Crash while in CALayer -hitTest:

Question

This issue really has me stumped...

I have an iPad project that I use UIPanGestureRecognizer and I am using the following method call within my handlePanGesture:

- (AIEnemyUnit *) hitTestForEnemyUnit:(CGPoint)where {
    CALayer * layer = [self hitTest:where];

    while (layer) {
        if ([layer isKindOfClass:[AIEnemyUnit class]]) {
            return (AIEnemyUnit *)layer;
        } else {
            layer = layer.superlayer;
        }
    }

    return nil;
}

Once I "find" an AIEnemyUnit layer, I continue with the drag and everything works fine. Except about around the 6th to 10th "drag" I get a crash with the debugger deep within only the CALayer -hitTest:

modifying layer that is being finalized - 0x124530
*** -[NSCFSet hitTest:]: unrecognized selector sent to instance 0x124530
*** Terminating app due to uncaught exception 'NSInvalidArgumentException', 
reason: 
'*** -[NSCFSet hitTest:]: unrecognized selector sent to instance 0x124530'

Answer 1

From the symptoms it looks like you have an over-release of a CALayer.

Two areas you should check:

1) Are you 'saving' this CALayer in a variable without retaining it? If you're hitting any autorelease pools (including the one supplied on the main thread) then these layers could be getting released unintentionally. As noted in the comments, since these aren't autoreleased, this can happen without hitting a pool. It can happen anytime the CALayer is released while you're holding a reference however.

2) You're explicitly calling release on this layer later on. Since you are given this layer as is (both hitTest: and superlayer return objects without an extra retain count) you do not have ownership, and thus should not release it.

Another useful tool for debugging is using NSZombies, and some other techniquies linked there. NSZombies basically allows your application to crash the moment you access a released object, which hopefully will give you a more meaningful stack trace.