What is the easiest to get the size class of a UIViewController?

Question

In the WWDC 2014 Platform State of the Union lecture Apple Engineer introduced size classes and he said we can think iPhone size UI as a compact size class and iPad size class as a regular size class. But size classes are not bounded to a particular device. They are a lot more general then that. If a viewcontroller look like an iPhone - its aspect ration is similar to it -, it will have a compact size class.

Is it a way to see at a certain time which size class was used by viewcontroller? I found contradictions between simulator content and Interface Builder preview and I would like to dig deeper into and see why it happens.

Answer 1

I am currently looking on how to use size classes programatically here are some finding that can be useful for you:

The easiest way to find out is to just check on the view controllers traitCollection

po self.traitCollection

Or listen to transitions:

First use the UIContentContainer protocol

@interface ViewController : UIViewController<UIContentContainer>

Then implement willTransitionToTraitCollection:

- (void)willTransitionToTraitCollection:(UITraitCollection *)newCollection withTransitionCoordinator:(id <UIViewControllerTransitionCoordinator>)coordinator
{

}

The newCollection element has information that can be used for your purpose, here is the info returned when rotating an iPhone 6 to landscape:

<UITraitCollection: 0x7f9ad152f320; _UITraitNameUserInterfaceIdiom = Phone,     _UITraitNameDisplayScale = 2.000000, _UITraitNameHorizontalSizeClass = Compact, _UITraitNameVerticalSizeClass = Compact, _UITraitNameTouchLevel = 0, _UITraitNameInteractionModel = 1>

And to portrait:

<UITraitCollection: 0x7f9ad142d770; _UITraitNameUserInterfaceIdiom = Phone, _UITraitNameDisplayScale = 2.000000, _UITraitNameHorizontalSizeClass = Compact, _UITraitNameVerticalSizeClass = Regular, _UITraitNameTouchLevel = 0, _UITraitNameInteractionModel = 1>

From there you can see it uses Compact Size Class for both horizontal and vertical in landscape, but uses Regular Size Class for Vertical when in portrait.