Future ignores await in flutter - firebase

Question

I'm trying to make a version control for my application. In other words, trying to connect to firebase documents, pick version number from there and compare it with value from shared preferences in order to decided if it's required to update the information.
I've followed the official tutorial on Futures.
here is my code:
#override
void initState(){
loadit();
}
Future _getIntVersionFromSharedPref() async {
await Firestore.instance.collection('DataIndex')
.document('dbversion').snapshots().listen((event) {
setState(() {
fbVersion = event['currentversion'];
});
print('_getIntVersionFromSharedPref() future -> $fbVersion');
return fbVersion;
});
}
Future<String> loadit() async {
var myFBversion = await _getIntVersionFromSharedPref();
print('loadit future -> $myFBversion');
}
However I endup with the following console output:
I/flutter: loadit future -> null
I/flutter: _getIntVersionFromSharedPref() future -> 1
Why would it fire null right away without waiting for the value? According to the official tutorial it should be executed in turn.

Answer 1

listen() doesn't return a Future, so you can't await it. It returns a StreamSubscription object asynchronously, and you must handle the ongoing results of the query within your stream handler function. When you are done with the Stream, you should unsubscribe from the stream using the StreamSubscription.
If you just want a single snapshot of the document, don't use snapshots() at all. Just use get() to get a single snapshot as shown in the documentation.

Answer 2

Listen isn't part of the dart Future API, so that listen callback isn't tied to the Promise that is implicitly returned from _getIntVersionFromSharedPref.
I think you need to manually return a Promise from that method, and resolve it from your listen callback. You can manually resolve a Future like this with the Completer class.
Something like this:
Future _getIntVersionFromSharedPref() async {
var completer = Completer<num>();
await Firestore.instance.collection('DataIndex')
.document('dbversion').snapshots().listen((event) {
setState(() {
fbVersion = event['currentversion'];
});
print('_getIntVersionFromSharedPref() future -> $fbVersion');
completer.complete($fbVersion);
});
// This will be returned right away, but anything that awaits it will block
// until the callback above has completed the future.
return completer.future;
}