title: How to get progress for multipart multiple files upload using OkHTTP3/Retrofit2?
body: <p>I am uploading a dynamic number of files in a single multipart request using Retrofit2. My retrofit interface looks like this - </p>
  
  <pre><code>public interface FileUploadService {  
      @Multipart
      @POST("upload")
      Call&lt;ResponseBody&gt; uploadMultipleFilesDynamic(
              @Part List&lt;MultipartBody.Part&gt; files);
  }
  </code></pre>
  
  <p>Now I want to track progress for this multi-file upload. <a href="https://stackoverflow.com/a/33384551/3811963">This solution</a> explains how to get progress while uploading a single file in a multipart request by extending <code>RequestBody</code>. Though I can't seem to comprehend how to apply this for my multiple files request. One solution I could think of was to create <code>ProgressRequestBody</code> by extending <code>OkHTTP MultipartBody</code> class instead of <code>RequestBody</code> but <code>OkHTTP3</code> implements <code>MultipartBody</code> as a final class making it impossible to extend. Can anyone point me in the right direction as it's a huge blocker for me to not be able to show the progress to the user for files upload. Or are there any work arounds that I can implement to achieve this functionality?</p>