“Future<List<Question>>”类型的值不能分配给“List<Question>”类型的变量答案

【问题标题】：A value of type 'Future<List<Question>>' can't be assigned to a variable of type 'List<Question>'“Future<List<Question>>”类型的值不能分配给“List<Question>”类型的变量
【发布时间】：2020-06-01 09:37:34
【问题描述】：

import 'dart:async';
import 'question.dart';
import 'package:http/http.dart' as http;
import 'dart:convert';

String opentdb = 'https://opentdb.com/api.php?amount=15&type=boolean';

class QuestionServices {
  Future<List<Question>> getData() async {
    List<Question> questions;
    String link = opentdb;
    var res = await http
        .get(Uri.encodeFull(link), headers: {"Accept": "application/json"});
    print(res.body);
    if (res.statusCode == 200) {
      var data = json.decode(res.body);
      var rest = data['results'] as List;
      print(rest);
      questions =
          rest.map<Question>((json) => Question.fromJson(json)).toList();
    }
    print("List Size: ${questions.length}");
    // _questions = questions;
    return questions;
  }

  List<Question> newQuestions = getData();
}

class Question {
  final String question;
  final bool answer;

  Question({this.question, this.answer});

  factory Question.fromJson(Map<String, dynamic> json) {
    return Question(
      question: json['question'] as String,
      answer: json['correct_answer'] as bool,
    );
  }
}

我正在尝试从 JSON 数据库创建问题列表，但每当我尝试获取返回的列表时，我都会收到错误消息：

"A value of type 'Future<List<Question>>' can't be assigned to a variable of type 'List<Question>'."

我不确定为什么我返回的 List 会发出该错误。是否有其他方法可以将 json 加入列表？

【问题讨论】：

列表 newQuestions = await getData();您可能错过了“等待”关键字

标签： json flutter dart

【解决方案1】：

List<Question> getData() async { // remove Future
    List<Question> questions;
    String link = opentdb;
 ...


//how to access now you will get instance of List<Question>

getData().then((List<Question> newQuestions){

})

【讨论】：

【解决方案2】：

getData 返回一个Future，因此您需要执行以下操作：

Future<List<Question>> newQuestions = getData();

【讨论】：

【解决方案3】：

Future<List<Question>>Listfetchdata() async{
   final responsein = await http.get('https://opentdb.com/api.php?amount=15&type=boolean');
   final getindata = json.decode(responsein.body);
   final getindata1 = json.encode(getindata['results']);
   return compute(listdata,getindata1);
 }

 List<Question> listdata(String creation){
   final parsed = jsonDecode(creation).cast<Map<String,dynamic>>();
   return parsed.map<Question>((json)=>Question.fromJson(json)).toList();
 }

List listitem;
 var maketolistarry=[''];
 void lastgettingdata() async{
   final king=await Listfetchdata();
listitem=king.map((e) => e.category).toList();
maketolistarry=listitem;
 }

在 maketolistarry 的帮助下你会得到一个列表

【讨论】：