google-cloud-firestore - Flutter Firestore 分页

Question

我正在尝试使用 Firestore 进行分页，我阅读了文档，它在 Swift

中像这样实现

let first = db.collection("cities")
    .order(by: "population")
    .limit(to: 25)

first.addSnapshotListener { (snapshot, error) in
    guard let snapshot = snapshot else {
        print("Error retrieving cities: \(error.debugDescription)")
        return
    }

    guard let lastSnapshot = snapshot.documents.last else {
        // The collection is empty.
        return
    }

    // Construct a new query starting after this document,
    // retrieving the next 25 cities.
    let next = db.collection("cities")
        .order(by: "population")
        .start(afterDocument: lastSnapshot)

    // Use the query for pagination.
    // ...
}

为了练习，我尝试获取三个文档，如果点击按钮，则再获取一个文档。

 Firestore.instance.collection('user').where('name', isEqualTo: 'Tom').orderBy('age').limit(3).getDocuments().then((snapshot) {
     _lastDocument = snapshot.documents.last;
     snapshot.documents.forEach((snap) {
        print(snap.data);
     });
   });

点击按钮后尝试这样。

 Firestore.instance.collection('user').where('name', isEqualTo: 'Tom').orderBy('age').startAfter(_lastDocument).limit(1).getDocuments().then((snapshot) {
     snapshot.documents.forEach((snap) {
        print(snap.data);
      });
     });

但是控制台这么说。

The following assertion was thrown while handling a gesture: type 'DocumentSnapshot' is not a subtype of type 'List[dynamic]'

为什么我必须通过列表？

有谁知道如何解决这个问题？

更新

我可以这样分页。

class PaginationExample extends StatefulWidget {
  @override
  _PaginationExampleState createState() => _PaginationExampleState();
}

class _PaginationExampleState extends State<PaginationExample> {
  var _restaurants = <Restaurant>[];
  var _nomore = false;
  var _isFetching = false;
  DocumentSnapshot _lastDocument;
  ScrollController _controller;


  void _fetchDocuments() async {
    final QuerySnapshot querySnapshot = await Firestore.instance.collection('restaurants').orderBy('likes').limit(8).getDocuments();
    // your logic here
  }

  Future<Null> _fetchFromLast() async {
    final QuerySnapshot querySnapshot = await Firestore.instance.collection('restaurants').orderBy('likes').startAfter([_lastDocument['likes']]).limit(4).getDocuments();
      if (querySnapshot.documents.length < 4) {
          _nomore = true;
          return;
      }
      _lastDocument = querySnapshot.documents.last;
      for (final DocumentSnapshot snapshot in querySnapshot.documents) {
        final Restaurant re = Restaurant(snapshot);
        _restaurants.add(re);
      }
      setState(() {});
  }

  void _scrollListener() async {
    if (_nomore) return;
    if (_controller.position.pixels == _controller.position.maxScrollExtent && _isFetching == false) {
        _isFetching = true;
        await _fetchFromLast();
        _isFetching = false;
    }
  }

@override
  void initState() {
    _fetchDocuments();
    _controller = new ScrollController()..addListener(_scrollListener);
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    return Container(

    );
  }
}

Answer 1

这里有错误:

     Firestore.instance.collection('user').where('name', isEqualTo: 'Tom').orderBy('age').startAfter(_lastDocument).limit(1).getDocuments().then((snapshot) {
         snapshot.documents.forEach((snap) {
            print(snap.data);
          });
         });

startAfter 方法需要一个 List 值参数，而您正在传递一个 DocumentSnapshot。

Takes a list of [values], creates and returns a new [Query] that starts after the provided fields relative to the order of the query.

你可以试试这样的:

 Firestore.instance.collection('user').where('name', isEqualTo: 'Tom').orderBy('age').startAfter([{'name': 'Tom'}]).limit(1).getDocuments().then((snapshot) {
         snapshot.documents.forEach((snap) {
            print(snap.data);
          });
         });