Question

Currently I have a horizontal UIScrollView populated with UIViews. The views are dequeued when moving off screen and reused. Each UIView is subclassed to draw images using drawInRect with [someView setNeedsDisplay] being called when the view enters the screen.

[_image drawInRect:imageRect];

My gradients, text, shapes, etc all load smoothly but as soon as I draw an image I suffer a noticeable performance hit. The size of the image doesn’t seem to matter because the scroll view always lags. (I get the same result when using a UIImageView as well.) All of my images are loaded beforehand too.

Is there a better way to draw images that won’t result in poor performance?

Answer 1

If you’re just drawing images, you might consider using CALayers with CGImageRef contents—drawRect: overrides are going to be slower than CALayer compositing. Another thing to watch out for is alpha-blended images, which require frequent recompositing, or images that aren’t aligned to integral pixels. The Improving Image Drawing Performance on iOS tech note is a great place to start on this kind of thing. Once you have the basics in hand, the Core Animation instrument in Instruments will be of great help to you—it watches out for blending, copied images, misaligned images, and screen updates.