通过 UIViewControllerAnimatedTransitioning 协议提供具体的动画效果

extension UITransitionContextViewControllerKey {
    @available(iOS 7.0, *)
    public static let from: UITransitionContextViewControllerKey

    @available(iOS 7.0, *)
    public static let to: UITransitionContextViewControllerKey
}
extension UITransitionContextViewKey {
    @available(iOS 8.0, *)
    public static let from: UITransitionContextViewKey

    @available(iOS 8.0, *)
    public static let to: UITransitionContextViewKey
}