在 FutureBuilder 中设置提供者值

Question

我有一个小部件向返回地图的 api 发出请求。我想做的不是每次加载小部件时都发出相同的请求并将列表保存到appState.myList。但。当我在FutureBuilder 中执行此操作appState.myList = snapshot.data; 时，出现以下错误：

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文件：

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文件：

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');
    }
  }
}

我了解myList 调用notifyListeners()，这就是导致错误的原因。希望我做对了。如果是这样，我如何设置appState.myList并在应用程序中使用而不会出现上述错误？

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();
  }
}

Answer 1

出现该异常是因为您正在从其后代同步修改一个小部件。

这很糟糕，因为它可能导致小部件树不一致。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：

• 在您的ChangeNotifier 上执行的突变总是在与创建您的ChangeNotifier 的版本相同的build 中完成。

  在这种情况下，您可以直接从 ChangeNotifier 的构造函数中调用：

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

• 执行的更改可能会“延迟”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 addPostFrameCallback 或 Future.microtask 中：

  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();
    }
  }
  

Answer 2

我在使用提供程序时遇到了与您类似的问题。我的解决方法是在获取数据的时候加上WidgetsBinding.instance.addPostFrameCallback()。

Answer 3

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