* 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.