Reordering CALayer sublayers without raping my performance?

FROM : Jonathan del Strother
DATE : Fri Nov 16 19:41:39 2007

I have a root CALayer with around 250 child layers.  Only 50 or so are 
visible at a time, the rest have their opacity set to 0 (which I've 
found to be far more performant that continuously removing and adding 
child layers).

Every 0.2 seconds, I want to move one of these child layers to be 
drawn on the top of everything else.  I'm doing this by altering the 
order of the root layer's sub layers.  I haven't been able to find an 
acceptable way of doing this.  Everything I've tried will continuously 
increase my virtual memory usage to ~3.5GB, before dropping back to 
1.1GB every 10 seconds or so.  While that drop happens, my application 
performance drops massively, displaying a frame a second at best, on a 
Mac Pro with 3GB RAM.

The obvious way of moving a sublayer to the front would appear to be :
   [rootLayer insertSublayer:child above:[[rootLayer sublayers] 
lastObject]];
where 'child' already exists within in the rootLayer's sublayers.

I've also tried variants along the lines of :
   NSMutableArray* sublayers = [[rootLayer sublayers] mutableCopy];
   [sublayers removeObject:child];
   [sublayers addObject:child];
   rootLayer.sublayers = sublayers;
   [sublayers release];

Everything I've tried results in the excessive virtual memory usage 
and a huge drop in performance.


If I give up on moving the child to the front, it runs perfectly : my 
app sits happily at 120MB real / 1.1GB virtual memory, and updates 
smoothly.  It's only when trying to alter the sublayers array that 
these problems kick in.


Any suggestions on getting around this?

Jon