FirebaseFirestore.get 返回 null - Flutter

Question

I'm trying to fetch data from Firestore.

class _TestState extends State<Test> {
bool abcd;
final adsRef = FirebaseFirestore.instance.collection('All ads');
getData(){
adsRef.get().then((snap) {
  snap.docs.forEach((doc) { 
    abcd=doc.data().isEmpty;
  });
});
}

@override
Widget build(BuildContext context) {
return Scaffold(
  appBar: new AppBar(
    title: Text("TestPage"),
  ),
  body: FlatButton(
    onPressed: () {
      print(abcd);
    },
    child: Text("data"),
  ),
  );
  }
  }

Here's the DB:

And I've tried getting doc.id , doc.exists , etc
but It's returning 'null'.

PS: I'm a newbie to firebase.

Answer 1

Data is loaded from Firebase/Firestore (and most modern cloud APIs) asynchronously, and needs to be passed to the UI through a setState() call (or another state management primitive).

You should:

1. start loading the data in the initState method of the class
2. then call setState() when the data is available (or changes)
3. this will trigger the build() method, where you can then use the data from the state

So something like:

class _TestState extends State<Test> {
  bool abcd;
  final adsRef = FirebaseFirestore.instance.collection('All ads');

  @override
  void initState() {
    super.initState();

    adsRef.get().then((snap) {
      snap.docs.forEach((doc) { 
        setState(() {
          abcd=doc.data().isEmpty;
        });
      });
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: new AppBar(
        title: Text("TestPage"),
      ),
      body: FlatButton(
        onPressed: () {
          print(abcd);
        },
        child: Text("data"),
      ),
    );
  }
}

Answer 2

You need to use "Async/Await" to get data from firebase

async getData()

and call

data = await getData() like this.