Programming iOS 6, 3rd Edition

Layer Efficiency

By now, you’re probably envisioning all sorts of compositing fun, with layers masking sublayers and laid semitransparently over other layers. There’s nothing wrong with that, but when an iOS device is asked to animate the movement of its drawing from place to place, the animation may stutter because the device lacks the necessary computing power to composite repeatedly and rapidly. This sort of issue is likely to emerge particularly when your code performs an animation (Chapter 17) or when the user is able to animate drawing through touch, as when scrolling a table view (Chapter 21). You may be able to detect these problems by eye, and you can quantify them on a device by using the Core Animation template in Instruments, which shows the frame rate achieved during animation. Also, both the Core Animation template and the Simulator’s Debug menu let you summon colored overlays that provide clues as to possible sources of inefficient drawing which can lead to such problems.

In general, opaque drawing is most efficient. (Nonopaque drawing is what Instruments marks in red as “blended layers.”) If a layer will always be shown over a background consisting of a single color, you can give the layer its own background of that same color; when additional layer content is supplied, the visual effect will be the same as if that additional layer content were composited over a transparent background. For example, instead of an image masked to a rounded rectangle (with a layer’s ...