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Over the years, App Kit has been improved and extended in many ways.In OS X Lion, Apple introduced Auto Layout, providing a more flexible and powerful way to position elements on screen.

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Crucially, the positions of views are now determined by the solution to the set of constraints set up by the developer (the Auto Layout engine uses the Cassowary constraint solver).

If you take one thing away from this article, it should be this: the position of a view is a function of the constraint system.

If that's not the case (i.e., a view referenced by constraints – meaning its position is determined by Auto Layout – does not have a well-defined position), then that view and any views that are directly or indirectly related to it (via the constraints system) would have an undefined position.

In practice, Auto Layout views are usually related to their superview (whether directly or are related to a sibling which in turn is related to their common superview).

Note that the behaviour is slightly different on OS X – App Kit will try to synthesise constraints for all views in the window, rather than just the minimum required set.

If Auto Layout is enabled for a particular window, the semantics of -set Frame: for any views that have -translates Autoresizing Mask Into Constraints == YES changes to: update the constraints associated with the receiver such that the output position is exactly equal to the input parameter of -set Frame: (remember that under Auto Layout, the position of a view is a function of the constraint system).

Furthermore, one might ask what determines whether Auto Layout is turned on for a window?

It gets enabled when there's at least one constraint in the view hierarchy in a window.

There's an extremely large amount of code written that does not use Auto Layout, thus the new approach needs to be backwards compatible.

More importantly, we need to know what the restrictions are on mixing code that uses Auto Layout and one that does not.

Auto Layout uses a constraint solver to find a solution to a system of linear equalities and inequalities (the set of NSLayout Constraints) – that solution determines the positions of the views.

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