Webclient在下载请求时无法设置请求超时时间和请求读写超时时间。WebClient在异步下载时遇到网络不通等问题时没有响应超时造成app挂起。-Webclient request can not be set in the download time and request to read and write request timeout timeout. WebClient in asynchronous download problems encountered when the network is not unreasonable timeout caused app to hang.