创建主入口 main.swift:

import UIKit

UIApplicationMain(
    Process.argc, Process.unsafeArgv,
    NSStringFromClass(MainApp), NSStringFromClass(MainAppDelegate)
)

创建 app.swift, 对应 MainApp 和 MainAppDelegate 的实现:

import UIKit

class MainApp: UIApplication {
    override func sendEvent(event: UIEvent) {
        super.sendEvent(event)
    }
}

class MainAppDelegate: UIResponder, UIApplicationDelegate {

    var window: UIWindow?

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

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

        self.window!.rootViewController = UIViewController()
        self.window!.backgroundColor = UIColor.whiteColor()
        self.window!.makeKeyAndVisible()

        self.window!.AddSubview {
            let label = UILabel(frame: self.window!.frame)
            label.textAlignment = .Center
            label.text = "你好, UIKit!"
            return label
        }
        
        return true
    }

}

extension UIView {
    func AddSubview(subview: ()->UIView) {
        self.addSubview(subview())
    }
}

运行效果:

Swift: 你好 UIKit.png