CALayer flickering when adding a foreground layer to IKImageBrowserView items with garbage collection on

Question

I'm trying to implement a technique similar to the one in the ImageBrowserViewAppearance sample code from Apple (located here: http://developer.apple.com/library/mac/#samplecode/ImageBrowserViewAppearance/Introduction/Intro.html ), where CALayers are generated on top of the items in the IKImageBrowserView to customize the appearances of the objects in the image browser.

However, I'm getting a weird problem when I turn on garbage collection, and I can reproduce it in the Apple sample code. Simply turn on Garbage Collection in the target, and build and launch the ImageBrowserAppearance sample app. Then, add some photos to the image browser using the "Add Photos..." button.

Now, click on an empty portion of the IKImageBrowserView, and click and drag to start selecting multiple items in the browser view. As you drag the selection box around, you should notice that sometimes the pin and gloss overlay for some of the items flicker and briefly appear in the bottom-left corner of the IKImageBrowserView. All of the CALayers seem to do this occasionally, I've seen the white surrounding slide area flicker down into the bottom-left corner as well.

When I mimic the technique in my own code, I (not surprisingly) also can reproduce this badge flickering. However, this problem disappears when garbage collection is off.

Anybody have a clue what could be going wrong here? I'd like to use garbage collection in my app in conjunction with this technique, but the flickering is kind of annoying.

Answer 1

I bookmarked this a while back but Apple's changed the URL and the text. Fortunately I quoted it when I bookmarked it:

The Core Graphics APIs (Quartz 2D) see an approximately 25% reduction in drawing performance for applications compiled to use garbage collection.

That "25% reduction in drawing performance" text has been rewritten into a "slight overhead in code execution" and that was for 10.5. Perhaps Apple fixed it for 10.6. And you're talking Core Animation, not Core Graphics.

Still, Core Animation eventually has to talk to Core Graphics, and perhaps that performance issue hasn't gone away, and you're being bitten by it.

Answer 2

I fooled around with this a bit and can confirm I get the same behavior running the project with GC turned on. In fact, if you're patient enough and slowly change the selection one image at a time using the arrow keys, eventually it'll trigger the behavior and you can see the layers from one image in the view are displayed in the lower left corner instead of on top of the image. I haven't been able to find any sort of pattern as to when it happens, or any relation between which image is selected and which image has its layers missing. I'm assuming that for whatever reason, those layers are getting their frame origin set to {0, 0}, but heck if I know why.