流供应商与扑火中的问题

Question

我有两个提供程序，并且我使用了MultiProvider，ChangeNotifierProvider正常运行，但是StreamProvider的问题是我的示例：

main.dart

class Makani extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    Db db = Db();
    return MultiProvider(
      providers: [
        StreamProvider(create: (context)=> db.getLocations(),),
        ChangeNotifierProvider(create: (context) => Model(),),
      ],
      child: MaterialApp(
        debugShowCheckedModeBanner: false,
        home: Directionality(
          textDirection: TextDirection.rtl,
          child: Scaffold(
            body: Map(),
          ),
        ),
      ),
    );
  }
}

db.dart

class Db {
  FirebaseFirestore  locationList =
  FirebaseFirestore.instance;

  Stream<List<Locations>> getLocations() {

    return locationList.collection('locations').snapshots()
        .map((snapshot) => snapshot.docs
        .map((doc) => Locations.fromFireStore(doc.data()))
        .toList());
  }
}

location.dart

class Locations{
   String title;
   String body;

  Locations({this.title, this.body});

    Locations.fromFireStore(Map<String, dynamic> data)
      : title = data['title'],
        body = data['body'];


}

tile.dart

这里（loc）返回null ！！！而且我不知道为什么吗？

class tiles extends StatelessWidget {

  @override
  Widget build(BuildContext context) {
    var loc = Provider.of<List<Locations>>(context);

    return Align(
      alignment: Alignment.bottomCenter,
      child: Container(
        margin: EdgeInsets.only(bottom: 16.0),
        height: 140.0,
        child: Directionality(
            textDirection: TextDirection.ltr,
            child: ScrollablePositionedList.builder(
              reverse: true,
              scrollDirection: Axis.horizontal,
              itemCount: loc.length,
              itemScrollController: tilesController,
              itemBuilder: (context, index) {
                addMarker(loc[index]);
                return tile(loc[index]);
              },

我在Map（）内部调用该类，而在main（）内部调用它