View source for Swift:Navigation between storyboards

===Navigate to Storyboard ID===
<pre class="brush:swift">
var vc = storyboard.instantiateViewControllerWithIdentifier("loginViewController") as UIViewController
window?.rootViewController = vc
</pre>
===Navigate Segue by Identifier===
First, drag a segue in storyboard and give it an identifier, then in the button action (or any action you like), do:
<pre class="brush:swift">
self.performSegueWithIdentifier("loginSegue", sender: self)
</pre>
===Get UIVewController from Navigation Controller===
Sometimes, you want to access the UIViewController from a navigation controller
<pre class="brush:swift">
var vc = storyboard?.instantiateViewControllerWithIdentifier("navigationID") as UINavigationController
var v = vc.viewControllers[0] as ViewController // [0] represents the first level UIViewController under NavigationController
</pre>