I got thinking about size classes in iOS 8 after the Apple Watch announcement. As of right now, there are two size classes in each dimension: "regular" and "compact." Broadly speaking, the compact size class desribes the iPhone, and the regular size class describes the iPad. Yes, I know that there's not a strict mapping between size class and device, but there's enough of a connection that we can mostly use size classes as a proxy for devices. (The iPhone 6 Plus muddies the waters here a bit.)
When they were announced at WWDC, I was surprised the classes weren't "regular" and "large" – regular for the iPhone and large for the iPad. That's even more true with the introdution of the Apple Watch. It's easy to imagine creating a size class matrix like this:
But instead, I think the Apple Watch implies something like this:
There's two things to consider here:
- Does the Apple Watch really fit into a size class smaller than "compact," and/or does it use size classes at all?
- Does the use of "regular" as the biggest current size class imply the existence of a "large" class in the future?
Where does the Apple Watch fit?
Apple hasn't released nearly enough technical information about the Apple Watch to talk about this with much certainty. Paul Sprangers made some educated guesses that the Apple Watch screen is roughly 280 x 350px, which would seem to place it in a size class smaller than "compact." Maybe this implies a new size class. ("Micro?")
It could be that the Apple Watch exists outside the size class system entirely. Apple's web site makes reference to something called the "Watch OS." We don't know much more about it than that, but even if it's distinct from iOS, it's likely to be strongly related. The odds are high that it's using size classes under the hood somewhere, even if it's not exposed to developers by name. After all, why create a system for flexibly adapting to different screen sizes if you're not going to use it on a new, differently-sized screen?
Either way, we'll know a lot more about this once we get a look at WatchKit.
Both the iPad Air and the iPad mini have a "regular" size class in both dimensions, which implies that Apple is at least leaving room for something larger than the iPad. The likeliest explanation is that they're keeping their options open for shipping larger devices in the future. Maybe a larger "iPad Pro"? Or perhaps an Apple TV SDK, in which the TV has a "large" size class. Time will tell.