Multipart File Uploads, Downloads & WorkManager

Transferring large files (camera photos, videos, PDF documents) over mobile networks requires robust engineering. If a user walks into an elevator and loses Wi-Fi mid-upload, the app must not crash or discard the transfer. In this class, we build resilient Multipart file uploads and background transfers powered by WorkManager.

Multipart Form Uploads with Retrofit & OkHttp

import okhttp3.MediaType.Companion.toMediaTypeOrNull
import okhttp3.MultipartBody
import okhttp3.RequestBody.Companion.asRequestBody
import java.io.File

interface FileUploadApiService {
    @retrofit2.http.Multipart
    @retrofit2.http.POST("v1/files/upload")
    suspend fun uploadDocument(
        @retrofit2.http.Part filePart: MultipartBody.Part
    ): Response<UploadResult>
}

fun prepareFilePart(file: File): MultipartBody.Part {
    val requestBody = file.asRequestBody("image/jpeg".toMediaTypeOrNull())
    return MultipartBody.Part.createFormData("file", file.name, requestBody)
}

Guaranteed Background Uploads via CoroutineWorker

By executing uploads inside a CoroutineWorker, Android guarantees that the file upload continues even if the user leaves the application or restarts their device:

class UploadFileWorker(
    context: Context,
    params: WorkerParameters
) : CoroutineWorker(context, params) {

    override suspend fun doWork(): Result {
        val filePath = inputData.getString("FILE_PATH") ?: return Result.failure()
        val file = File(filePath)

        return try {
            val filePart = prepareFilePart(file)
            val response = apiService.uploadDocument(filePart)
            if (response.isSuccessful) Result.success() else Result.retry()
        } catch (e: Exception) {
            Result.retry()
        }
    }
}