该viewWillTransitionToSize委托方法被调用了UIViewControllerTransitionCoordinator符合对象。协议声明的方法是animateAlongsideTransition(_:animation, completion:)。您可以使用它来使代码在转换完成后执行。

override func viewWillTransitionToSize(size: CGSize, withTransitionCoordinator coordinator: UIViewControllerTransitionCoordinator) {
    super.viewWillTransitionToSize(size, withTransitionCoordinator: coordinator)
    coordinator.animateAlongsideTransition(nil, completion: {
        _ in

        // Your code here
    })
}