Programming iOS 6, 3rd Edition

Layers and Key–Value Coding

All of a layer’s properties are accessible through key–value coding by way of keys with the same name as the property. Thus, to apply a mask to a layer, instead of saying this:

layer.mask = mask;

we could have said:

[layer setValue: mask forKey: @"mask"];

In addition, CATransform3D and CGAffineTransform values can be expressed through key–value coding and key paths. For example, instead of writing this earlier:

self.rotationLayer.transform = CATransform3DMakeRotation(M_PI/4.0, 0, 1, 0);

we could have written this:

[self.rotationLayer setValue:[NSNumber numberWithFloat:M_PI/4.0]
                  forKeyPath:@"transform.rotation.y"];

This notation is possible because CATransform3D is key–value coding compliant for a repertoire of keys and key paths. These are not properties, however; a CATransform3D doesn’t have a rotation property. It doesn’t have any properties, because it isn’t even an object. You cannot say:

self.rotationLayer.transform.rotation.y = //... No, sorry

The transform key paths you’ll use most often are rotation.x, rotation.y, rotation.z, rotation (same as rotation.z), scale.x, scale.y, scale.z, translation.x, translation.y, translation.z, and translation (two-dimensional, a CGSize).

The Quartz Core framework also injects KVC compliance into CGPoint, CGSize, and CGRect, allowing you to use keys and key paths matching their struct component names. For a complete list of KVC compliant classes related to CALayer, along with the keys and key paths they implement, plus rules ...