files'),
),
body: Center(
child: Text(
'Button tapped $_counter time${_counter == 1 ? '' : 's'}.',
),
),
floatingActionButton: FloatingActionButton(
onPressed: _incrementCounter,
tooltip: 'Increment',
child: const Icon(Icons.add),
),
);
}
}
counter_test.dart
import 'package:write_file_test/main.dart';
void main() {
test('Check file save works', () async {
final CounterStorage storage;
var counter = 6;
var t = storage.writeCounter(counter);
expect(find.text('1'), findsOneWidget);
});
}
pubspec.yaml
name: write_file_test
description: Flutter test file saving
publish_to: 'none' # Remove this line if you wish to publish to pub.dev
version: 1.0.0+1
environment:
sdk: ">=2.12.0 <3.0.0"
dependencies:
flutter:
sdk: flutter
path_provider: ^2.0.5
cupertino_icons: ^1.0.2
dev_dependencies:
flutter_test:
sdk: flutter
flutter_lints: ^1.0.0
flutter:
uses-material-design: true