iOS- Detect current size classes on viewDidLoad

As of iOS 8 UIViewController adopts the UITraitEnvironment protocol. This protocol declares a property named traitCollection which is of type UITraitCollection. You can therefor access the traitCollection property simply by using self.traitCollection

UITraitCollection has two properties that you want to access named horizontalSizeClass and verticalSizeClass Accessing these properties return an NSInteger. The enum that defines the returned values is declared in official documentation as follows- (this could potentially be added to in the future!)

typedef NS_ENUM (NSInteger, UIUserInterfaceSizeClass {
   UIUserInterfaceSizeClassUnspecified = 0,
   UIUserInterfaceSizeClassCompact     = 1,
   UIUserInterfaceSizeClassRegular     = 2,
};

So you could get the class and use say a switch to determine your code direction. An example could be –

NSInteger horizontalClass = self.traitCollection.horizontalSizeClass;
NSInteger verticalCass = self.traitCollection.verticalSizeClass;

switch (horizontalClass) {
    case UIUserInterfaceSizeClassCompact :
        // horizontal is compact class.. do stuff...
        break;
    case UIUserInterfaceSizeClassRegular :
        // horizontal is regular class.. do stuff...
        break;
    default :
        // horizontal is unknown..
        break;
}
// continue similarly for verticalClass etc.