问如何从firebase获取文档？

EN

 var videoCollectionQuery = await FirebaseFirestore.instance.collection('sendvideos').where("previewimage",isEqualTo: widget.videofile).get(); 
//this will get all the documents that satisfy your 'where' condition, you need to use'get()' and 'await' also.

String videoUrl = videoCollectionQuery.docs.isNotEmpty ?videoCollectionQuery.docs[0].data()['videourl'] : 'No results found';
// this will get you the map you need, unless the query is empty, it'll return 'No results found'.

  final FirebaseAuth _firebaseAuth = FirebaseAuth.instance;
  final FirebaseUser currentUser = await _firebaseAuth.currentUser();

  if (currentUser != null) {
    String uidCur = currentUser.uid;
    QuerySnapshot querySnapshot = await Firestore.instance
        .collection("user_category")
        .document(uidCur)
        .collection('categories')
        .getDocuments();

    var x = querySnapshot.documents;

    _myCategoryData.clear();
    notifyListeners();
    x.forEach((f) => _myCategoryData.add(f));