Use generics to get a child view controller of a given type

Example.swift

if let tabViewController = self.contentViewController?.childViewControllerWithType(NSTabViewController) {
    // tabViewController is an NSTabViewController, woop!
}

NSViewController+ChildViewController.swift

import Cocoa

extension NSViewController {

    func childViewControllerWithType<ViewController>(type: ViewController.Type) -> ViewController? {

        if let vc = self as? ViewController {
            return vc
        }

        for viewController in self.childViewControllers {
            if let vc = viewController as? NSViewController {
                if let child = vc.childViewControllerWithType(type) {
                    return child
                }
            }
        }

        return nil
    }
}