Improving drawing performance on custom UIView

Question

I have a custom UIView which is composed of many images, their positions are changing in response to the user touch. The view must track the user touch and i'm experiencing a performance bottleneck in the drawing of such view, preventing me to follow the input in realtime.

At the beginning i was drawing everything in the [UIView drawRect:] method and of course it was way too slow because everything was redrawn even if not necessary. Then, i used more CALayers to update only the layer that was changing and this gave me much better responsiveness. But still, when i have to draw the same image many times on a layer it takes up to 500ms.

Since the images are placed at fixed positions it there a way to pre-draw them? Should i consider putting them in many CALayers and just hide/show them? Also, i don't understand why a [CALayer setNeedsDisplayInRect:] exists but the delegate has (apparently) no way to know what the invalid rect is to optimize the drawing.

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Solution

Following the advice in the answer I finally created many CALayers for the images and set the contents property the first time the layer was being shown. This is a lazy-loading compromise: in a first attempt i set the contents of every layer at the creation time but this caused to pre-draw any possible image on the program launch, freezing the application for seconds.

Answer 1

From the documentation for -[CALayer drawInContext:]:

Default implementation does nothing. The context may be clipped to protect valid layer content. Subclasses that wish to find the actual region to draw can call CGContextGetClipBoundingBox. Called by the display method when the contents property is being updated.

The default implementation of display calls drawInContext: on an automatically-created context; presumably setting the bounding box as well (which is presumably passed to drawRect:).

If you're drawing several static images, I'd just stick each one in its own UIView; I don't think the overhead is that big (if it is, the CALayer overhead should be smaller). If they all animate, I'd definitely use UIView/CALayer. If some of them don't animate (much) and you notice significant slowness, you can pre-render those. It's a trade-off between rendering in drawRect: (or similar) and layer compositing on the GPU, but in general I'd assume that the latter is much faster.