I can access the value like this:

NSNumber* rotationZ = [myLayer valueForKeyPath:@"transform.rotation.z"];

But for some reason, if I try to KV-observe that key path, I get a compiler error. First, this is how I try to do it:

[myLayer addObserver:self forKeyPath:@"transform.rotation.z" options:0 context:nil];

The compiler tells me:

*** Terminating app due to uncaught exception 'NSUnknownKeyException', reason: '[ addObserver: forKeyPath:@"rotation.z" options:0x0 context:0x528890] was sent to an object that is not KVC-compliant for the "rotation" property.'

what I don't get is, why I can access that z value by KVC key path, but not add an observer to it. Does this make sense?

How else could I observe the z value of that matrix? I don't care about the other values of the matrix. Only the z rotation. Any other way to access and observe it?