feat(dio): Allow ResponseDecoder and RequestEncoder to be async

dio/CHANGELOG.md

@@ -7,6 +7,7 @@ See the [Migration Guide][] for the complete breaking changes list.**
 
 - Raise warning for `Map`s other than `Map<String, dynamic>` when encoding request data.
 - Improve exception messages
+- Allow `ResponseDecoder` and `RequestEncoder` to be async
 
 ## 5.3.3
 

dio/lib/src/dio_mixin.dart

@@ -627,7 +627,13 @@ abstract class DioMixin implements Dio {
           // Call the request transformer.
           final transformed = await transformer.transformRequest(options);
           if (options.requestEncoder != null) {
-            bytes = options.requestEncoder!(transformed, options);
+            final encoded = options.requestEncoder!(transformed, options);
+
+            if (encoded is Future) {
+              bytes = await encoded;
+            } else {
+              bytes = encoded;
+            }
           } else {
             // Converts the data to UTF-8 by default.
             bytes = utf8.encode(transformed);

dio/lib/src/options.dart

@@ -1,3 +1,5 @@
+import 'dart:async';
+
 import 'package:meta/meta.dart';
 
 import 'adapter.dart';
@@ -76,14 +78,14 @@ enum ListFormat {
 typedef ValidateStatus = bool Function(int? status);
 
 /// The type of a response decoding callback.
-typedef ResponseDecoder = String? Function(
+typedef ResponseDecoder = FutureOr<String?> Function(
   List<int> responseBytes,
   RequestOptions options,
   ResponseBody responseBody,
 );
 
 /// The type of a response encoding callback.
-typedef RequestEncoder = List<int> Function(
+typedef RequestEncoder = FutureOr<List<int>> Function(
   String request,
   RequestOptions options,
 );

dio/lib/src/transformers/sync_transformer.dart

@@ -125,11 +125,17 @@ class SyncTransformer extends Transformer {
     );
     final String? response;
     if (options.responseDecoder != null) {
-      response = options.responseDecoder!(
+      final decodeResponse = options.responseDecoder!(
         responseBytes,
         options,
         responseBody..stream = Stream.empty(),
       );
+
+      if (decodeResponse is Future) {
+        response = await decodeResponse;
+      } else {
+        response = decodeResponse;
+      }
     } else if (!isJsonContent || responseBytes.isNotEmpty) {
       response = utf8.decode(responseBytes, allowMalformed: true);
     } else {

dio/test/options_test.dart

@@ -1,4 +1,6 @@
 @TestOn('vm')
+import 'dart:convert';
+
 import 'package:dio/dio.dart';
 import 'package:test/test.dart';
 
@@ -399,6 +401,50 @@ void main() {
     expect(response.data, null);
   });
 
+  test('responseDecoder can return Future<String?>', () async {
+    final dio = Dio();
+    dio.options.responseDecoder = (_, __, ___) => Future.value('example');
+    dio.options.baseUrl = EchoAdapter.mockBase;
+    dio.httpClientAdapter = EchoAdapter();
+
+    final Response response = await dio.get('');
+
+    expect(response.data, 'example');
+  });
+
+  test('responseDecoder can return String?', () async {
+    final dio = Dio();
+    dio.options.responseDecoder = (_, __, ___) => 'example';
+    dio.options.baseUrl = EchoAdapter.mockBase;
+    dio.httpClientAdapter = EchoAdapter();
+
+    final Response response = await dio.get('');
+
+    expect(response.data, 'example');
+  });
+
+  test('requestEncoder can return Future<List<int>>', () async {
+    final dio = Dio();
+    dio.options.requestEncoder = (data, _) => Future.value(utf8.encode(data));
+    dio.options.baseUrl = EchoAdapter.mockBase;
+    dio.httpClientAdapter = EchoAdapter();
+
+    final Response response = await dio.get('');
+
+    expect(response.statusCode, 200);
+  });
+
+  test('requestEncoder can return List<int>', () async {
+    final dio = Dio();
+    dio.options.requestEncoder = (data, _) => utf8.encode(data);
+    dio.options.baseUrl = EchoAdapter.mockBase;
+    dio.httpClientAdapter = EchoAdapter();
+
+    final Response response = await dio.get('');
+
+    expect(response.statusCode, 200);
+  });
+
   test('invalid response type throws exceptions', () async {
     final dio = Dio(
       BaseOptions(