UploadEither.swift

import YPImagePicker

struct NewPost {
  /// YPMediaItem translates to either a UIImage for photo or Data/URL for video
  var media: Either<[YPMediaItem], [NewPost.Medium]>
  
  struct Medium {
    let url: String
    let thumbnailUrl: String?
    let mediumType: Post.Medium.MediumType
  }
}

enum BackendAPI {
  func post(_ newPost: NewPost) -> Observable<NetworkResult<Post>> {
    let media: Observable<[NewPost.Medium]> = {
      switch post.media {
      case .left(let mediaItems):
        return Observable<NewPost.Medium>.combineLatest(mediaItems.map { mediaItem -> Observable<NewPost.Medium> in
          switch mediaItem {
          case .photo(p: let photo): return StorageService.upload(photo)
          case .video(v: let video): return StorageService.upload(video)
          }
        })
      case .right(let media):
        return Observable.just(media)
      }
    }()

    return media.flatMap { media in
      return requestObject(.newPost(post, media: media))
    }
  }
}