CFAbsoluteTime start = CFAbsoluteTimeGetCurrent();
// do something
CFAbsoluteTime end = CFAbsoluteTimeGetCurrent();
NSLog(@"time cost: %0.3f", end - start);
類型說明：
CFAbsoluteTime為double類型
typedef double CFTimeInterval;
typedef CFTimeInterval CFAbsoluteTime;
/* absolute time is the time interval since the reference date */
/* the reference date (epoch) is 00:00:00 1 January 2001. */
