Lazy Loading view controllers in Swift

MainViewController.swift

/**
 * Lazy load view controller example
 */
class MainViewController: UIViewController {
     
    lazy var firstViewController: FirstViewController = {
        return self.storyboard!.instantiateViewControllerWithIdentifier("FirstViewController") as! FirstViewController
    }()
    
    lazy var secondViewController: SecondViewController = {
        return self.storyboard!.instantiateViewControllerWithIdentifier("SecondViewController") as! SecondViewController
    }()
    
    @IBOutlet weak var promptContainerView: UIView
    
    var visiblePrompt: UIViewController?
    
    func viewWillAppear(animated: Bool) {
        let foo = true
        if foo { // if some condition is true
            self.presentPrompt(self.firstViewController)
        } else {
            self.presentPrompt(self.secondViewController)
        }
    }

    // add our lazily loaded view controller to the container outlet and match the container constraints.
    // Also, you can optionally throw in a view animation
    func presentPrompt(viewController: UIViewController, animate: ((UIView?, UIView, () -> Void) -> Void) = { _, _, block in block() }){
        
        self.visiblePrompt?.willMoveToParentViewController(nil)
        self.addChildViewController(viewController)
        
        viewController.hostViewController = self
        
        let contentView = viewController.view
        let containerView = self.promptContainerView
        
        containerView.addSubview(contentView)
        
        contentView.translatesAutoresizingMaskIntoConstraints = false
        NSLayoutConstraint.activateConstraints([
            NSLayoutConstraint(item: contentView, attribute: .Top, relatedBy: .Equal, toItem: containerView, attribute: .Top, multiplier: 1.0, constant: 0.0),
            NSLayoutConstraint(item: contentView, attribute: .Bottom, relatedBy: .Equal, toItem: containerView, attribute: .Bottom, multiplier: 1.0, constant: 0.0),
            NSLayoutConstraint(item: contentView, attribute: .Leading, relatedBy: .Equal, toItem: containerView, attribute: .Leading, multiplier: 1.0, constant: 0.0),
            NSLayoutConstraint(item: contentView, attribute: .Trailing, relatedBy: .Equal, toItem: containerView, attribute: .Trailing, multiplier: 1.0, constant: 0.0),
            ])
        
        animate(self.visiblePrompt?.view, viewController.view) {
            self.visiblePrompt?.view.removeFromSuperview()
            self.visiblePrompt?.removeFromParentViewController()
            
            viewController.didMoveToParentViewController(self)
            self.visiblePrompt = viewController
        }
    }
}