Question

I have a basic Cocoa app with a custom NSCollectionView that overrides drawRect: to draw a red background (for illustration purposes).

I noticed that at seemingly random times, drawRect: would not be called although I invoked setNeedsDisplay:YES. Other times it worked and the collection view had a red background.

It took me a few hours, but then I finally noticed that high up in the view hierarchy one of the parent views was a layer-backed view (I had “Core Animation Layer” checked in Interface Builder to add a shadow to an element).

Changing that parent view to not use layer-backing immediately solved the problem and drawRect: is then always called as expected.

What puzzles me is that with layer-backing enabled on a parent view, drawRect: would be called sometimes, but not always.

Should I assume that I can’t reliably override drawRect: in any subview if a parent view is layer-backed?

Answer 1

Watch the WWDC 2012 video on layer-backed views, and read the documentation for -layerContentsRedrawPolicy. Sounds like you want NSViewLayerContentsRedrawOnSetNeedsDisplay.