如何设置从另一个 StreamProvider 获取参数值的 StreamProvider？

Question

我正在为我的应用程序使用 FirebaseAuth 和 Firestore。

'User' 是一个模型，它表示 firestore 上的文档，并使用 FirebaseUser 的 uid 作为其 DocumentId。

为了获得 'User'，我必须从 FirebaseUser 传入 uid。

我的想法是设置一个FirebaseUser的StreamProvider，然后设置另一个User的StreamProvider，它以FirebaseUser为参数。

但是出现错误，我找不到解决这个问题的方法，任何建议都会对我有很大帮助。

List<SingleChildCloneableWidget> uiConsumableProviders = [
  StreamProvider<FirebaseUser>(
    builder: (context) => Provider.of<LoginNotifier>(context, listen: false).streamFirebaseUser,
  ),
  StreamProvider<User>(
    builder: (context) =>
        Provider.of<UserNotifier>(context, listen: false).streamUser(Provider.of<FirebaseUser>(context)),
  ),
];

I/flutter (15813): ══╡ EXCEPTION CAUGHT BY WIDGETS LIBRARY ╞═══════════════════════════════════════════════════════════
I/flutter (15813): The following assertion was thrown building InheritedProvider<FirebaseUser>:
I/flutter (15813): inheritFromWidgetOfExactType(InheritedProvider<FirebaseUser>) or inheritFromElement() was called
I/flutter (15813): before BuilderStateDelegate<Stream<User>>.initDelegate() completed.
I/flutter (15813): 
I/flutter (15813): When an inherited widget changes, for example if the value of Theme.of()
I/flutter (15813): changes, its dependent widgets are rebuilt. If the dependent widget's reference
I/flutter (15813): to the inherited widget is in a constructor or an initDelegate() method, then
I/flutter (15813): the rebuilt dependent widget will not reflect the changes in the inherited
I/flutter (15813): widget.
I/flutter (15813): 
I/flutter (15813): Typically references to inherited widgets should occur in widget build()
I/flutter (15813): methods. Alternatively, initialization based on inherited widgets can be placed
I/flutter (15813): in the didChangeDependencies method, which is called after initDelegate and
I/flutter (15813): whenever the dependencies change thereafter.
I/flutter (15813):

Answer 1

您不能在 builder 内调用 Provider.of。

但是您可以使用小部件将 Provider.of 转换为流，然后在 StreamProvider 的 builder 中使用该流。

这是一个可以为您完成的小部件：

class ProviderToStream<T> extends StatefulWidget
    implements SingleChildCloneableWidget {
  const ProviderToStream({Key key, this.builder, this.child}) : super(key: key);

  final ValueWidgetBuilder<Stream<T>> builder;
  final Widget child;

  @override
  _ProviderToStreamState<T> createState() => _ProviderToStreamState<T>();

  @override
  ProviderToStream<T> cloneWithChild(Widget child) {
    return ProviderToStream(
      key: key,
      builder: builder,
      child: child,
    );
  }
}

class _ProviderToStreamState<T> extends State<ProviderToStream> {
  final StreamController<T> controller = StreamController<T>();

  @override
  void dispose() {
    controller.close();
    super.dispose();
  }

  @override
  void didChangeDependencies() {
    super.didChangeDependencies();
    controller.add(Provider.of<T>(context));
  }

  @override
  Widget build(BuildContext context) {
    return widget.builder(context, controller.stream, widget.child);
  }
}

那么你可以这样做：

MultiProvider(
  providers: [
    StreamProvider<Foo>(builder: (_) => Stream.empty()),
    ProviderToStream<Foo>(
      builder: (_, foo, child) => StreamProvider<String>(
        builder: (_) async* {
          await for (final value in foo) {
            yield value.toString();
          }
        },
        child: child,
      ),
    ),
  ],
  child: MaterialApp(),
);

Answer 2

经过一些变通办法后，我通过将 rxdart 中的 Observable 与 Provider 组合解决了我的问题。像这样：

Stream<User> get userStream {
return Observable(_firebaseAuth.onAuthStateChanged).switchMap((user) {
  if (user != null) {
    return userCollection
        .document(user.uid)
        .snapshots()
        .map((doc) => User.fromDocumentSnapshot(doc))
        .handleError(_catchError);
  } else {
    return Observable<User>.just(null);
  }
});}

外部 MultiProvider：

StreamProvider<User>(builder: (context) => Provider.of<FireStoreService>(context, listen: false).userStream,),

希望这对遇到同样问题的人有所帮助。

如何设置从另一个 StreamProvider 获取参数值的 StreamProvider？

How to setup StreamProvider which take a param value from another StreamProvider?

flutter

flutter-provider