Intro

For Mobile App developer consuming data from API is one of skill set that you must have. Flutter it self have many package to help developer consuming data from API.

DIO

DIO is a popular package for retrive data from API on flutter, it's powefull and easy to use.

GET Example

For example we would using reqres.in API to perform GET, POST and PUT

First we should make a data model, this is to store the data from API before we consume it with flutter.
You can make data model based on response data from API, here the response data from method GET reqres.in :
GET:https://reqres.in/api/users?page=2

class UserModel {
// this is the data from json response,
// each type data based on success json response
  int? page;
  int? perPage;
  int? total;
  int? totalPages;
  List<Data>? data;

// this is constructor
  UserModel({
    this.page,
    this.perPage,
    this.total,
    this.totalPages,
    this.data,
  });

// this is function to store data base on key-value response
  UserModel.fromJson(Map<String, dynamic> json) {
    page = json['page'];
    perPage = json['per_page'];
    total = json['total'];
    totalPages = json['total_pages'];
    data = json['data'] == null
        ? []
        : (json['data'] as List).map((e) => Data.fromJson(e)).toList();
// on this data we should mapping each response from List to 'class Data'
  }
}

// this class for store data from each response on List
class Data {
  int? id;
  String? email;
  String? firstName;
  String? lastName;
  String? avatar;

  Data({
    this.id,
    this.email,
    this.firstName,
    this.lastName,
    this.avatar,
  });

  Data.fromJson(Map<String, dynamic> json) {
    id = json['id'];
    email = json['email'];
    firstName = json['first_name'];
    lastName = json['last_name'];
    avatar = json['avatar'];
  }
}

then we would make function to Get API with DIO.
Using GET method on DIO is easy, we just need to declare DIO package on a class then make a function to use method GET and passing the URL.

class ApiServices {
  Dio dio = Dio();

  Future<Map<String, dynamic>> getUser() async {
    const String url = 'https://reqres.in/api/users?page=2';
    try {
      Response response = await dio.get(url);
      return response.data;
    } on DioError catch (error, trace) {
      print('DIO error : $error, on : $trace');
      return error.response!.data;
    }
  }
}

Explanation:

Future<Map<String, dynamic>> getUser() async {

on this code, we use Future Type data because Dart only allow asyncronous on Future type data function. Then we use Type data Map of String, and dynamic because the response from GET is still array of json, we need to mapping it later.

Then we make another function to cast Map of String, dynamic to List of DataModel:

Future<List<DataModel>> userServices() async {
    List<DataModel> userList = [];
    var response = await getUser();
    if (response.containsKey('data') && response['data'] is List) {
      userList =
          (response['data'] as List).map((e) => DataModel.fromJson(e)).toList();
    }
    return userList;
  }

Explanation:

Future<List<DataModel>> userServices() async {

The return data from getUser is Map of json data, we should convert it to List of DataModel before we consume it on Flutter.

if (response.containsKey('data') && response['data'] is List) {

Because we just want to consume array of 'data' we should ensure that the response has it and then we mapping it to DataModel.

userList =
(response['data'] as List).map((e) => DataModel.fromJson(e)).toList();

And then we have List of DataModel, we can consume it with ListView.builder() on Flutter.

Common DIO GET Method

Beside of 'path' on dio.get() there another parameter on method get() you can use:
queryParameters: to give param to API,
options: this required Options() and have many function like setting header etc.