Why can't i get data value from json - flutter - json

Question

Am trying to get values from my json file categories.json using flutter but am always getting error or its not showing and I don't really know what is wrong
This is my main.dart
Future<List<Category>> loadData() async {
String jString = await rootBundle.loadString("assets/categories.json");
List<dynamic> jRes = jsonDecode(jString);
List<Category> datas = jRes.map((e) => Category.fromJson(e)).toList();
return datas;
}
Container(
child: FutureBuilder<List<Category>>(
future: loadData(),
builder: (context, data) {
if (data.connectionState != ConnectionState.waiting &&
data.hasData) {
var userList = data.data;
return ListView.builder(
itemCount: userList.length,
itemBuilder: (context, index) {
var userData = userList[index];
return Column(
children: [
Text("Category: ${userData.catName}"),
],
);
});
} else {
return Center(
child: CircularProgressIndicator(),
);
}
})),
and my model.dart
class Category {
String catId;
String catName;
SubCategory subcat;
Category({this.catId, this.catName, this.subcat});
factory Category.fromJson(Map<String, dynamic> json) {
return Category(
catId: json['cat_id'],
catName: json['category'],
subcat: SubCategory.fromJson(json['cat_subcategory']),
);
}
Map<String, dynamic> toJson() {
return {
"cat_id": catId,
"category": catName,
};
}
}
class SubCategory {
String subName, subImage;
SubCategory({this.subName, this.subImage});
factory SubCategory.fromJson(Map<String, dynamic> json) {
return SubCategory(subName: json['sub_name'], subImage: json['sub_image']);
}
Map<String, dynamic> toJson() {
return {
"sub_name": subName,
"sub_image": subImage,
};
}
}
and lastly my categories.json file
[{
"category": "Design & Creativity",
"cat_id": "1",
"cat_subcategory": [
{
"sub_name": "Ads",
"sub_image": "https://images.unsplash.com/photo-1589838017489-9198a27bd040?ixid=MXwxMjA3fDB8MHxzZWFyY2h8Mnx8YWR2ZXJ0aXNlbWVudHxlbnwwfHwwfA%3D%3D&ixlib=rb-1.2.1&auto=format&fit=crop&w=500&q=60"
}
]
}]
//There are more of them
The problem am facing when I run it is that it only shows me the CircularProgressIndicator() in my main.dart and when I remove the if statement, it says Another exception was thrown: NoSuchMethodError: The getter 'length' was called on null. Please how do I go about solving this problem and if you need more explanation then tell me
PS: When I check the loadData() value it says type 'List<dynamic>' is not a subtype of type 'Map<String, dynamic>'
EDIT: Thank you all for the answers but what i did was to generate a model using this website model generator
and i was able to get the value

Answer 1

Personally, when I want to load data from json file, I do this:
Future<List<Category>> loadData() async {
List<Category> datas = [];
return rootBundle.loadString("assets/categories.json").then((value) {
List<dynamic> jRes = jsonDecode(value);
jRes.forEach((element) {
datas.add(Category.fromJson(element));
});
return datas;
});
}
Also, in SubCategory class
factory SubCategory.fromJson(Map<String, dynamic> json) {
return SubCategory(subName: json['sub_name'], subImage: json['sub_image']);
}
Your fromJson() require Map<String, dynamic>.
But if you look at your file
"cat_subcategory": [
{
"sub_name": "Ads",
"sub_image": "https://images.unsplash.com/photo-1589838017489-9198a27bd040?ixid=MXwxMjA3fDB8MHxzZWFyY2h8Mnx8YWR2ZXJ0aXNlbWVudHxlbnwwfHwwfA%3D%3D&ixlib=rb-1.2.1&auto=format&fit=crop&w=500&q=60"
}
]
You can see that cat_subcategory is a List
So you are giving a List to your SubCategory.fromJson() instead of a Map.
If you want to give a Map, you can simply give the first index of your list
Your Category.fromJson() become
factory Category.fromJson(Map<String, dynamic> json) {
return Category(
catId: json['cat_id'],
catName: json['category'],
subcat: SubCategory.fromJson(json['cat_subcategory'][0]),
);
}

Answer 2

Change your model.dart class
class Category {
String category;
String catId;
List<CatSubcategory> catSubcategory;
Category({this.category, this.catId, this.catSubcategory});
Category.fromJson(Map<String, dynamic> json) {
category = json['category'];
catId = json['cat_id'];
if (json['cat_subcategory'] != null) {
catSubcategory = new List<CatSubcategory>();
json['cat_subcategory'].forEach((v) {
catSubcategory.add(new CatSubcategory.fromJson(v));
});
}}
Map<String, dynamic> toJson() {
final Map<String, dynamic> data = new Map<String, dynamic>();
data['category'] = this.category;
data['cat_id'] = this.catId;
if (this.catSubcategory != null) {
data['cat_subcategory'] =
this.catSubcategory.map((v) => v.toJson()).toList();
}
return data;
}}
class CatSubcategory {
String subName;
String subImage;
CatSubcategory({this.subName, this.subImage});
CatSubcategory.fromJson(Map<String, dynamic> json) {
subName = json['sub_name'];
subImage = json['sub_image'];
}
Map<String, dynamic> toJson() {
final Map<String, dynamic> data = new Map<String, dynamic>();
data['sub_name'] = this.subName;
data['sub_image'] = this.subImage;
return data;
}}
And Replace
List<Category> datas = jRes.map((e) => Category.fromJson(e)).toList();
with
List<Category> datas = List<Category>.from(jRes.map((category)=> Category.fromJson(category)));