HttpProgressEventlink

Progress event type is either upload or download.

进度事件的类型或者是上传或者是下载。

Number of bytes uploaded or downloaded.

已经上传或下载的字节数。

Total number of bytes to upload or download. Depending on the request or response, this may not be computable and thus may not be present.

要上传或下载的总字节数。它可能是无法计算出来的，因此也就可能不存在，取决于是请求还是响应。