1.  *  动画的开始与结束的快慢,有五个预置分别为(下同): 
2.      *  kCAMediaTimingFunctionLinear            线性,即匀速 
3.      *  kCAMediaTimingFunctionEaseIn            先慢后快 
4.      *  kCAMediaTimingFunctionEaseOut           先快后慢 
5.      *  kCAMediaTimingFunctionEaseInEaseOut     先慢后快再慢 
6.      *  kCAMediaTimingFunctionDefault           实际效果是动画中间比较快.

timingFunction