在 FutureBuilder 中设置提供者值

Question

我有一个小部件可以向 api 发出请求，其中 returns 是一张地图。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 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');
    }
  }
}

我知道 myList 调用了 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同步修改其后代的小部件。

这很糟糕，因为它可能导致小部件树不一致。一些小部件。可能是使用突变前的值构建的小部件，而其他人可能正在使用突变后的值。

解决方法是消除不一致。使用ChangeNotifierProvider，通常有2种情况：

对您的 ChangeNotifier 执行的突变始终在与创建您的 ChangeNotifier 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
  }
}
```

执行的更改可能会发生 "lazily"（通常在更改页面之后）。

在那种情况下，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provider的时候遇到了和你类似的问题。我的解决办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，这就是我解决问题的方法。

在 FutureBuilder 中设置提供者值

Setting provider value in FutureBuilder

dart

flutter

flutter-provider