Flutter/Dart 未处理的异常：类型“String”不是类型转换中“Map<String, dynamic>”类型的子类型

Question

我将 JSON 作为字符串从 response.body 发送，但得到“未处理的异常：类型 'String' 不是类型转换中的 'Map' 类型的子类型 " 在 Flutter/Dart 中映射后出错。 如果我手动编写 JSON，代码可以工作。但是，如果我从参数中获取 JSON 作为字符串，则会出现转换错误。

我尝试将返回值转换为 List，但我猜 extractData 应该改变，卡在这里。 "s" 是 JSON 字符串。

getVacDates 函数仅用于通过映射从 JSON 中获取日期或总值，例如 [10/14/21, 10/15/21, 10/16/21...]。

static List<String> getVacDates(String s) {
    final newList = jsonEncode(s);
    final extractedData = jsonDecode(newList) as Map<String, dynamic>;
    final china= extractedData['timeline'].map((e) => e["date"]);

    List<String> StringList = china.cast<String>();
    print(StringList);
    return StringList;
  }

JSON 信息如下：

{"country":"USA","timeline":

[{"total":409571117,"daily":757824,"totalPerHundred":121,"dailyPerMillion":2253,"date":"10/14/21"},

    {"total":410559043,"daily":743873,"totalPerHundred":122,"dailyPerMillion":2212,"date":"10/15/21"},    

    {"total":411028977,"daily":737439,"totalPerHundred":122,"dailyPerMillion":2193,"date":"10/16/21"},    

    {"total":411287235,"daily":731383,"totalPerHundred":122,"dailyPerMillion":2175,"date":"10/17/21"}]}

简化程序：

responseVac = await http.get('https://disease.sh/v3/covid-19/vaccine/coverage/countries/usa?lastdays=3');
        var data = MyFile.myFunction(xxx, yyy, responseVac.body);

在我的文件中：

static ChartsData myFunction(String xxx, bool yyy, String responseVac) {

List<String> getVacDates(String s) =>
          [for (final data in jsonDecode(s)['timeline']) data['date'] as String];

      print(getVacDates(responseVac));

Answer 1

不完全确定你想要得到什么，但它是这样的吗？

import 'dart:convert';

void main() {
  print(getVacDates(jsonString)); // [10/14/21, 10/15/21, 10/16/21, 10/17/21]
}

List<String> getVacDates(String s) => [
      for (final timeline in jsonDecode(s)['timeline'])
        timeline['date'] as String
    ];

const jsonString = '''{
  "country": "USA",
  "timeline": [
    {
      "total": 409571117,
      "daily": 757824,
      "totalPerHundred": 121,
      "dailyPerMillion": 2253,
      "date": "10/14/21"
    },
    {
      "total": 410559043,
      "daily": 743873,
      "totalPerHundred": 122,
      "dailyPerMillion": 2212,
      "date": "10/15/21"
    },
    {
      "total": 411028977,
      "daily": 737439,
      "totalPerHundred": 122,
      "dailyPerMillion": 2193,
      "date": "10/16/21"
    },
    {
      "total": 411287235,
      "daily": 731383,
      "totalPerHundred": 122,
      "dailyPerMillion": 2175,
      "date": "10/17/21"
    }
  ]
}''';