NoSuchMethodError（NoSuchMethodError：方法'[]'在null上被调用。接收者：null尝试调用：[]（“title”））

Posted 2023-03-07

【中文标题】NoSuchMethodError（NoSuchMethodError：方法\'[]\'在null上被调用。接收者：null尝试调用：[]（“title”））

【英文标题】：NoSuchMethodError (NoSuchMethodError: The method '[]' was called on null. Receiver: null Tried calling: []("title"))NoSuchMethodError（NoSuchMethodError：方法'[]'在null上被调用。接收者：null尝试调用：[]（“title”））

【发布时间】：2020-09-15 19:07:41

【问题描述】：

从 json 获取数据后出现此错误；

我的模型是这样的：

class JobModel 
 final String title;
 final int id;
 final String slug;
 final String content;

JobModel(this.title, this.id, this.slug, this.content);

JobModel.fromJson(Map<String, dynamic> json)
  : title = json['title'],
    id = json['id'],
    slug = json['slug'],
    content = json['content'];

这是我从 json 文件中获取数据的函数：

List<JobModel> jobList;
Map<String, dynamic> jsonResponse;
Future<List<JobModel>> _getJobs() async 
var response = await http.get(
    "https://www.eradauti.ro/api/context?pathname=/anunturi/pagina-1&userID=");
this.setState(() 
  jsonResponse = json.decode(response.body);
);
jobList = List<JobModel>();
jsonResponse.forEach((key, value) 
  JobModel job = JobModel.fromJson(value);
  jobList.add(job);
);
print(jsonResponse[0]['title']); //shows null
return jobList;

我的 json 文件如下所示：

  "context": [
    // first JobModel Map
    
      "title": "some title here",
      "id": 1234,
      "slug": "some slug value",
      "content": "the rest of the object content",
    ,
    // second JobModel Map
    
      "title": "some other title here",
      "id": 5678,
      "slug": "some other slug value",
      "content": "blah blah,",
    ,
  ]

你能告诉我我做错了什么吗？

【参考方案1】：

您的响应格式与您编码的格式不同。

结构是这样的：

"context": 
    "categories": [
        
            "id": 15,
            "title": "Auto-Moto",
            "slug": "auto-moto",
            "content": "Vanzari-cumparari de automobile, piese acte. Anunţurile sunt destinate zonei Radauti-Suceava<br />",
            "related": 
                "link": "/anunturi/auto-moto-15",
                "archiveLink": "/arhiva-anunturi/c/auto-moto-15",
                "deletedPostCount": 387,
                "postCount": 22
            
        ,

因此，要获取作业列表，您应该相应地对其进行解析。进入工作列表的正确语句应该是这样的：

print(jsonResponse['context']['categories'])

为了能够创建作业模型列表，请尝试以下操作：

var json = jsonResponse['context']['categories'];
var jobList = List<JobModel>();
json.forEach((key, value) 
  print(key);
  JobModel job = JobModel.fromJson(value);
  jobList.add(job);
);
print(jobList);

试试这个 sn-p：

Future<List<JobModel>> _getJobs() async 
     var response = await http.get(
    "https://www.eradauti.ro/api/context?pathname=/anunturi/pagina-1&userID=");
     jsonResponse = json.decode(response.body);
     var json = jsonResponse['context']['categories'];
     var jobList = List<JobModel>();
     json.forEach((key, value) 
       print(key);
       JobModel job = JobModel.fromJson(value);
       jobList.add(job);
     );
     print(jobList);

【讨论】：

如果我把var json = jsonResponse['context']['categories'] 放在jsonResponse = json.decode(response.body) 之前，我似乎不知道我应该把它放在哪里，没有任何反应，没有错误，如果我把它放在我收到错误Local variable 'json' can't be referenced before it is declared. 之后。但是print(jsonResponse['context']['categories']); 起作用了。