Custom Modal 만들기 (alpha tip)

In your main view controller:

func showModal() {
    let modalViewController = ModalViewController()
    modalViewController.modalPresentationStyle = .OverCurrentContext
    presentViewController(modalViewController, animated: true, completion: nil)
}

In your modal view controller:

class ModalViewController: UIViewController {
    override func viewDidLoad() {
        view.backgroundColor = UIColor.clearColor()
        view.opaque = false
    }
}

If you are working with a storyboard:

//In storyBoard
Background = Clear Color
Drawing = Uncheck the Opaque checkbox
Presentation = Over Current Context

Alpha가 view에 지정되었을 때는, view 계층 아래에 속한 자식을 모두에게 alpha가 적용된다.