The CA Programming Guide is talking about Constraints Layout Managers. However, the CALayer in the iPhone SDK doesn't have any constraints property or addConstraint method.
They say iPhone OS just doesnt provide custom layout managers. But how about the standard ones?
Unfortunately, despite the documentation saying so, even the springs-and-struts layout of layers is missing from the iPhone implementation of CALayer. This can be seen by the missing autoresizingMask property on CALayer on the iPhone.
For Core Plot, we implemented our own custom layer layout via the -layoutSublayers method of CALayer. The CPLayer class provides an example of this approach.
The only method you can use for positioning views/layers relative to their superview/superlayer is via the springs & struts model. Custom layout managers aren't available on the iPhone.
The Springs and Struts layout support is available via UIView's autoresizingMask property.
来源：https://stackoverflow.com/questions/1181725/are-there-constraint-layout-managers-on-iphone-os