timingFunction

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