🐛 Only produce null response body when the response is a JSON type (#1874)

Resolves #1834.

### New Pull Request Checklist

- [x] I have read the
[Documentation](https://pub.dev/documentation/dio/latest/)
- [x] I have searched for a similar pull request in the
[project](https://github.com/cfug/dio/pulls) and found none
- [x] I have updated this branch with the latest `main` branch to avoid
conflicts (via merge from master or rebase)
- [x] I have added the required tests to prove the fix/feature I'm
adding
- [x] I have updated the documentation (if necessary)
- [x] I have run the tests without failures
- [x] I have updated the `CHANGELOG.md` in the corresponding package

Author: AlexV525

dio/CHANGELOG.md

@@ -6,6 +6,7 @@ See the [Migration Guide][] for the complete breaking changes list.**
 ## Unreleased
 
 - Remove `http` from `dev_dependencies`.
+- Only produce null response body when `ResponseType.json`.
 
 ## 5.2.1+1
 

dio/lib/src/dio_mixin.dart

@@ -537,10 +537,9 @@ abstract class DioMixin implements Dio {
   // Initiate Http requests
   Future<Response<dynamic>> _dispatchRequest<T>(RequestOptions reqOpt) async {
     final cancelToken = reqOpt.cancelToken;
-    ResponseBody responseBody;
     try {
       final stream = await _transformData(reqOpt);
-      responseBody = await httpClientAdapter.fetch(
+      final responseBody = await httpClientAdapter.fetch(
         reqOpt,
         stream,
         cancelToken?.whenCancel,
@@ -559,7 +558,19 @@ abstract class DioMixin implements Dio {
       );
       final statusOk = reqOpt.validateStatus(responseBody.statusCode);
       if (statusOk || reqOpt.receiveDataWhenStatusError == true) {
-        ret.data = await transformer.transformResponse(reqOpt, responseBody);
+        Object? data = await transformer.transformResponse(
+          reqOpt,
+          responseBody,
+        );
+        // Make the response as null before returned as JSON.
+        if (data is String &&
+            data.isEmpty &&
+            T != dynamic &&
+            T != String &&
+            reqOpt.responseType == ResponseType.json) {
+          data = null;
+        }
+        ret.data = data;
       } else {
         await responseBody.stream.listen(null).cancel();
       }

dio/lib/src/transformer.dart

@@ -22,9 +22,10 @@ abstract class Transformer {
   /// [transformResponse] allows changes to the response data  before
   /// it is passed to [ResponseInterceptor].
   ///
-  /// **Note**: As an agreement, you must return the [response]
+  /// **Note**: As an agreement, you must return the [responseBody]
   /// when the Options.responseType is [ResponseType.stream].
-  Future transformResponse(RequestOptions options, ResponseBody response);
+  // TODO(AlexV525): Add generic type for the method in v6.0.0.
+  Future transformResponse(RequestOptions options, ResponseBody responseBody);
 
   /// Deep encode the [Map<String, dynamic>] to percent-encoding.
   /// It is mostly used with the "application/x-www-form-urlencoded" content-type.

dio/lib/src/transformers/sync_transformer.dart

@@ -42,86 +42,97 @@ class SyncTransformer extends Transformer {
     }
   }
 
-  /// As an agreement, we return the [response] when the
-  /// Options.responseType is [ResponseType.stream].
   @override
   Future<dynamic> transformResponse(
     RequestOptions options,
-    ResponseBody response,
+    ResponseBody responseBody,
   ) async {
+    final responseType = options.responseType;
+    // Do not handled the body for streams.
     if (options.responseType == ResponseType.stream) {
-      return response;
+      return responseBody;
     }
-    int length = 0;
-    int received = 0;
+
     final showDownloadProgress = options.onReceiveProgress != null;
+    final int totalLength;
     if (showDownloadProgress) {
-      length = int.parse(
-        response.headers[Headers.contentLengthHeader]?.first ?? '-1',
+      totalLength = int.parse(
+        responseBody.headers[Headers.contentLengthHeader]?.first ?? '-1',
       );
+    } else {
+      totalLength = 0;
     }
-    final completer = Completer();
-    final stream = response.stream.transform<Uint8List>(
+
+    int received = 0;
+    final stream = responseBody.stream.transform<Uint8List>(
       StreamTransformer.fromHandlers(
         handleData: (data, sink) {
           sink.add(data);
           if (showDownloadProgress) {
             received += data.length;
-            options.onReceiveProgress?.call(received, length);
+            options.onReceiveProgress?.call(received, totalLength);
           }
         },
       ),
     );
+
+    final streamCompleter = Completer<void>();
+    int finalLength = 0;
     // Keep references to the data chunks and concatenate them later.
     final chunks = <Uint8List>[];
-    int finalSize = 0;
-    final StreamSubscription subscription = stream.listen(
+    final subscription = stream.listen(
       (chunk) {
-        finalSize += chunk.length;
+        finalLength += chunk.length;
         chunks.add(chunk);
       },
       onError: (Object error, StackTrace stackTrace) {
-        completer.completeError(error, stackTrace);
+        streamCompleter.completeError(error, stackTrace);
+      },
+      onDone: () {
+        streamCompleter.complete();
       },
-      onDone: () => completer.complete(),
       cancelOnError: true,
     );
     options.cancelToken?.whenCancel.then((_) {
       return subscription.cancel();
     });
-    await completer.future;
-    // Copy all chunks into a final Uint8List.
-    final responseBytes = Uint8List(finalSize);
+    await streamCompleter.future;
+
+    // Copy all chunks into the final bytes.
+    final responseBytes = Uint8List(finalLength);
     int chunkOffset = 0;
     for (final chunk in chunks) {
       responseBytes.setAll(chunkOffset, chunk);
       chunkOffset += chunk.length;
     }
 
