弹性动画方法animateWithDuration: delay: usingSpringWithDamping: initialSpringVelocity: options: animations

这个动画效果是从IOS7开始有的，个人觉得动画效果的用户体验很好，用起来也方便，但是这里面的dampingRatio和velocity涉及到物理力学的知识，比较难理解和计算，只能通过Demo试验来理解它所代表的意思。通过形象生动的比喻来理解它。

附上原英文文档解释
/* Performs animations using a timing curve described by the motion of a spring. When dampingRatio is 1, the animation will smoothly decelerate to its final model values without oscillating. Damping ratios less than 1 will oscillate more and more before coming to a complete stop. You can use the initial spring velocity to specify how fast the object at the end of the simulated spring was moving before it was attached. It’s a unit coordinate system, where 1 is defined as travelling the total animation distance in a second. So if you’re changing an object’s position by 200pt in this animation, and you want the animation to behave as if the object was moving at 100pt/s before the animation started, you’d pass 0.5. You’ll typically want to pass 0 for the velocity. */

其他参数不再介绍
dampingRatio（阻尼系数）
范围 0~1 当它设置为1时，动画是平滑的没有振动的达到静止状态，越接近0 振动越大

velocity （弹性速率）
就是形变的速度，从视觉上看可以理解弹簧的形变速度，到动画结束，该速度减为0，所以，velocity速度越大，那么形变会越快，当然在同等时间内，速度的变化（就是速率）也会越快，因为速度最后都要到0。

如果不明白可以自己手写Demo进行比较