flutter - 在FutureBuilder中设置提供者的值(value)

我有一个小部件，它向返回 map 的api发出请求。我想做的是每次加载小部件时都不会发出相同的请求，并将列表保存到appState.myList。但。当我在appState.myList = snapshot.data;中执行此FutureBuilder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会调用notifyListeners()，这就是导致错误的原因。希望我说对了。如果是这样，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();
  }
}

最佳答案

之所以出现该异常，是因为您正在从其后代同步修改窗口小部件。

这很糟糕，因为它可能导致小部件树不一致。一些小部件。可能会使用突变前的值来构建窗口小部件，而其他窗口小部件可能会使用突变后的值。

解决的办法是消除不一致之处。使用ChangeNotifierProvider，通常有两种情况:

对ChangeNotifier执行的变异总是在与创建ChangeNotifier的构建相同的构建中完成。

在这种情况下，您可以直接从ChangeNotifier的构造函数进行调用:

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

所执行的更改可以“懒惰地”发生(通常在更改页面之后)。

在这种情况下，您应该将突变包装在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();
  }
}