A minimal implementation of `AppDelegate` for Swift 2.x.

AppDelegate.swift

import UIKit

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

    func application(
        application: UIApplication,
        didFinishLaunchingWithOptions launchOptions: [NSObject: AnyObject]?
    ) -> Bool {
        window = UIWindow(frame: UIScreen.mainScreen().bounds)
        window?.backgroundColor = UIColor.whiteColor()
        window?.rootViewController = UIViewController()
        window?.makeKeyAndVisible()
        return true
    }
}