✨ 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 restoring MultipartFile from `FormData`.
 
 ## 5.2.1+1
 
@@ -28,7 +29,7 @@ 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 +140,14 @@ class MultipartFile {
     _isFinalized = true;
     return _stream;
   }
+
+  /// Restore MultipartFile, returning a new instance of the same object. This is useful if your request failed and you wish
+  /// to sistematically retry it, for example a 401 error that can be solved by refreshing the token.
+  MultipartFile restoreMultipartFile() => MultipartFile(
+        _stream,
+        length,
+        filename: filename,
+        contentType: contentType,
+        headers: headers,
+      );
 }

dio/test/formdata_test.dart

@@ -94,6 +94,73 @@ void main() async {
       testOn: 'vm',
     );
 
+    // Restored multipart files should be able to read again and be the same as the original ones.
+    test(
+      'complex with restoration',
+      () 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();
+
+        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.restoreMultipartFile(),
+          ),
+        );
+        fm1.files.add(
+          MapEntry(
+            'files',
+            multipartFile2.restoreMultipartFile(),
+          ),
+        );
+        fm1.files.add(
+          MapEntry(
+            'files',
+            multipartFile3.restoreMultipartFile(),
+          ),
+        );
+        expect(fmStr.length, fm1.length);
+        expect(fm1.files[0].value.isFinalized, false);
+        expect(fm1.files[1].value.isFinalized, false);
+        expect(fm1.files[2].value.isFinalized, false);
+      },
+      testOn: 'vm',
+    );
+
     test('encodes maps correctly', () async {
       final fd = FormData.fromMap(
         {