Controlling Animation with CAAnimation - iOS Programming: The Big Nerd Ranch Guide - page 645

Bouncing with a CAKeyframeAnimation
For practice with CAKeyframeAnimation , you are going to make the time label grow
and shrink to give it a bouncing effect ( Figure 23.7 ).
Figure 23.7 Current time mid-bounce
Declare a new method in TimeViewController.h .
- (void)bounceTimeLabel;