问题
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?
回答1:
The transform property for the CALayer is a struct, not an object, so it isn't KVC compliant.
What you should be able to do is, instead of binding to the Z rotation, bind to the transform property and pull the Z value out whenever you get the KVO notification.
I think the confusion here is that when you use dot notation on an NSObject, you're really using that object's - (id)property and - (void)setProperty methods, which are KVC compliant. When you use dot notation on a struct, you're accessing a member of that struct, not calling a method.
来源：https://stackoverflow.com/questions/1260704/how-to-key-value-observe-the-rotation-of-a-calayer