Flutter：如何在子小部件的构建上下文之外的函数中访问“提供者”值？

Question

提供者值来自父窗口小部件。我可以在构建上下文下使用提供者值。但是我需要 getHomeCampaigns 函数中的提供者值。一旦构建了小部件，我尝试定义局部变量并将它们分配给提供程序值，但该函数声称这些变量是在 null 上调用的。我猜它们是在提供者的构建上下文下设置之前被使用的。

class HomeScreen extends StatefulWidget {
  @override
  _HomeScreenState createState() => _HomeScreenState();
}

class _HomeScreenState extends State<HomeScreen> {
  // Variables to be set inside the function and then used by the last widget
  User currentUser;
  String country;
  List<CampaignWidget> friendsCamps;
  List<CampaignWidget> featuredCamps;

  // Function that needs the provider values
  getHomeCampaigns() async {
    // Get all campaigns where their owner is in the list of friends for the current user
    QuerySnapshot friendsSnapshot = await Firestore.instance
        .collection('campaigns')
        .where('user.uid', whereIn: currentUser.friends)
        .where('attributes.active', isEqualTo: true)
        .orderBy('created', descending: true)
        .limit(20)
        .getDocuments();
    // Get featured campaigns documents in the current country
    QuerySnapshot featuredSnapshot = await Firestore.instance
        .collection('campaigns')
        .where('attributes.featured', isEqualTo: true)
        .where('funders.${currentUser.uid}', isEqualTo: false)
        .where('country', isEqualTo: country)
        .orderBy('user.rating', descending: true)
        .limit(5)
        .getDocuments();
    // Make 2 lists of CampaignWidget out of the documents retrieved
    List<CampaignWidget> campaigns = friendsSnapshot.documents
        .map((doc) => CampaignWidget(campaign: Campaign.fromDocument(doc)))
        .toList();
    List<CampaignWidget> featured = featuredSnapshot.documents
        .map((doc) => CampaignWidget(campaign: Campaign.fromDocument(doc)))
        .toList();
    setState(() {
      // Set the featured and friends lists of CampaignWidget to the newly made lists
      this.featuredCamps = featured;
      this.friendsCamps = campaigns;
    });
  }

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

  @override
  Widget build(BuildContext context) {
    this.currentUser = Provider.of<User>(context);
    this.country = Provider.of<String>(context);
    return Scaffold(
      backgroundColor: Color(0xFFE8E8E8),
      appBar: AppBar(
        centerTitle: false,
        title: Text("Home"),
        actions: <Widget>[
          /// Search users
          IconButton(
            icon: Icon(
              Icons.search,
            ),
            onPressed: () {},
          ),
        ],
      ),
      body: RefreshIndicator(
        onRefresh: () => getHomeCampaigns(),
        child: // Some widget that uses the variables,
      ),
    );
  }
}

Answer 1

您可以像这样从您的提供程序读取变量：Provider.of(context, listen: false).yourVariable in getHomeCampaigns。您必须拥有 listen: false 才能使其正常工作，因为它不在小部件树中，因此无论何时 yourVariable 更改它都无法更新 - 请参阅 .如果这不起作用，则在您声明您的提供者时会出现问题。如果可行，请投票并接受作为答案，否则请在下方评论。

Answer 2

我的 api 从作为一个继承的小部件转移到一个提供者。 “The Flutter Complete Reference”建议避免由于复杂性而使用inheritWidget。我从 materialapp 开始，将它包裹在我的 api 类中，其中包含我的 restful 调用。我使用计数器查看按钮已被按下。现在，我可以在 materialapp 树中的所有对象中访问我的 api 调用。

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
@override
Widget build(BuildContext context) {
return MaterialApp(
    title: 'My app',
    theme: ThemeData(
         primarySwatch: Colors.blue,
    ),
    debugShowCheckedModeBanner: false,
    home: Provider<Api>(
        create: (context) => Api(),
        child: Home(title: 'my title')));
  }
}

class Home extends StatefulWidget {
 Home({Key key, @required this.title}) : super(key: key);
 final String title;
 @override
 _HomeState createState() => _HomeState();
 }

class _HomeState extends State<Home> {
 int _counter = 0;
 onLogin(BuildContext context) {
   LoginView param = new LoginView("abc@com",
    "xxx");

   Provider.of<Api>(context, listen: false)
      .addLogin(context, param)
    .then((value) {
    setState(() {
    _counter++;
    });
   }).catchError((error) {
    DialogCaller.showErrorDialog(context, 
error.toString()).then((value) {});
});
}

 @override
 Widget build(BuildContext context) {
  return Scaffold(
        appBar: AppBar(
          title: const Text('Example'),
        ),
        body: Container(
            child: Column(
          children: [
            TextButton.icon(
                label: Text("abc"),
                icon: Icon(Icons.web),
                onPressed: () {
                  onLogin(context);
                  setState(() {
                    _counter++;
                  });
                }),
            Text(
              '$_counter',
              style: 
 Theme.of(context).textTheme.headline4,
            )
          ],
        )));
 }
}