Chapter 4. Animation

Animation is an attribute changing over time. In general, this will usually be a visible attribute of something in the interface. The changing attribute might be positional: something moves or changes size, not jumping abruptly, but sliding smoothly. Other kinds of attribute can animate as well. A view’s background color might change from red to green, not switching colors abruptly, but fading from one to the other. A view might change from opaque to transparent, not vanishing abruptly, but 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 = // animate to red

Animation is easy because Apple wants to facilitate your use of it. Animation isn’t just cool and fun; it clarifies that something is changing or responding. It is crucial to the character of the iOS interface.

For example, one of my first apps was based on a macOS game in which the user clicks cards to select them. In the macOS version, a card was highlighted to show it was selected, and the computer would ...

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