✨ Make MultipartFile recoverable to enable retrying FormData requests

dio/CHANGELOG.md

@@ -6,6 +6,7 @@ See the [Migration Guide][] for the complete breaking changes list.**
 ## Unreleased
 
 - Remove `http` from `dev_dependencies`.
+- Add support for cloning `MultipartFile` from `FormData`.
 - Only produce null response body when `ResponseType.json`.
 
 ## 5.2.1+1
@@ -29,7 +30,7 @@ See the [Migration Guide][] for the complete breaking changes list.**
   Dio 6.0.0 - Please use the replacement `IOHttpClientAdapter.createHttpClient` instead.
 - Using `CancelToken` no longer closes and re-creates `HttpClient` for each request when `IOHttpClientAdapter` is used.
 - Fix timeout handling for browser `receiveTimeout`.
-- Improve performance when sending binary data (`List<int>`/`Uint8List`). 
+- Improve performance when sending binary data (`List<int>`/`Uint8List`).
 
 ## 5.1.2
 

dio/lib/src/multipart_file.dart

@@ -140,4 +140,15 @@ class MultipartFile {
     _isFinalized = true;
     return _stream;
   }
+
+  /// Clone MultipartFile, returning a new instance of the same object.
+  /// This is useful if your request failed and you wish to retry it,
+  /// such as an unauthorized exception can be solved by refreshing the token.
+  MultipartFile clone() => MultipartFile(
+        _stream,
+        length,
+        filename: filename,
+        contentType: contentType,
+        headers: headers,
+      );
 }

dio/test/formdata_test.dart

@@ -94,6 +94,99 @@ void main() async {
       testOn: 'vm',
     );
 
+    // Cloned multipart files should be able to be read again and be the same
+    // as the original ones.
+    test(
+      'complex with cloning',
+      () async {
+        final multipartFile1 = MultipartFile.fromString(
+          'hello world.',
+          headers: {
+            'test': <String>['a']
+          },
+        );
+        final multipartFile2 = await MultipartFile.fromFile(
+          'test/mock/_testfile',
+          filename: '1.txt',
+          headers: {
+            'test': <String>['b']
+          },
+        );
+        final multipartFile3 = MultipartFile.fromFileSync(
+          'test/mock/_testfile',
+          filename: '2.txt',
+          headers: {
+            'test': <String>['c']
+          },
+        );
+
+        final fm = FormData.fromMap({
+          'name': 'wendux',
+          'age': 25,
+          'path': '/图片空间/地址',
+          'file': multipartFile1,
+          'files': [
+            multipartFile2,
+            multipartFile3,
+          ]
+        });
+        final fmStr = await fm.readAsBytes();
+
+        // Files are finalized after being read.
+        try {
+          multipartFile1.finalize();
+          fail('Should not be able to finalize a file twice.');
+        } catch (e) {
+          expect(e, isA<StateError>());
+          expect(
+            (e as StateError).message,
+            'The MultipartFile has already been finalized. This typically '
+            'means you are using the same MultipartFile in repeated requests.',
+          );
+        }
+
+        final fm1 = FormData();
+        fm1.fields.add(MapEntry('name', 'wendux'));
+        fm1.fields.add(MapEntry('age', '25'));
+        fm1.fields.add(MapEntry('path', '/图片空间/地址'));
+        fm1.files.add(
+          MapEntry(
+            'file',
+            multipartFile1.clone(),
+          ),
+        );
+        fm1.files.add(
+          MapEntry(
+            'files',
+            multipartFile2.clone(),
+          ),
+        );
+        fm1.files.add(
+          MapEntry(
+            'files',
+            multipartFile3.clone(),
+          ),
+        );
+        expect(fmStr.length, fm1.length);
+
+        // The cloned multipart files should be able to be read again.
+        expect(fm.files[0].value.isFinalized, true);
+        expect(fm.files[1].value.isFinalized, true);
+        expect(fm.files[2].value.isFinalized, true);
+        expect(fm1.files[0].value.isFinalized, false);
+        expect(fm1.files[1].value.isFinalized, false);
+        expect(fm1.files[2].value.isFinalized, false);
+
+        // The cloned multipart files' properties should be the same as the
+        // original ones.
+        expect(fm1.files[0].value.filename, multipartFile1.filename);
+        expect(fm1.files[0].value.contentType, multipartFile1.contentType);
+        expect(fm1.files[0].value.length, multipartFile1.length);
+        expect(fm1.files[0].value.headers, multipartFile1.headers);
+      },
+      testOn: 'vm',
+    );
+
     test('encodes maps correctly', () async {
       final fd = FormData.fromMap(
         {