iOS.GetCurrentTimestamp

Cocoa 中测量时间的方法 

1. The Methods to Get Current Timestamp

iOS中获取时间戳的方法:

A. CACurrentMediaTime()

B. gettimeofday

C. CFAbsoluteTimeGetCurrent

D. [NSDate date] timeIntervalSince1970]

E. mach_absolute_time() 

Ref[1]对比了以上方法的耗时，但是其准确性需要验证。

2. Clock Time

时钟时间

"CFAbsoluteTimeGetCurrent [NSDate timeIntervalSinceReferenceDate] return clock time.

Clock time is synchronized with the network time signal. So, they are not suitable for timestamp." Ref[2]

3. 各种方法的对比以及适用场景

3.1 对于测量App中某些操作的所消耗的时间，用CACurrentMediaTime()来说很合适。

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Reference

1. http://stackoverflow.com/questions/358207/iphone-how-to-get-current-milliseconds

2. Tip: CACurrentMediaTime

http://www.alexcurylo.com/blog/2009/09/05/tip-cacurrentmediatime/

3. iOS.mach_absolute_time

4. Benchmarking

http://nshipster.com/benchmarking/

http://nshipster.cn/benchmarking/

5. ‘CACurrentMediaTime()’

http://bendodson.com/weblog/2013/01/29/ca-current-media-time/

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Todo

1. Timers, Clocks, and Cocoa

http://devetc.org/code/2014/01/21/timers-clocks-and-cocoa.html

2. 

http://www.raywenderlich.com/forums/viewtopic.php?f=2&t=1019

3. All in the Timing: Keeping Track of Time Passed on iOS

http://blog.spacemanlabs.com/2011/09/all-in-the-timing-keeping-track-of-time-passed-on-ios/

4. How to convert mach_absolute_time() into seconds?