프로젝트 xib로 시작하기.

@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {
    var window: UIWindow?

    func application(application: UIApplication, didFinishLaunchingWithOptions launchOptions: [NSObject: AnyObject]?) -> Bool {

        window = UIWindow(frame: UIScreen.mainScreen().bounds)

        var mainViewController = ViewController(nibName: "ViewController", bundle: nil)

        window?.rootViewController = mainViewController
        window?.makeKeyAndVisible()
        return true
    }

    ...
}

다음과 같이 AppDelegate.swift파일에 넣어주면 된다.