Prevent Concurrent Access to File in Dart - flutter

Question

I am creating a disk cache and I faced problem while running flutter tests cause by json.decode() but I found that the problem is that the same file is being read twice at the same time.
Here is the code for the disk cache class:
import 'dart:convert';
import 'dart:io';
import 'package:path_provider/path_provider.dart';
import 'package:the_chef/repositories/cache/base_cache.dart';
class DiskCache<V> extends BaseCache<List<String>, V> {
String directoryPath;
DiskCache({
BaseCache<List<String>, V> other,
}) {
next = other;
}
Future<V> _get(List<String> path) async {
print("get $path");
var val;
var file;
file = await _exists(path);
if (file != null) {
print("start");
var str = await file.readAsString();
val = json.decode(str);
print("end get $path");
return val;
}
val ??= next?.get(path);
if (val != null) {
await _createFile(file);
var bytes = json.encode(val);
await file.writeAsString(bytes);
}
if(val == null) {
print("notfound");
}
return val;
}
Future<V> get(List<String> path) async {
var x = await _get(path);
return x;
}
#override
Future<void> set(List<String> path, V value) async {
print("set $path");
final file = await _createFileFromPath(path);
var bytes = json.encode(value);
await file.writeAsString(bytes);
next?.set(path, value);
print("end set $path");
}
#override
Future<void> evict(List<String> path) async {
print("evict $path");
final file = await _localFile(path);
if (await file.exists()) {
await file.delete();
} else {
await file.delete(recursive: true);
}
await next?.evict(path);
}
Future<File> _exists(List<String> path) async {
var file = await _localFile(path);
return (await file.exists()) ? file : null;
}
Future<File> _localFile(List<String> filePath) async {
final path = await _localPath();
return File('$path/${filePath.join('/')}');
}
Future<String> _localPath() async {
if (directoryPath != null) {
return directoryPath;
}
directoryPath = (await getApplicationDocumentsDirectory()).path;
return directoryPath;
}
Future<File> _createFileFromPath(List<String> path) async {
var file = await _localFile(path);
await _createFile(file);
return file;
}
Future<void> _createFile(File file) async {
file.createSync(recursive: true);
}
}
It would be very great if you have any suggestion of how to improve the cache or have any other advices.
Thanks in advance.