worldhopper/lib/services/dio_factory.dart

import 'package:dio/dio.dart';

/// Creates Dio instances that automatically follow redirects for all HTTP
/// methods (including POST). The default Dio/dart:io behaviour only follows
/// redirects for GET, so POST→301/302 would otherwise throw.
Dio createDio([BaseOptions? options]) {
  final dio = Dio(options);
  dio.interceptors.add(_RedirectInterceptor());
  return dio;
}

/// Interceptor that follows 3xx redirects for any HTTP method.
class _RedirectInterceptor extends Interceptor {
  static const _maxRedirects = 5;

  @override
  void onResponse(Response response, ResponseInterceptorHandler handler) {
    handler.next(response);
  }

  @override
  void onError(DioException err, ErrorInterceptorHandler handler) async {
    final statusCode = err.response?.statusCode;
    if (statusCode == null ||
        statusCode < 300 ||
        statusCode > 399 ||
        statusCode == 304) {
      return handler.next(err);
    }

    final location = err.response?.headers.value('location');
    if (location == null) {
      return handler.next(err);
    }

    final originalUri = err.requestOptions.uri;
    final redirectUri = Uri.parse(location).isAbsolute
        ? Uri.parse(location)
        : originalUri.resolve(location);

    // Preserve method for 307/308, change to GET for 301/302/303
    final preserveMethod = statusCode == 307 || statusCode == 308;
    final method = preserveMethod ? err.requestOptions.method : 'GET';

    final dio = Dio();
    dio.interceptors.add(this); // follow further redirects
    var redirects = 0;

    var opts = Options(
      method: method,
      headers: Map.of(err.requestOptions.headers),
      responseType: err.requestOptions.responseType,
      validateStatus: err.requestOptions.validateStatus,
      followRedirects: false, // we handle it ourselves
    );

    var currentUri = redirectUri;
    Response? response;

    while (redirects < _maxRedirects) {
      redirects++;
      try {
        response = await dio.requestUri(
          currentUri,
          data: preserveMethod ? err.requestOptions.data : null,
          options: opts,
        );
        break;
      } on DioException catch (e) {
        final sc = e.response?.statusCode;
        final loc = e.response?.headers.value('location');
        if (sc != null && sc >= 300 && sc <= 399 && sc != 304 && loc != null) {
          currentUri = Uri.parse(loc).isAbsolute
              ? Uri.parse(loc)
              : currentUri.resolve(loc);
          if (sc == 301 || sc == 302 || sc == 303) {
            opts = opts.copyWith(method: 'GET');
          }
          continue;
        }
        return handler.next(e);
      }
    }

    if (response != null) {
      return handler.resolve(response);
    }
    return handler.next(err);
  }
}