-    if (options.responseType == ResponseType.bytes) {
+    // Return the finalized bytes if the response type is bytes.
+    if (responseType == ResponseType.bytes) {
       return responseBytes;
     }
 
-    final String? responseBody;
+    final isJsonContent = Transformer.isJsonMimeType(
+      responseBody.headers[Headers.contentTypeHeader]?.first,
+    );
+    final String? response;
     if (options.responseDecoder != null) {
-      responseBody = options.responseDecoder!(
+      response = options.responseDecoder!(
         responseBytes,
         options,
-        response..stream = Stream.empty(),
+        responseBody..stream = Stream.empty(),
       );
-    } else if (responseBytes.isNotEmpty) {
-      responseBody = utf8.decode(responseBytes, allowMalformed: true);
+    } else if (!isJsonContent || responseBytes.isNotEmpty) {
+      response = utf8.decode(responseBytes, allowMalformed: true);
     } else {
-      responseBody = null;
+      response = null;
     }
-    if (responseBody != null &&
-        responseBody.isNotEmpty &&
-        options.responseType == ResponseType.json &&
-        Transformer.isJsonMimeType(
-          response.headers[Headers.contentTypeHeader]?.first,
-        )) {
-      return jsonDecodeCallback(responseBody);
+
+    if (response != null &&
+        response.isNotEmpty &&
+        responseType == ResponseType.json &&
+        isJsonContent) {
+      return jsonDecodeCallback(response);
     }
-    return responseBody;
+    return response;
   }
 }

dio/test/mock/http_mock.mocks.dart

@@ -672,8 +672,8 @@ class MockTransformer extends _i1.Mock implements _i5.Transformer {
           returnValue: Future<String>.value('')) as _i4.Future<String>);
   @override
   _i4.Future<dynamic> transformResponse(
-          _i6.RequestOptions? options, _i7.ResponseBody? response) =>
+          _i6.RequestOptions? options, _i7.ResponseBody? responseBody) =>
       (super.noSuchMethod(
-          Invocation.method(#transformResponse, [options, response]),
+          Invocation.method(#transformResponse, [options, responseBody]),
           returnValue: Future<dynamic>.value()) as _i4.Future<dynamic>);
 }

dio/test/options_test.dart

@@ -346,6 +346,14 @@ void main() {
       r10.requestOptions.contentType,
       startsWith(Headers.multipartFormDataContentType),
     );
+
+    // Regression: https://github.com/cfug/dio/issues/1834
+    final r11 = await dio.get('');
+    expect(r11.data, '');
+    final r12 = await dio.get<Map>('');
+    expect(r12.data, null);
+    final r13 = await dio.get<Map<String, Object>>('');
+    expect(r13.data, null);
   });
 
   test('default content-type 2', () async {

dio/test/transformer_test.dart

@@ -0,0 +1,52 @@
+import 'package:dio/dio.dart';
+import 'package:test/test.dart';
+
+void main() {
+  group(BackgroundTransformer(), () {
+    test('transformResponse transforms the request', () async {
+      final transformer = BackgroundTransformer();
+      final response = await transformer.transformResponse(
+        RequestOptions(responseType: ResponseType.json),
+        ResponseBody.fromString(
+          '{"foo": "bar"}',
+          200,
+          headers: {
+            Headers.contentTypeHeader: ['application/json'],
+          },
+        ),
+      );
+      expect(response, {'foo': 'bar'});
+    });
+  });
+
+  // Regression: https://github.com/cfug/dio/issues/1834
+  test('null response body only when the response is JSON', () async {
+    final transformer = BackgroundTransformer();
+    final r1 = await transformer.transformResponse(
+      RequestOptions(responseType: ResponseType.json),
+      ResponseBody.fromBytes([], 200),
+    );
+    expect(r1, '');
+    final r2 = await transformer.transformResponse(
+      RequestOptions(responseType: ResponseType.bytes),
+      ResponseBody.fromBytes([], 200),
+    );
+    expect(r2, []);
+    final r3 = await transformer.transformResponse(
+      RequestOptions(responseType: ResponseType.plain),
+      ResponseBody.fromBytes([], 200),
+    );
+    expect(r3, '');
+    final r4 = await transformer.transformResponse(
+      RequestOptions(responseType: ResponseType.json),
+      ResponseBody.fromBytes(
+        [],
+        200,
+        headers: {
+          Headers.contentTypeHeader: [Headers.jsonContentType],
+        },
+      ),
+    );
+    expect(r4, null);
+  });
+}

dio/test/transformers/background_transformer_test.dart

@@ -26,10 +26,10 @@ class MyTransformer extends BackgroundTransformer {
   @override
   Future transformResponse(
     RequestOptions options,
-    ResponseBody response,
+    ResponseBody responseBody,
   ) async {
     options.extra['self'] = 'XX';
-    return super.transformResponse(options, response);
+    return super.transformResponse(options, responseBody);
   }
 }