It's not a private API, the docs are just out of date. You can technically set any object as the contents of a CALayer (the property's type is `id` or `Any`),

It's not a private API, the docs are just out of date. You can technically set any object as the contents of a CALayer (the property's type is `id` or `Any`), it's just that most objects won't do anything productive. The docs call out using images specifically because that's the 99% use-case; they need to be updated saying that `IOSurfaceRef`s can be used as layer content as well. `IOSurfaceRef` itself was actually private at one point, IIRC, but isn't anymore.