从alamofire上传多个文件时，如何获取文件上传完成计数？

Question

我有数据要显示在进度栏中，但是如何获取已完成文件的计数？下面的代码不起作用。

upload.uploadProgress(closure: { (progress) in
    print("Upload Progress: \(progress.fractionCompleted)") // worked
    print("Number of uploaded file \(upload.uploadProgress.fileCompletedCount) \(progress.fileCompletedCount)") // always nil
})

请帮助我。