NSURLSessionTaskTransactionMetrics
An object that encapsualtes the performance metrics collected by the URL Loading System during the execution of a session task.
用于封装在执行会话任务期间URL加载系统收集的性能指标。
Apple官方文档参考

Timeline of temporal metrics for a URL session task
URL会话任务的时间指标时间轴

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fetchStartDate
The time when the task started fetching the resource, from the server or locally.

domainLookupStartDate
The time immediately before the task started the name lookup for the resource.

domainLookupEndDate
The time after the name lookup was completed.

connectStartDate
The time immediately before the task started establishing a TCP connection to the server.

secureConnectionStartDate
The time immediately before the task started the TLS security handshake to secure the current connection.

secureConnectionEndDate
The time immediately after the security handshake completed.

connectEndDate
The time immediately after the task finished establishing the connection to the server.

requestStartDate
The time immediately before the task started requesting the resource, regardless of whether it is retrieved from the server or local resources.

requestEndDate
The time immediately after the task finished requesting the resource, regardless of whether it was retrieved from the server or local resources.

responseStartDate
The time immediately after the task received the first byte of the response from the server or from local resources.

responseEndDate
The time immediately after the task received the last byte of the resource.