xcode6 - How can Xcode 6 adaptive UIs be backwards-compatible with iOS 7 and iOS 6?

Question

I just watched the WWDC video #216, "Building Adaptive UI with UIKit."

At about 45:10 Tony Ricciardi talks about changes to IB in Xcode 6 to support the new changes.

He says "You can deploy these documents backwards to older versions of iOS".

(where "These documents" presumably means XIBs and storyboards that have specific settings for different size classes.)

I'm not making this up. Go watch the WWDC video.

How is that possible? Trait collections and size classes are only defined in iOS 8. How can runtime behavior that's dependent on UI constructs that are new to iOS 8 work in previous iOS versions?

If it is possible it would be wonderful. You could build apps that will run on iOS 6, 7, and 8, and take advantage of the new flexible UI layout abilities that Apple has added to Xcode 6. I've created adaptive UI logic myself in code, and it's quite a bit of work.

Answer 1

Changes made to the UI with Size Classes in Interface Builder DO appear correctly on iOS 7 devices and the Preview in Xcode. For example, I changed some Auto Layout constraints and font sizes for Regular height Regular width and those changed constraints are visible in the iPad Simulator running iOS 7.0.

All size class optimizations are made available to iOS 7, except size classes that have a Compact Height. This has been confirmed by Apple and is now stated directly in the documentation:

For apps supporting versions of iOS earlier than iOS 8, most size classes are backward compatible.

Size classes are backward compatible when:
- The app is built using Xcode version 6 or later
- The deployment target of the app is earlier than iOS 8
- Size classes are specified in a storyboard or xib
- The value of the height component is not compact

Because iOS 7 doesn't respect a couple of size classes, if you use them you'll run into issues. For example: When you have Compact w Any h defined and then Compact w Compact h defined, on iOS 7 it will respect the Compact w Any h but on iOS 8 it renders the Compact w Compact h appearance.

So, if you would like to utilize those two size classes and maintain compatibility with iOS 7, I would do any optimizations you desire for iPhone in landscape in Any w Any h or Compact w Any h, then perform your other optimizations for different size classes as necessary, and that way you won't need to use any size class with compact height and will avoid running into issues.