Flutter API client class with http package

api_client.dart

import 'package:http/http.dart' as http;

import './api_request.dart';
import './api_error.dart';
import './api_method.dart';

abstract class APIClientInterface {
  Future<T> call<T>(APIRequest<T> request);
}

class APIClient implements APIClientInterface {
  APIClient._();

  static APIClient _instance;
  static APIClient get instance {
    if (_instance == null)
      _instance = APIClient._();
    
    return _instance;
  }

  final _client = http.Client();

  Future<T> call<T>(APIRequest<T> request) async {
    final url = request.baseURL + request.path;
    final headers = request.headers;

    http.Response response;
    switch (request.method) {
      case APIMethod.GET:
        response = await _client.get(url, headers: headers);
        break;
      case APIMethod.POST:
        response = await _client.post(url, headers: headers);
        break;
    }

    if (response.statusCode != 200) {
      throw APIError(statusCode: response.statusCode, message: response.body);
    }

    return request.decode(response);
  }
}

api_error.dart

import 'package:flutter/foundation.dart';

class APIError extends Error {
  APIError({
    @required this.statusCode,
    @required this.message,
  });

  final int statusCode;
  final String message;

  @override
  String toString() => '${statusCode}: ${message}';
}

api_method.dart

enum APIMethod {
  GET,
  POST,
}

api_request.dart

import 'package:http/http.dart' as http;

import './api_method.dart';

abstract class APIRequest<T> {
  String get baseURL => 'http://endpoint';
  String get path;
  APIMethod get method => APIMethod.GET;
  Map<String, String> get headers => {
    'Content-Type': 'application/json',
  };
  T decode(http.Response response);
}