Programming iOS 6, 3rd Edition

Scroll View Performance

At several points in earlier chapters I’ve mentioned performance problems and ways to increase drawing efficiency. Nowhere are you so likely to need these as in connection with a scroll view. As a scroll view scrolls, views must be drawn very rapidly as they appear on the screen. If the view-drawing system can’t keep up with the speed of the scroll, the scrolling will visibly stutter.

Performance testing and optimization is a big subject, so I can’t tell you exactly what to do if you encounter stuttering while scrolling. But certain general suggestions (mostly extracted from a really great WWDC 2010 video) should come in handy:

• Everything that can be opaque should be opaque: don’t force the drawing system to composite transparency, and remember to tell it that an opaque view or layer is opaque by setting its opaque property to YES. If you really must composite transparency, keep the size of the nonopaque regions to a minimum; for example, if a large layer is transparent at its edges, break it into five layers — the large central layer, which is opaque, and the four edges, which are not.
• If you’re drawing shadows, don’t make the drawing system calculate the shadow shape for a layer: supply a shadowPath, or use Core Graphics to create the shadow with a drawing. Similarly, avoid making the drawing system composite the shadow as a transparency against another layer; for example, if the background layer is white, your opaque drawing can itself include a shadow ...