Thinking In Terms Of iOS 8 Size Classes

One of the least sexy features of iOS 8’s new developer SDK may in fact be its most important: size classes.

A History Lesson

When iOS was first introduced, we had one device: the iPhone. It had a 320×480 pixel screen and it came in any color you liked, as long as it was black. Even in this single iteration, it supported two orientations: portrait and landscape.

Designing an app that supports two orientations isn’t always as simple as letting views stretch and resize like Mobile Safari or Messages do. In a lot of cases, buttons and other controls need to be moved around to better take into account the altered sizing of landscape (480×320).

Fast forward several years later and we’ve got retina displays, iPads, and 5″ iPhones. Oh and all those devices also support multiple orientations of course.

The current/old way of solving this problem has been a mix of listening for orientation changes in your view controllers and custom views, as well as using multiple Xibs or Storyboards.

Let’s say I was building a universal version of Glassboard that supported the iPad. With iOS 7 and older, I would need to create a secondary Storyboard that had each of my view controllers re-established, its outlets redone, and its target/action sequences reconnected. Duplicate work. Any seasoned programmer knows this isn’t ideal. Having to make the same change in two different places sucks.

Now, let’s add in the areas where I wasn’t necessarily using a Storyboard or Xib. There are likely going to be checks in the code to detect orientation and device size so that I can adjust my constraints or frame-based layout code (like an animal) manually.

Your code is likely littered with a lot of this:

Size Classes

Obviously the above is not ideal, and it’s likely only going to get worse as Apple introduces new devices going forward.

WWDC is a wonderful opportunity to read tea leaves and see where Apple is going by their new APIs and how hard they are pounding concepts into your heads each year. In addition to pushing Auto Layout this year, you also saw many instances of a resizable iOS simulator and a new technology for handling all this crazy idiom and orientation stuff: size classes.

A size class is a new technology used by iOS to allow you to custom your app for a given device class, based on its orientation and screen size.

There are two goals with size classes:

  1. To get developers and designers out of the habit of thinking in terms of specific devices, and instead thing of a more generalized approach.
  2. To plan for the future.

The first one obviously leads to the second. You’ve no doubt seen the countless rumors that the iPhone 6, 7 or whatever is going to be a larger device. You’ve probably heard that Apple is working on wearables. Guess what will make developing for these new devices easier? Size classes.

There are presently four size classes.

Size Class Chart

At any time your device is going to have a horizontal size class and a vertical size class. Together, those are what is used to define which set of layout attributes and traits to show the user on screen.


The horizontal and vertical size classes are considered traits. Together with the current interface idiom and the display scale they make up what is called a trait collection. This doesn’t just include where on screen specific controls should be placed.

Traits can also be used for things like image assets, assuming you are using Asset Catalogs (remember when I said to use the new stuff Apple introduces?). Instead of just having a 1x and 2x version of your asset, you can specify different image assets for a horizontal and vertically compact size class, as well as one for horizontally regular and vertically compact. In code, it still looks like the same UIImage call. The Asset Catalog takes care of rendering the appropriate image basd on the current trait collection.

Designing For Size Classes

Size classes are a nice addition for developers because it helps simplify the development experience when we want to support multiple devices and orientations. By making our job easier, Apple can more easily introduce new device sizes and sell developers on making their apps universal rather than just being an iPhone only product.

The biggest change for us developers is redoing our orientation code for the third or fourth time since iOS was created. We’re developers. We’re used to it.

For designers, trait collections means thinking less about the specific device you are designing for and instead thinking about the attributes of that device. The biggest attribute designers think in terms of now is physical screen size. Their mockups are pixel perfect for 320×480, 480×960 and the like.

Designing for those scenarios is no longer tenable as we move more towards a world where not every device is guaranteed to have the exact screen dimensions your Photoshop or Sketch mockups account for. Instead, the goal should be thinking about designing in the general sense for the class of device, which is presently:

If an iPhone 6 comes along and it has a bigger screen, my guess is that it’s going to have the same trait collections as the existing iPhone 4s and 5. Sure, the dimensions of the device will be bigger than the existing phones, but the UI should adaptively handle those changes based on your already defined interfaces for those specific trait collections.

This is likely the point where the designers have flipped over their bamboo work benches, but it’s the reality. Just like software development, software design is about adapting to the constraints. The new constraint is that we no longer have a world where we’re building towards a specific screen size. We’re not approaching Android levels of craziness, but this absolutely preps us for where Apple is headed.

Adopting Size Classes

The good news is that Interface Builder makes working with size classes easy. The better news is these Interface Builder changes are backwards compatible so you can simplify and consolidate Storyboards and Xibs where appropriate without leaving behind any legacy users.

The not so good news is that if you want to work with trait collections in code, it’s iOS 8 only. This shouldn’t come as a shock as Apple rarely backports new APIs to legacy versions of the OS. What it does mean is that you’re going to have to add some additional branching in your code to account for when you want to, for example, adjust the intrinsicContentSize of your custom UIView. If the device is on iOS 8, you can use the horizontal or vertical size class to help determine that value. If the device is still on iOS 7 or whatever, the existing code you’re using will have to remain deployed.

Since I use and recommend Interface Builder, my work load is significantly less than those that are still living by the “write everything in code” mantra. If you find yourself still in that camp, I’d highly recommend using iOS 8, Xcode 6, and trait collections as an opportunity to get on the Interface Builder bandwagon. It’s not only going to lessen the amount of boilerplate code you have to write, but it’s also going to simplify your code base by extracting out a lot of the trait handling into a visual UI.