UITraitCollections in iOS 7

Goal

The goal is to have some of the useful UITraitCollection API available to iOS 7, in order to reduce branching codepaths when writing layout code.

API

Currently the following API should be available to iOS 7:

• -traitCollection - The current UITraitCollection for the UIView or UIViewController, on iOS 7 an instance of DDTraitCollection will be returned (it should be API-compatible with UITraitCollection)
• -traitCollectionDidChange: - This method will be called in any iOS 7 view or viewController that has previously called -dd_registerForTraitCollectionUpdates

Example

First and foremost, you should #import <DDTraitCollection/UIView[Controller]+DDTraitCollection.h> in any files where you would like to access the traitCollection

If you would like to create UITraitCollection (or DDTraitCollection) instances, you should #import <DDTraitCollection/DDTraitCollection.h>

Here is a quick example of how you could use this:

- (void)updateConstraints {
  UITraitCollection *currentTraitCollection = self.traitCollection;

  if(currentTraitCollection.horizontalSizeClass == UIUserInterfaceSizeClassCompact) {
    // Update constraints for compact horizontal size class
    ...
  } else {
    // Do stuff for regular
    ...
  }

  [super updateConstraints];
}

Warning

If you need to create UITraitCollection instances in code shared between iOS7 and 8 you should do so via the DDTraitCollection class methods (using UITraitCollection in iOS 7 will just nil stuff out).

In iOS 7 it will return an instance of DDTraitCollection whereas in iOS 8 it will correctly return UITraitCollection instances.