iOS9新出现的
/** Subclass for mass-spring animations. */
@interface CASpringAnimation : CABasicAnimation
/* The mass of the object attached to the end of the spring. Must be greater
than 0. Defaults to one. */
@property CGFloat mass;
/* The spring stiffness coefficient. Must be greater than 0.
* Defaults to 100. */
@property CGFloat stiffness;
/* The damping coefficient. Must be greater than or equal to 0.
* Defaults to 10. */
@property CGFloat damping;
/* The initial velocity of the object attached to the spring. Defaults
* to zero, which represents an unmoving object. Negative values
* represent the object moving away from the spring attachment point,
* positive values represent the object moving towards the spring
* attachment point. */
@property CGFloat initialVelocity;
/* Returns the estimated duration required for the spring system to be
* considered at rest. The duration is evaluated for the current animation
* parameters. */
@property(readonly) CFTimeInterval settlingDuration;
@end