Flutter-添加UserID之前正在使用的Firebase数据库参考

Question

我正在尝试从FirebaseAuth获取当前用户ID，然后将该ID用作Firebase数据库参考路径的一部分。我不确定如何让构建版本等到我的当前用户ID返回并且设置了数据库引用之后。

在下面的代码中，由于watchRef为空，因此在首次构建时将出错，但是如果引用位置中有数据，则最终将起作用。如果引用位置中没有数据，则侦听器将永远不会setState()数据，而我只会收到错误消息：

  

断言失败：第31行pos 12：'query！=   I / flutter（11449）：null'：不正确。

我尝试过FutureBuilders和Streams，但是我做的事情不正确。

任何帮助将不胜感激。

class WatchedList extends StatefulWidget {
  WatchedList({this.app, this.category});
  final FirebaseApp app;
  final String category;

  @override
  State<StatefulWidget> createState() {
    return WatchedListState();
  }
}

enum Category { watch, read, heard, done }

class WatchedListState extends State<WatchedList> {
  int count = 20;
  List<FirebaseItem> items = List();
  FirebaseItem item;
  String title = '';
  DatabaseReference watchRef;
  FirebaseUser currentUser;
  TextStyle textStyle = TextStyle(
    fontWeight: FontWeight.bold,
    fontSize: 20.0,
  );

  final GlobalKey<FormState> formKey = GlobalKey<FormState>();

  @override
  void initState() {
    super.initState();
    item = FirebaseItem('', '', '', '');
    _initDB();
  }

  void _initDB() async {
    final FirebaseDatabase database = FirebaseDatabase.instance;
    final FirebaseAuth firebaseAuth = FirebaseAuth.instance;
    currentUser = await firebaseAuth.currentUser();
    switch (widget.category) {
      case 'watch':
        watchRef = database
            .reference()
            .child('users')
            .child(currentUser.uid)
            .child('watch');
        title = '...WATCHED';
        break;
      case 'read':
        watchRef = database
            .reference()
            .child('users')
            .child(currentUser.uid)
            .child('read');
        title = '...READ';
        break;
      case 'heard':
        watchRef = database
            .reference()
            .child('users')
            .child(currentUser.uid)
            .child('heard');
        title = '...HEARD';
        break;
      case 'done':
        watchRef = database
            .reference()
            .child('users')
            .child(currentUser.uid)
            .child('done');
        title = '...DONE';
        break;
    }
    watchRef.onChildAdded.listen(_onEntryAdded);
    watchRef.onChildChanged.listen(_onEntryChanged);
    print(currentUser.uid);
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(title),
        actions: <Widget>[
          IconButton(
            icon: Icon(Icons.filter_list),
            onPressed: () {},
          ),
          // action button
          IconButton(
            icon: Icon(Icons.settings),
            onPressed: () {},
          ),
        ],
      ),
      body: Column(
        children: <Widget>[
          Flexible(
            child: FirebaseAnimatedList(
              query: watchRef,
              itemBuilder: (BuildContext context, DataSnapshot snapshot,
                  Animation<double> animation, int index) {
                return _buildItemSlide(index);
              },
            ),
          ),
        ],
      ),
      floatingActionButtonLocation: FloatingActionButtonLocation.centerFloat,
      floatingActionButton: FloatingActionButton.extended(
        onPressed: () {
          Navigator.push(
            context,
            MaterialPageRoute(
              builder: (context) => NewSuggestion(
                    app: widget.app,
                  ),
            ),
          );
        },
        label: Text('Add Suggestion'),
        icon: Icon(Icons.add),
      ),
    );
  }

  Widget _buildItemCard() {
    //Todo - Setting to change between cards and flat list?
    return Card(
      color: Colors.white,
      elevation: 2.0,
      child: ListTile(
        leading: CircleAvatar(
          child: Icon(Icons.ondemand_video),
        ),
        title: Text('Awesome Movie'),
        subtitle: Text('Describing awesome movie'),
        trailing: GestureDetector(
          child: Icon(
            Icons.delete,
            color: Colors.grey,
          ),
          onTap: () {},
        ),
        onTap: () {
          debugPrint('List Tile Tapped');
        },
      ),
    );
  }

  Widget _buildItemSlide(int index) {
    return ListTile(
      leading: CircleAvatar(
        child: Icon(Icons.ondemand_video),
      ),
      title: Text(items[index].title),
      subtitle: Text(items[index].body),
      trailing: GestureDetector(
        child: Icon(
          Icons.delete,
          color: Colors.grey,
        ),
        onTap: () {
          _deleteEntry(context, items[index], index);
        },
      ),
      onTap: () {
        Navigator.push(
            context,
            MaterialPageRoute(
                builder: (context) => ItemDetails(title: items[index].title)));
      },
    );
  }

  _onEntryAdded(Event event) {
    setState(() {
      items.add(FirebaseItem.fromSnapshot(event.snapshot));
    });
  }

  _onEntryChanged(Event event) {
    var old = items.singleWhere((entry) {
      return entry.key == event.snapshot.key;
    });
    setState(() {
      items[items.indexOf(old)] = FirebaseItem.fromSnapshot(event.snapshot);
    });
  }

  void _deleteEntry(
      BuildContext context, FirebaseItem item, int position) async {
    await watchRef.child(item.key).remove().then((_) {
      setState(() {
        items.removeAt(position);
      });
    });
  }
}