Programming iOS 4

Chapter 17. Animation

Animation is the visible change of an attribute over time. The changing attribute might be positional, but not necessarily. For example, a view’s background color might change from red to green, not instantly, but perceptibly fading from one to the other. Or a view’s opacity might change from opaque to transparent, not instantly, but perceptibly fading away.

Without help, most of us would find animation beyond our reach. There are just too many complications — complications of calculation, of timing, of screen refresh, of threading, and many more. Fortunately, help is provided. You don’t perform an animation yourself; you describe it, you order it, and it is performed for you. You get animation on demand.

Asking for an animation can be as simple as setting a property value; under some circumstances, a single line of code will result in animation:

myLayer.backgroundColor = [[UIColor redColor] CGColor]; // animate change to red

And this is no coincidence. Apple wants to facilitate your use of animation. Animation is crucial to the character of the iOS interface. It isn’t just cool and fun; it clarifies that something is changing or responding. For example, one of my first apps was based on a Mac OS X game in which the user clicks cards to select them. In the Mac OS X version, a card was highlighted to show it was selected, and the computer would beep to indicate a click on an ineligible card. On iOS, these indications were insufficient: the highlighting felt weak, ...