Starting with Dio

For REST Api request, here we will use DIO Package.

Dio Setup with Riverpod This setup ensures a single Dio instance (Singleton), centralizes configuration (BaseOptions), and maps low-level DioException to your custom AppException classes automatically.

Core Dio Client (handles exceptions)

dart

import 'package:dio/dio.dart';
import 'package:flutter_riverpod/flutter_riverpod.dart';

// Reuse AppExceptions
class NetworkException extends AppException { /* ... */ }
class UnauthorizedException extends AppException { /* ... */ }
class ServerException extends AppException { /* ... */ }

class ApiClient {
  late final Dio _dio;

  ApiClient() {
    _dio = Dio(BaseOptions(
      baseUrl: 'https://api.example.com/',
      connectTimeout: const Duration(seconds: 5),
      receiveTimeout: const Duration(seconds: 3),
      headers: {'Content-Type': 'application/json'},
    ));

    _dio.interceptors.addAll([
      LogInterceptor(responseBody: true, error: true),
      InterceptorsWrapper(
        onRequest: (options, handler) {
          // token: options.headers['Authorization'] = ...;
          handler.next(options);
        },
        onError: (e, handler) => handler.next(e),
      ),
    ]);
  }

  Future<T> get<T>(String path, {Map<String, dynamic>? query}) async {
    try {
      final res = await _dio.get(path, queryParameters: query);
      return res.data as T;
    } on DioException catch (e, st) {
      _mapDioError(e, st);
    } catch (e, st) {
      throw NetworkException('Unexpected', cause: e, stackTrace: st);
    }
    throw const NetworkException('Unknown'); // unreachable
  }

  Future<T> post<T>(String path, dynamic data) async {
    try {
      final res = await _dio.post(path, data: data);
      return res.data as T;
    } on DioException catch (e, st) {
      _mapDioError(e, st);
    } catch (e, st) {
      throw NetworkException('Unexpected', cause: e, stackTrace: st);
    }
  }

  void _mapDioError(DioException e, StackTrace st) {
    final status = e.response?.statusCode;

    if (status == 401) throw UnauthorizedException('Unauthorized', cause: e, stackTrace: st);
    else if ([408, 500, 502, 503, 504].contains(status))
      throw ServerException('Server error', cause: e, stackTrace: st);
    else if (e.type == DioExceptionType.connectionTimeout ||
             e.type == DioExceptionType.receiveTimeout ||
             e.type == DioExceptionType.connectionError)
      throw NetworkException('Network', cause: e, stackTrace: st);

    throw ServerException(e.message ?? 'Failed', cause: e, stackTrace: st);
  }
}

2) Providers (providers.dart)

dart

// Singleton-like ApiClient (keepAlive: true for app-life)
final apiProvider = Provider<ApiClient>((_) => ApiClient());

// Repo (depends on ApiClient)
final userRepoProvider = Provider<UserRepo>((ref) =>
  UserRepo(ref.read(apiProvider))
);

Now Do Dependency injection with your service or repo classes.