SpeedMeasure
Speed Measure Tool on iOS platform.
Adopt NSURLSessionDataTask and NSURLSessionDownloadTask to perform the actually download job.
Firstly, the task was NSURLSessionDataTask, it prompt us if it received the first response, we'll treat it as the timestamp that download jobs actually happen.
then switch the task to NSURLSessionDownloadTask, it updates the progress periodically and enable us to calculate the download rate.