返回地图时 FutureBuilder 中的空快照

Question

我不明白为什么我在未来的构建器中总是得到空返回，而我确定应该有一个值

//global variables
Map<String,bool> students;
List<String> studentsList ;
List<bool> studentsCheck ;

FutureBuilder(
  future: GetInfoFromBackEnd(),
  builder: (BuildContext context, AsyncSnapshot snapshot) {
    return snapshot.hasData?
    CustomScrollView(....some not important widgets here....):CircularProgressIndicator();

},)
Future GetInfoFromBackEnd() async {
students = {
  'this info should be taken from backend0':false,
  'this info should be taken from backend0':false,
  'this info should be taken from backend0':false,
  'this info should be taken from backend0':false,
  'this info should be taken from backend0':false,
  };
studentsList = students.keys;
studentsCheck = students.values;
return students;
 }

我对此代码有一个解决方法，但我需要了解为什么会这样以及发生了什么

Answer 1

提供了包含您的代码的基本 HelloWorld Flutter 示例。我向未来添加了延迟，以便 ProgressIndicator 显示两秒钟。像我预期的那样工作：

import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);
  final String title;

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  int _counter = 0;
  Map<String, bool> students;

  void _incrementCounter() {
    setState(() {
      _counter++;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: FutureBuilder(
          future: GetInfoFromBackEnd(),
          builder: (BuildContext context, AsyncSnapshot<Map> snapshot) {
            return snapshot.hasData
                ? new Text(snapshot.data.toString())
                : new CircularProgressIndicator();
          }),
      floatingActionButton: FloatingActionButton(
        onPressed: _incrementCounter,
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ),
    );
  }

  Future<Map> GetInfoFromBackEnd() async {
    return Future.delayed(Duration(seconds: 2), () {
      students = {
        'this info should be taken from backend0': false,
        'this info should be taken from backend1': false,
        'this info should be taken from backend2': false,
        'this info should be taken from backend3': false,
        'this info should be taken from backend4': false,
      };
      return students;
    });
  }
}

Answer 2

问题不在 FutureBuilder 或地图中，实际上，当我试图在 List 中获取学生的 Map 值和键时，它会破坏 Future 并阻止它返回正确的值，所以我在快照中得到了 null。数据;这是我修复它的方法：

studentsList = students.keys.toList();
studentsCheck = students.values.toList();
return students;

附： : 错误是type '_CompactIterable<String>' is not a subtype of type 'List<String>'