Using Package Dio in Flutter - sending multiple images(multipart) under one key

Summary

The issue at hand is sending multiple images as multipart files under a single key (“images”) using the Dio package in Flutter. The current implementation results in two separate field entries, one for each image, instead of a single key holding both files.

Root Cause

The root cause of this issue is the way FormData and MultipartFile are being used in the code. Specifically:

The formData.files.addAll method is used to add multiple files to the form data, but this results in separate field entries for each file.
The ListFormat.multiCompatible parameter is not being utilized correctly to achieve the desired outcome.

Why This Happens in Real Systems

This issue occurs in real systems because of the following reasons:

Misunderstanding of the Dio package documentation: The documentation may not provide clear examples or guidance on how to send multiple files under a single key.
Incorrect usage of FormData and MultipartFile: The code may not be using the correct methods or parameters to achieve the desired outcome.
Lack of testing and debugging: Insufficient testing and debugging may lead to unforeseen issues and errors.

Real-World Impact

The impact of this issue in real-world systems can be significant, including:

Failed API requests: The backend server may reject the request due to the incorrect format of the multipart files.
Data corruption or loss: The files may not be uploaded correctly, resulting in data corruption or loss.
User frustration: The issue may cause frustration for users who are trying to upload multiple files.

Example or Code

import 'package:dio/dio.dart';
import 'package:path/path.dart';

void sendMultipleImages() async {
  final dio = Dio();
  final formData = FormData.fromMap({
    'name': 'example',
  });

  final image1 = await MultipartFile.fromFile(
    'path/to/image1.jpg',
    filename: basename('path/to/image1.jpg'),
    contentType: MediaType('image', 'jpeg'),
  );

  final image2 = await MultipartFile.fromFile(
    'path/to/image2.jpg',
    filename: basename('path/to/image2.jpg'),
    contentType: MediaType('image', 'jpeg'),
  );

  formData.files.add(MapEntry('images', image1));
  formData.files.add(MapEntry('images', image2));

  // Use the 'files' parameter to send multiple files under a single key
  final response = await dio.post('https://example.com/upload', data: {
    'images': [image1, image2],
  });

  print(response.data);
}

How Senior Engineers Fix It

Senior engineers fix this issue by:

Carefully reading the Dio package documentation: Understanding the correct usage of FormData and MultipartFile.
Using the correct parameters and methods: Utilizing the ‘files’ parameter to send multiple files under a single key.
Thoroughly testing and debugging: Ensuring that the code is working as expected and handling any potential errors.

Why Juniors Miss It

Junior engineers may miss this issue due to:

Lack of experience with the Dio package: Limited understanding of the package’s capabilities and correct usage.
Insufficient testing and debugging: Not thoroughly testing and debugging the code to catch potential issues.
Misunderstanding of multipart file uploads: Not fully understanding how to send multiple files under a single key.

Using Package Dio in Flutter – sending multiple images(multipart) under one key