Prevent concurrent access to the same data in Dart - dart

Question

I'm trying to create a file cache in Dart (Flutter), where a file only gets downloaded once and then cached for future requests. (Yes, I know there are existing packages for this, but my needs are more specific.)
Problem is, if I have two widgets on the same page trying to display the same image, they're both making the same request at the same time, downloading the file twice.
I tried turning the cache into a singleton, handing out a single instance of itself, but that seems to have no effect:
class FileCache {
final _fileList = List<File>();
static FileCache _instance;
factory FileCache() {
if (_instance == null) {
_instance = FileCache._internal();
}
return _instance;
}
FileCache._internal();
bool add(File file) {
if (_fileList.contains(file)) {
return false;
}
_fileList.add(file);
return true;
}
void remove(File file) {
_fileList.remove(file);
}
}
I did see another package that does synchronization (here), but looking at the Dart code I have no idea how it is enforcing the synchronous access.
How, in Dart, can you force a specific class or member variable to be accessed serially for this purpose?

Answer 1

The Flutter UI runs in a single isolate. Memory isn't shared across isolates (hence the name), so you don't need to worry about parallel operations (as you would with multiple threads on a multi-core system). However, you do need to worry about concurrent operations that can be interleaved when execution yields from await.
This means that you don't need special atomic primitives. You could set a flag when downloading a file to avoid downloading it again.
You don't use Futures anywhere, so there are no places for your code (as shown) to be interrupted. However, you also don't show the code where you're actually downloading files, and presumably you have asynchrony there. You could do something like:
final pendingDownloads = <String, Future<void>>{};
Future<void> downloadFile(String url) {
if (pendingDownloads.containsKey(url)) {
return pendingDownloads[url];
}
Future<void> downloadFileInternal() async {
final request = await HttpClient().getUrl(...);
...
}
pendingDownloads[url] = downloadFileInternal();
return pendingDownloads[url];
}