Navigation "in code"

Swift-ViewController-Navigation.swift

// in appDelegate:
let controllerId = "target VC Storyboard ID"
let storyboard: UIStoryboard = UIStoryboard(name: "Main", bundle: nil)
let initViewController: UIViewController = storyboard.instantiateViewControllerWithIdentifier(controllerId)
self.window?.rootViewController = initViewController

// *replace* current ViewController by anotherone:
let storyboard: UIStoryboard = UIStoryboard(name: "Main", bundle: nil)
let targetViewController: UIViewController = storyboard.instantiateViewControllerWithIdentifier("target VC StoriboardID") as UIViewController
let navigationController: UINavigationController = self.navigationController!;

var controllerStack = navigationController.viewControllers;
let index = controllerStack.indexOf(self);
controllerStack[index!] = targetViewController
navigationController.setViewControllers(controllerStack, animated: true);

// to replace ALL VC stack, replaces ll. 12+ with:
self.navigationController?.setViewControllers([targetViewController], animated: true)

// to present modally:
self.presentViewController(targetViewController, animated: true, completion: nil)

// ...
showViewController()

self.window?.rootViewController?.presentViewController(importAlert, animated: true, completion: nil)