Programming iOS 4

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 presentation at the 2010 WWDC) 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. (The Core Animation module of Instruments, available when testing on a device, has a Color Blended Layers option that will show you where transparency is being composited.) 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.

Note

Apple’s documentation also says that setting a view’s clearsContextBeforeDrawing to NO may make a difference. I can’t confirm or deny this; it may be true, but I haven’t encountered a case ...