1. Basic code for model

1. define model name 'User'

2. define model parameters/variables

3. define constructor with class name

4. define fromJson()

5. define Map()

              class User {
                  //1. variable declaration
                  int? userId;
                  int? id;
                  String? title;
                  String? body;

                  //2. constructor
                  User({
                    this.userId,
                    this.id,
                    this.title,
                    this.body,
                  });

                  //3. fromJson()
                  User.fromJson(Map<String, dynamic> json) {
                    id = json['id'];
                    userId = json['userId'];
                    title = json['title'];
                    body = json['body'];
                  }


                  //4. Map()
                  Map<String, dynamic> toJson() {
                    final Map<String, dynamic> data = new Map<String, dynamic>();
                    data['userId'] = this.userId;
                    data['id'] = this.id;
                    data['title'] = this.title;
                    data['body'] = this.body;
                    return data;
                  }

                }


                

2. use model in the controller

import user model

                import 'package:getx_restaurant/models/user.dart';
              

create object of User model

              User? user;
              

set the data to model using API

              try {
                  isLoading(true);
                  http.Response response = await http.get(Uri.tryParse(
                      'https://jsonplaceholder.typicode.com/posts/1')!);
                  if (response.statusCode == 200) {
                    ///data successfully
                    var result = jsonDecode(response.body);
                   
                    user = User.fromJson(result);


                  } else {
                    print('error fetching data');
                  }
                } catch (e) {
                  print('Error while getting data is $e');
                } finally {
                  isLoading(false);
                }

  

complete code

                import 'package:flutter/cupertino.dart';
import 'dart:convert';
import 'package:get/get.dart';
import 'package:getx_restaurant/models/user.dart';
import 'package:http/http.dart' as http;


class LoginController extends GetxController {
  final name="manoj";
  var isLoading = false.obs;
  User? user;

  @override
  void onInit() {
      super.onInit();
      fetchData();
  }


  fetchData() async {
    try {
      isLoading(true);
      http.Response response = await http.get(Uri.tryParse(
          'https://jsonplaceholder.typicode.com/posts/1')!);
      if (response.statusCode == 200) {
        ///data successfully
        var result = jsonDecode(response.body);

        user = User.fromJson(result);
        //print(user?title);

      } else {
        print('error fetching data');
      }
    } catch (e) {
      print('Error while getting data is $e');
    } finally {
      isLoading(false);
    }
  }



}

Access the data from view

child: Text("${logincontroller.user?.title}"),

              class Geo {
  final double lat;
  final double lng;

  Geo({
    this.lat = 0.0,
    this.lng = 0.0,
  });

  Map<String, dynamic> toJson() {
    return {
      'lat': lat,
      'lng': lng,
    };
  }
}

class Address {
  final String? street;
  final String? suite;
  final String? city;
  final String? zipcode;

  Address({
    this.street,
    this.suite,
    this.city,
    this.zipcode,
  });

  Map<String, dynamic> toJson() {
    return {
      'street': street,
      'suite': suite,
      'city': city,
      'zipcode': zipcode,
    };
  }
}

class Company {
  final String? name;
  final String? catchPhrase;
  final String? bs;

  Company({
    this.name,
    this.catchPhrase,
    this.bs,
  });

  Map<String, dynamic> toJson() {
    return {
      'name': name,
      'catchPhrase': catchPhrase,
      'bs': bs,
    };
  }
}

class User {
  final int id;
  final String? username;
  final String? email;
  final Address? address;
  final String? phone;
  final String? website;
  final Company? company;

  User({
    required this.id,
    this.username,
    this.email,
    this.address,
    this.phone,
    this.website,
    this.company,
  });

  Map<String, dynamic> toJson() {
    return {
      'id': id,
      'username': username,
      'email': email,
      'address': address?.toJson(),
      'phone': phone,
      'website': website,
      'company': company?.toJson(),
    };
  }
}