class FileDownloader {
    func downloadFile(_ fileType: FileType, from urlString: String, as fileName: String, completion: @escaping (String?) -> Void) -> DownloadTask? {
        let filePath = LocalStorage.documentPath(of: fileType).appendingPathComponent(fileName)
        if !LocalStorage.fileExists(of: fileType, fileName: fileName) {
            #if DEBUG
            print("file not found at \(filePath)")
            #endif

            let storage = Storage.storage()
            let storageRef = storage.reference(forURL: urlString)
            let downloadTask = storageRef.write(toFile: filePath) { (localUrl, error) in
                if let err = error {
                    print("Error writing file: \(err)")
                } else {
                    print("file saved at \(filePath.path)")
                    completion(filePath.relativePath)
                }
            }
            return DownloadTask(firebaseDownloadTask: downloadTask)
        } else {
            memoireLogger.debug("file found at \(filePath)")
            completion(filePath.relativePath)
            return DownloadTask(firebaseDownloadTask: nil)
        }
    }

    class DownloadTask {
        typealias Status = StorageTaskStatus

        var firebaseDownloadTask: StorageDownloadTask?
        private var _id: String
        var id: String {
            get {
                return _id
            }
        }

        init(firebaseDownloadTask: StorageDownloadTask?) {
            self.firebaseDownloadTask = firebaseDownloadTask
            self._id = UUID().uuidString
        }

        func observe(_ status: Status, handler: @escaping (Snapshot) -> Void) {
            if firebaseDownloadTask == nil {
                let snapshot = Snapshot()
                handler(snapshot)
            } else {
                firebaseDownloadTask?.observe(status) { snapshot in
                    handler(Snapshot(snapshot.progress))
                }
            }
        }

        struct Snapshot {
            var progress: Progress?

            init(_ progress: Progress? = nil) {
                if progress == nil {
                    self.progress = Progress(totalUnitCount: 1)
                    self.progress?.completedUnitCount = 1
                } else {
                    self.progress = progress
                }
            }
        }
    }
}