Flutter: The argument type 'dynamic' can't be assigned to the parameter type 'Map<String, dynamic>'

Question

Suppose I have a json object like this:

{
 "sort": "asc",
 "page": {
    "next": "asdfg",
    "prev": "kljekj"
 }
}

I wrote a datamodel in dart like this:

class AllPage{
 String sort;
 Page page;

 AllPage({this.sort, this.page});

 AllPage.fromJson(Map<String, dynamic> json) {
   return AllPage(
     sort = json['sort'],
     page = Page.fromJson(json['page']));
 }
}

class Page {
 String next;
 String prev;

 Page({this.next, this.prev});

 Page.fromJson(Map<String, dynamic> json) {
  return Page(
    next = json['next'],
    prev = json['prev']);
 }
}

I have seen countless examples in which this works. However, I am getting an error in my AppPage class, inside Page.FromJson(json['page']). The error says: "The argument type 'dynamic' can't be assigned to the parameter type 'Map>string, dynamic>'.

I am new to flutter/dart. So this could be something simple that I am missing. Any help is greatly appreciated. Thank you!

aqwert aqwert · Accepted Answer · 2020-12-01T03:28:42

json['page'] returns a dynamic since it could be really anything. You need to do some casting

class AllPage {
  String sort;
  Page page;

  AllPage({this.sort, this.page});

  factory AllPage.fromJson(Map<String, dynamic> json) {
    return AllPage(
      sort: json['sort'].toString(),
      page: Page.fromJson(json['page'] as Map),
    );
  }
}

class Page {
  String next;
  String prev;

  Page({this.next, this.prev});

  factory Page.fromJson(Map json) {
    return Page(
      next: json['next'].toString(),
      prev: json['prev'].toString(),
    );
  }
}

Note that it is assuming a lot about the structure of the json. You should add checks in such as what if json['page'] or json['next'] is null.

Also fixed up some of the compile issues around the use of constructors

Flutter: The argument type 'dynamic' can't be assigned to the parameter type 'Map<String, dynamic>'

1 Answers