-[CALayer setNeedsDisplayInRect:] causes the whole layer to be redrawn

Question

I'm subclassing CALayer to provide my own drawing in method. For optimization I call -[MyLayer setNeedsDisplayInRect:] instead of -[MyLayer setNeedsDisplay]. In the drawing method I get the rect which should be redrawn via CGContextGetClipBoundingBox().

If I use this layer as the base layer of an UIView every thing works as expected. The problem arises, as soon as I use my custom layer as a sublayer of an other CALayer. Than CGContextGetClipBoundingBox() always returns the rect of the bounds of that layer.

Any ideas?

[EDIT]

It seems, that there is no guaranty, that the content of the CALayer is cached and only the dirty part gets redrawn. I did a small test and stored the rect that needs display as a separate property. The result was, that only this part was visible on the screen.

I'll now render to an image context and keep that image as a cache. In the draw method I'll only display the cached image.

Answer 1

Apple's documentation is unfortunately conflicting as the docs on -setNeedsDisplayInRect do not indicate whether the method works in practice. Based on my own experience, this technote sets it straight:

Note that, because of the way that iPhone/iPod touch/iPad updates its screen, the entire view will be redrawn if you call -setNeedsDisplayInRect: or -setNeedsDisplay:.

That being said, there are a number of things you can look into if you think that you are hitting a wall due to redundant drawing.

• If drawing images, the biggest performance improvement you can make is to use images of the same dimensions at which you draw. If they're not, try to cache your image by rendering it to some offscreen bitmap context and bring it back later on.
• Check out the shouldRasterize property on CALayer. This can be a godsend if you are trying to manipulate a layer whose sublayers potentially constitute a complex layer hierarchy. Be sure to check out how you're doing in Instruments by ticking the Color Hits Green and Misses Red box in the Core Animation instrument. If you see a lot of red, chances are using shouldRasterize is hurting more than it's helping.
• Even better than shouldRasterize is to flatten your layer hierarchy, as then you can avoid the extra overhead that shouldRasterize incurs when flattening your layer hierarchy real time. Of course this is not always possible, but don't be afraid to try :)
• If you're drawing images, try experimenting with your blending mode. If you happen to be drawing opaque images, there's no need to be using normal source over methods (which use both read/write bandwidth). Try kCGBlendModeCopy, which allows you to eliminate read bandwidth overhead.
• Check out CGLayerRef, which allows you to cache Core Graphics output across various calls to your drawing methods. My experience is that, unless you're doing some hardcore pixel pushing, that this ends up being more costly than just redrawing. See this for an interesting read.
• Above all, Instruments is your friend. Check out a couple videos from past WWDCs (2012, 2011, and 2010); they all have great info about how to fine-tune performance.

Please feel free to ask any further questions if something I've said makes little sense.