调试函数耗时的利器CFAbsoluteTimeGetCurren

    CFAbsoluteTime start = CFAbsoluteTimeGetCurrent();
    // do something
    CFAbsoluteTime end = CFAbsoluteTimeGetCurrent();
    NSLog(@"time cost: %0.3f", end - start);

类型说明：
CFAbsoluteTime为double类型

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. */