[SOLVED] How to solve exception "toJson() method have to add to MultipartFile" when uploading image?

How to solve exception "toJson() method have to add to MultipartFile" when uploading image?

I'm using the retrofit package with dio in Flutter to upload a file using @MultiPart() and MultipartFile. But when I run build_runner, I get this error:

retrofit_generator on lib/features/schedule/data/datasources/schedule_remote_datasource.dart:
Exception: toJson() method have to add to MultipartFile

Here’s my API method:

@POST(ScheduleConstants.createMeal)
@MultiPart()
Future<void> createMeal({
  @Part(name: 'meal_type') required String mealType,
  @Part(name: 'description') required String description,
  @PartFile(name: 'image') required MultipartFile image,
});

I'm importing MultipartFile from Dio:

import 'package:dio/dio.dart';

I've run

flutter pub get
flutter pub run build_runner build --delete-conflicting-outputs

But the error persists. Is there something I’m missing?

Solution

Even though dio internally uses MultipartFile, you don’t need to manually use it in your Retrofit method. Instead, use File type, and it will automatically convert it into MultipartFile.

@POST(ScheduleConstants.createMeal)
@MultiPart()
Future<void> createMeal({
  @Part(name: 'meal_type') required String mealType,
  @Part(name: 'description') required String description,
  @Part(name: 'image') required File image,
});

After switching to File, the retrofit_generator builds your method and automatically converts the File to MultipartFile in the *.g.dart generated file like this:

  _data.files.add(
    MapEntry(
      'image',
      MultipartFile.fromFileSync(
        image.path,
        filename: image.path.split(Platform.pathSeparator).last,
      ),
    ),
  );

For more information on defining different HTTP methods, refer to the official example of retrofit - https://github.com/trevorwang/retrofit.dart/blob/master/example/lib/example.dart