Dart : Parse object of Generic type - flutter

Question

Response 1:
{
"status" :"ok",
"message":"found",
"data" : {
"key1" :"value1",
"key2" :"value2"
}
}
Response 2 :
{
"status": "ok",
"message": "found",
"data": {
"users": [
{
"key1": "value1",
"key2": "value2"
}
]
}
}
Want to achieve for creating common class with type T.
it can be parsed like this :
Common<List<Data>> commonRes = Common<List<Data>>.fromJson(jsonDecode(res.body));
or
Common<Data> commonRes = Common<Data>.fromJson(jsonDecode(res.body));
Wanted to achieve same as java generic type T.
Anyone knows how to achieve this in dart ?
Tried to create common class like this. but, no luck.
class Common<T> {
String? status;
String? message;
T? data;
Common({
this.status,
this.message,
this.data,
});
Common.fromJson(Map<String, dynamic> json) {
status = json['status'];
message = json['message'];
data = json['data'] as T;
}
Map<String, dynamic> toJson() {
final Map<String, dynamic> data1 = Map<String, dynamic>();
data1['status'] = this.status;
data1['message'] = this.message;
data1['data'] = this.data;
return data1;
}
}

Answer 1

It becomes apparent rather quickly that what you were trying to do is impossible, as dart has no idea how to convert json['data'] to T and it can't know, a similar thing is the case in the toJson() function.
Usually the way to solve this is by helping them out and providing generator functions, which some of the code generator libraries also do, which I would suggest checking out. Anyway, here is the general idea behind it if you want to implement it yourself, but I recommend using a codegen library as this gets pretty old rather quickly once you have many API calls etc.
import 'dart:convert';
class Common<T> {
String? status;
String? message;
T? data;
Common({
this.status,
this.message,
this.data,
});
Common.fromJson(Map<String, dynamic> json, T Function(Map<String, dynamic>) fromTJSON) {
status = json['status'];
message = json['message'];
data = fromTJSON(json['data']);
}
Map<String, dynamic> toJson(Map<String, dynamic> Function(T?) toTJSON) {
final Map<String, dynamic> data1 = Map<String, dynamic>();
data1['status'] = status;
data1['message'] = message;
data1['data'] = toTJSON(data);
return data1;
}
}
class ClassA {
String? a;
ClassA({this.a});
ClassA.fromJson(Map<String, dynamic> json) {
a = json['a'];
}
Map<String, dynamic> toJson() {
final Map<String, dynamic> data1 = Map<String, dynamic>();
data1['a'] = a;
return data1;
}
}
class ClassB {
String? b;
ClassB({this.b});
ClassB.fromJson(Map<String, dynamic> json) {
b = json['b'];
}
static ClassB? generatorFoo(Map<String, dynamic> json) {
return null;
}
Map<String, dynamic> toJson() {
final Map<String, dynamic> data1 = Map<String, dynamic>();
data1['b'] = b;
return data1;
}
}
int main() {
String c1JSON = '''
{
"status" :"ok",
"message":"found",
"data" : {
"a" :"someA"
}
}
''';
String c2JSON = '''
{
"status" :"ok",
"message":"found",
"data" : {
"b" :"someb"
}
}
''';
Map<String,dynamic> map1 = jsonDecode(c1JSON);
Common<ClassA> cA = Common.fromJson(map1, (map) => ClassA.fromJson(map));
print(cA.toJson((obj) => obj != null ? obj.toJson() : {}));
Map<String,dynamic> map2 = jsonDecode(c2JSON);
Common<ClassB> cB = Common.fromJson(map2, (map) => ClassB.fromJson(map));
print(cB.toJson((obj) => obj != null ? obj.toJson() : {}));
return 0;
}
In your case you will also need to write a function to help with the mapping of the lists, the general idea is to provide a generator function so you can encode and decode the nested generic types.