17

I have a widget that makes a request to an api which returns a map. What I would like to do is not make the same request every time the widget is loaded and save the list to appState.myList. But. when I do this appState.myList = snapshot.data; in the FutureBuilder, I get the following error:

flutter: ══╡ EXCEPTION CAUGHT BY FOUNDATION LIBRARY ╞════════════════════════════════════════════════════════
flutter: The following assertion was thrown while dispatching notifications for MySchedule:
flutter: setState() or markNeedsBuild() called during build.
flutter: This ChangeNotifierProvider<MySchedule> widget cannot be marked as needing to build because the
flutter: framework is already in the process of building widgets. A widget can be marked as needing to be
flutter: built during the build phase only if one of its ancestors is currently building. ...

sun.dart file:

class Sun extends StatelessWidget {
  Widget build(BuildContext context) {
    final appState = Provider.of<MySchedule>(context);
    var db = PostDB();

    Widget listBuild(appState) {
      final list = appState.myList;
      return ListView.builder(
        itemCount: list.length,
        itemBuilder: (context, index) {
          return ListTile(title: Text(list[index].title));
        },
      );
    }

    Widget futureBuild(appState) {
      return FutureBuilder(
        future: db.getPosts(),
        builder: (BuildContext context, AsyncSnapshot snapshot) {
          if (snapshot.hasData) {
            // appState.myList = snapshot.data;
            return ListView.builder(
              itemCount: snapshot.data.length,
              itemBuilder: (context, index) {
                return ListTile(title: Text(snapshot.data[index].title));
              },
            );
          } else if (snapshot.hasError) {
            return Text("${snapshot.error}");
          }
          return Center(
            child: CircularProgressIndicator(),
          );
        },
      );
    }

    return Scaffold(
        body: appState.myList != null
            ? listBuild(appState)
            : futureBuild(appState));
  }
}

postService.dart file:

class PostDB {
  var isLoading = false;

  Future<List<Postmodel>> getPosts() async {
    isLoading = true;
    final response =
        await http.get("https://jsonplaceholder.typicode.com/posts");

    if (response.statusCode == 200) {
      isLoading = false;
      return (json.decode(response.body) as List)
          .map((data) => Postmodel.fromJson(data))
          .toList();
    } else {
      throw Exception('Failed to load posts');
    }
  }
}

I understand that the myList calls notifyListeners() and that's what causes the error. Hope I got that right. If so, how do I set appState.myList and use in the app without getting the above error?

import 'package:flutter/foundation.dart';
import 'package:myflutter/models/post-model.dart';

class MySchedule with ChangeNotifier {
  List<Postmodel> _myList;

  List<Postmodel> get myList => _myList;

  set myList(List<Postmodel> newValue) {
    _myList = newValue;
    notifyListeners();
  }
}
4

3 回答 3

12

出现该异常是因为您正在从其后代同步修改小部件。

这很糟糕，因为它可能导致不一致的小部件树。一些小部件。可以使用突变前的值构建小部件，而其他小部件可能正在使用突变值。

解决方案是消除不一致。使用ChangeNotifierProvider，通常有两种情况：

  • 对您执行的突变ChangeNotifier总是在与创建您的ChangeNotifier.

    在这种情况下，您可以直接从您的构造函数进行调用ChangeNotifier：

    class MyNotifier with ChangeNotifier {
  MyNotifier() {
    // TODO: start some request
  }
}

  • 执行的更改可能会“懒惰地”发生（通常在更改页面之后）。

    在这种情况下，您应该将您的突变包装在 anaddPostFrameCallback或 a中Future.microtask：

    class Example extends StatefulWidget {
  @override
  _ExampleState createState() => _ExampleState();
}

class _ExampleState extends State<Example> {
  MyNotifier notifier;

  @override
  void didChangeDependencies() {
    super.didChangeDependencies();
    final notifier = Provider.of<MyNotifier>(context);

    if (this.notifier != notifier) {
      this.notifier = notifier;
      Future.microtask(() => notifier.doSomeHttpCall());
    }
  }

  @override
  Widget build(BuildContext context) {
    return Container();
  }
}
于 2019-06-06T15:37:52.770 回答
1

只需notifyListeners();从代码中删除。我遇到了这个错误，这就是我为解决问题所做的。

于 2020-11-06T08:44:55.247 回答
1

我在使用提供程序时遇到了与您类似的问题。我的解决方案是WidgetsBinding.instance.addPostFrameCallback()在获取数据时添加。

于 2019-08-23T01:53:30.217 回答