未处理的异常：类型“String”不是类型转换中“Future<String>”类型的子类型

Question

我遇到了这个错误，谁能帮我解决这个错误

static Future<String> get_video_lecture_subject(int schoolId,int classroom) async {

  var body;
   body = jsonEncode({
  "school_id": schoolId,
  "classroom": classroom,
  });

  final response = await http.post(
      'https://b5c4tdo0hd.execute-api.ap-south-1.amazonaws.com/testing/get-video-lecture-subjects',
      headers: {"Content-Type": "application/json"},
      body: body,
  );

   print(response.body.toString());
   return response.body.toString();
 }

我在 getpref() 函数中使用了上述函数

 getpref() async {
    SharedPreferences prefs = await SharedPreferences.getInstance();
    int classNo = int.parse(prefs.getString("class")[0]);
    int schoolId = prefs.getInt("school_id");

      print("hello");
      response=(await ApiService.get_video_lecture_subject(schoolId, classNo)) as Future<String> ;
      print(response);
  }

Answer 1

表达式：

(await ApiService.get_video_lecture_subject(schoolId, classNo)) as Future<String>

调用你的方法get_video_lecture_subject。这将返回一个Future<String>。 然后你等待那个未来，这会产生一个String。

然后您尝试将 String 转换为 Future<String>。那失败了，因为字符串不是未来。

尝试删除as Future<String>。

Answer 2

查看这个示例，如果它有效，请告诉我

import 'dart:async';

import 'package:flutter/material.dart';

void main() {
  getPrefs();
  runApp(MyApp());
}

void getPrefs() async {
  String value = await yourfunction();
  print(value);
}

Future<String> yourfunction() {
  final c = new Completer<String>();

  // Below is the sample example you can do your api calling here
  Timer(Duration(seconds: 1), () {
    c.complete("you should see me second");
  });
  // Process your thigs above and send the string via the complete method.
  // in you case it will be c.complete(response.body);

  return c.future;
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
        home: Scaffold(
      body: SafeArea(
          child:
              /* FutureBuilder<String>(
            future: yourfunction(),
            builder: (context, snapshot) {
              if (snapshot.hasData) {
                return Text(snapshot.data);
              }

              return CircularProgressIndicator();
            }), */
              Text('')),
    ));
  }
}

Answer 3

用这个替换该行

      final res = await ApiService.get_video_lecture_subject(schoolId, classNo);

更新：更改变量名称（可能它的类型被初始化为 Future）