Flutter - How to parse JSON data to a base class? - dart

Question

I'm wondering how can I parse a JSON data to a base class, because I'm trying to write, but get Unhandled Exception: type 'Future' is not a subtype of type 'WeatherModel' in type cast.
Here is my code:
abstract class BaseModel {
fromJson(Map<String, dynamic> json);
Map<String, dynamic> toJson();
}
import 'BaseModel.dart';
class WeatherModel extends BaseModel {
String success;
Result result;
Records records;
WeatherModel({this.success, this.result, this.records});
#override
fromJson(Map<String, dynamic> json) {
//...
}
#override
Map<String, dynamic> toJson() {
final Map<String, dynamic> data = new Map<String, dynamic>();
//... parse to Json
return data;
}
}
}
import 'dart:async';
import 'package:dio/dio.dart';
import 'package:flightinfo/model/BaseModel.dart';
class HttpUtils {
static Future<BaseModel> get(String url, Map<String, dynamic> params, BaseModel baseModel) async {
try {
print("url:$url,params:$params");
Response response = await Dio().get(url, queryParameters: params);
if (response != null && response.statusCode == 200) {
baseModel.fromJson(response.data);
print(baseModel);
return baseModel;
}
print(response);
} catch (exception) {
print(exception);
}
return null;
}
}
class WeatherRequest {
Future<WeatherModel> get() async {
return HttpUtils.get(_url, _params,new WeatherModel());
}
}
I think HttpUtils.get direct to BaseModel for extensibility. This is very common in Java, but in Dart I get a exception in WeatherRequest in below line:
return HttpUtils.get(_url, _params,new WeatherModel());
Unhandled Exception: type 'Future' is not a subtype of type 'WeatherModel' in type cast
In Dart, how to take care of this cast?

Answer 1

The error might be occurring because get() method of WeatherRequest returns a Future to WeatherModel but you have returned a Future<BaseModel> instead.
Try adding await to resolve Future converting it to BaseModel before returning.
return (await HttpUtils.get(_url, _params,new WeatherModel()) );
Also, as per this SO answer:
You can cast an object to its parent class, but you can't cast an object to any of its child class.
So implicit casting of BaseModel to WeatherModel may not work as expected. Instead, you can add a helper method to BaseModel like BaseModel.toWeatherModel and return a WeatherModel
abstract class BaseModel {
toWeatherModel(){
//...
}
}
Or modify get method of WeatherRequest to return Future<BaseModel>.
class WeatherRequest {
Future<BaseModel> get() async {
return (await HttpUtils.get(_url, _params,new WeatherModel()));
}
}