The iOS family continues to grow. Although iOS targets are not nearly as splintered as Android’s multitude, iOS apps must adapt interfaces to numerous configurations for universal deployment. Routines specific to iPads or iPhones, to landscape or portrait orientation, and to retina or non-retina screens have transformed many iOS apps into a tangle of special-purpose code.

To push back, iOS 6 introduced Auto Layout, a descriptive system for interface design. Auto Layout enabled developers to use rule-based view placement that automatically adjusts to screen dimensions and orientation.

Now iOS 8 adds the notion of adaptive deployment. New classes and protocols enable apps to retrieve the specifics of the current runtime environment. This enables them to adapt not only to hardware limitations but also to whatever screen space has been allocated to their presentations. A truly adaptive app gracefully responds with a well designed and engaging interface, ready for the user at any size.

A iOS 8 trait collection describes a single point in deployment space. This space represents the range of possible conditions an interface might encounter in the real world. By making these traits concrete, Apple enters into a somewhat implicit contract with the developer – these are the types and ranges of flexibility you must design for.

Even if Apple introduces multi-windowing, you only have to worry about “compact” and “regular” interface sizes under the current system. There’s a (vague and legally unenforcable) guarantee that for at least the next year Apple will not add some “Ultra Compact” game-changer where you suddenly have to worry about designing forms for 100-pixel destinations. And, if Apple does introduce that windowing, the traitCollectionDidChange callback means you can switch your tablet presentations to phone ones for side-by-side execution and then back again.

No matter what kind of apps you design, a tension always exists between pixel-perfect control and adaptability. An interface that looks stunning on a 4″ iPhone may look cramped on a 3.5″ screen and sparse on a tablet. This has led some developers to build what is essentially multiple codebases under a single app umbrella.

With trait collections and standardized callbacks, iOS 8 is attempting to bring sanity back to those apps. So long as the presentation specifics don’t get too ridiculous, Auto Layout and trait-driven assets should be able to handle those specifics both at launch time and during run time.

Will Apple introduce a 3x display resolution? I think it unlikely but if they do, the technology is already in-place to handle that trait collection with a 3.0 display scale. Will Apple add a new geometry to the phone family? That’s a rumor that seems to have more appeal — but Auto Layout is ready for that, and the compact-regular layout traits will support any baby-dolphin-sized phablet that might hit the market. Will iOS 8 support multi-app display? The technology to support it is ready for developers if they take advantage of those features.

I admit that some of Apple’s design choices for trait collections baffle me. For example, how is it that a 480-point height is “regular” and a 586-point width is “compact”? But despite that, I find it comforting to  receive a concrete set of interface expectations via the trait API.

What do you think? Please do let me know.