// Swift
let s = self.window!.rootViewController as! UISplitViewController
let n = s.viewControllers[s.viewControllers.count-1] as! UINavigationController
n.topViewController!.navigationItem.leftBarButtonItem = s.displayModeButtonItem()


// Objective-C
id s = self.window.rootViewController;
id n = [s viewControllers][[s viewControllers].count-1];
[n topViewController].navigationItem.leftBarButtonItem = s.displayModeButtonItem;