Race condition with stream listen - flutter

Question

I have an async function like below. However content is being returned null well before the stream listening is done.
I started playing out with Future.delayed, but thought better of it and wanted to ask if there is a better approach to ensure this is async?
import 'package:googleapis/drive/v3.dart' as ga;
static Future<String> getContentsFromFile() async {
String content;
ga.Media mediaResponse = await drive.files.get(fileId);
mediaResponse.stream.listen((data) {
print("DataReceived: "+data);
content = data
}, onDone: () async {
print("Task Done");
}, onError: (error) {
print("Some Error");
});
return content;
}
Im calling the function like so..
String content = await getContentsFromFile();

Answer 1

EDIT: Made the example more complete, with handling of errors and partial content.
You can use Completer for this sort of control flow:
import 'dart:async';
import 'package:googleapis/drive/v3.dart' as ga;
static Future<String> getContentsFromFile() async {
Completer<String> completer = Completer();
String content = "";
ga.Media mediaResponse = await drive.files.get(fileId);
mediaResponse.stream.listen((data) {
print("DataReceived: "+data);
content += data;
}, onDone: () async {
print("Task Done");
completer.complete(content);
}, onError: (error) {
print("Some Error");
completer.completeError(error);
});
return completer.future;
}