颤振提供者 "uid was caled on null"

Question

在过去的几个月里，我一直在研究 Flutter，该死的，我被困在了这个上。

我想设置一个提供商，它接受经过身份验证的用户的 ID，但是每当我运行我的应用程序时，它都会给我一个错误：

The getter 'uid' was called on null.

这是我的代码：

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);
  final String title;

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  // ignore: deprecated_member_use
  AuthService _auth = new AuthService();
  TimerService _timer;
  User user;

  List<Timing> timers = [];

  initUser() async {
    await _auth.signInAnon();
    user = await _auth.currentUser();
    print("THE UID IS ${user.uid}");
    setState(() {});
  }

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

  @override
  Widget build(BuildContext context) {
    //TODO: implement firebase data req
    return StreamProvider<List<Timing>>.value(
      value: TimerService(user.uid).timingStream,
      child: Scaffold(
        backgroundColor: Styles.backgroundColor,
        floatingActionButton: FloatingActionButton(
          onPressed: () {
            Navigator.push(context,
                MaterialPageRoute(builder: (context) => CreateTimer()));
          },
          child: Icon(Icons.add),
        ),
        appBar: AppBar(
          elevation: 0.1,
          backgroundColor: Styles.backgroundColor,
          centerTitle: true,
          title: Text(widget.title),
          actions: <Widget>[
            IconButton(
              icon: Icon(Icons.list),
              onPressed: () {
                //TODO
              },
            )
          ],
        ),
        body: Column(children: [
          Expanded(
            child: SingleChildScrollView(
              child: Column(
                  crossAxisAlignment: CrossAxisAlignment.stretch,
                  mainAxisAlignment: MainAxisAlignment.start,
                  children: [
                    TimerList(),
                  ]),
            ),
          ),
        ]),
      ),
    );
  }
}

它还会因此抛出另一个错误：

Another exception was thrown: NoSuchMethodError: The getter 'length' was called on null.

这是由于我使用了所提供的数据：

class TimerList extends StatefulWidget {
  TimerListState createState() => TimerListState();
}

class TimerListState extends State<TimerList> {
  @override
  Widget build(BuildContext context) {
    final timingStream = Provider.of<List<Timing>>(context);

    return ListView.builder(
        itemCount: timingStream.length,
        itemBuilder: (context, index) {
          return Column(
            crossAxisAlignment: CrossAxisAlignment.stretch,
            mainAxisAlignment: MainAxisAlignment.start,
            children: <Widget>[TimerWidget(timingStream[index])],
          );
        });
  }
}

请向我解释为什么以及如何为提供商提供用户 uid 的正确方法。如果我发布了过多的代码，我深表歉意，为了安全起见，抱歉。

Answer 1

在 Dart 中，默认情况下，如果您未分配任何值，则该值为 null 所以在你的情况下 User 是 null 所以创建一个新对象。

  User user=new User();

现在，如果您想使用用户实例，那么您可以

initUser() async {
    await _auth.signInAnon();
    user = await _auth.currentUser();
    print("THE UID IS ${user.uid}");
    setState(() {});
  }

颤振提供者 "uid was caled on null"

Flutter Provider "uid was caled on null"

provider

flutter