AppDelegate.swift

import UIKit

@UIApplicationMain
class AppDelegate : UIResponder, UIApplicationDelegate {
    var window : UIWindow?
    func application(application: UIApplication, didFinishLaunchingWithOptions launchOptions: [NSObject: AnyObject]?) -> Bool {
            self.window = UIWindow(frame:UIScreen.mainScreen().bounds)
            self.window!.rootViewController = UIViewController()
            
            let mainView = self.window!.rootViewController!.view
            
            let newView = UIView(frame: CGRectMake(100, 100, 50, 50))
            newView.backgroundColor = UIColor.redColor()
            mainView.addSubview(newView)
            
            self.window!.backgroundColor = UIColor.whiteColor()
            self.window!.makeKeyAndVisible()
            return true
    }
}