flutter Why doesn't async and await work? - firebase

Question

_getstockList
_getstockList( List<dynamic> nlist) async {
Map<String, dynamic> userdocdata;
var userdata = await firestore.collection('users').doc('NVPjZEAZneKblrubGZSW').get();
userdocdata = userdata.data() as Map<String, dynamic>;
nlist = userdocdata['favorite'];
}
Main Code
Widget build(BuildContext context) {
List<dynamic> list = [];
List<Map<String, dynamic>> stockcardlist = [];
_getstockList(list);
print(list); // output
_getstockInfo(list, stockcardlist);
~~~
}
_getstockInfo
_getstockInfo(List<dynamic> nlist, List<Map<String,dynamic>> stockcardlist){
print(nlist.length); // output
}
Desired result
print(list)
print(nlist.length)
valid value
BUT
result
print(list) = []
print(nlist.length) = 0
please help me i use Future, sync, unawait but i cant solve

Answer 1

It looks like _getStockList doesn't return anything. When you pass it, the original object remains unaffected. You could try to fix that:
Future<List<dynamic>> _getstockList() async {
Map<String, dynamic> userdocdata;
var userdata = await firestore.collection('users').doc('NVPjZEAZneKblrubGZSW').get();
userdocdata = userdata.data() as Map<String, dynamic>;
List<dynamic> nlist = userdocdata['favorite'];
return nlist;
}
Now you need to call this function to fill the list
Widget build(BuildContext context) {
List<Map<String, dynamic>> stockcardlist = [];
List<dynamic> list = await _getstockList(); // DOES NOT WORK!
print(list);
_getstockInfo(list, stockcardlist);
...
}
However, this does not work, since you are not allowed to use await in build, which is not async. To get around this, you can use a FutureBuilder:
https://api.flutter.dev/flutter/widgets/FutureBuilder-class.html
Widget build(BuildContext context) {
return FutureBuilder<List<dynamic>>(
future: _getstockList(),
builder: (BuildContext context, AsyncSnapshot<List<dynamic>> snapshot) {
List<dynamic> list = snapshot.data ?? [];
print(list);
return MyAwesomeScreenWithAList(list);
}
);
}
Now if you want to use the results of the first async function in a second one, the easiest will probably be, though not ideal, to use two nested FutureBuilders.