UIPageViewController子视图控制器无法在viewDidAppear上执行分段

class MyPageViewController: UIPageViewController {
    // 子视图控制器数组
    var viewControllers: [UIViewController] = []

    override func viewDidLoad() {
        super.viewDidLoad()
        
        // 初始化子视图控制器数组
        let viewController1 = UIViewController()
        let viewController2 = UIViewController()
        let viewController3 = UIViewController()
        viewControllers = [viewController1, viewController2, viewController3]
        
        // 设置UIPageViewController的数据源和代理
        dataSource = self
        delegate = self
        
        // 设置初始显示的子视图控制器
        setViewControllers([viewController1], direction: .forward, animated: true, completion: nil)
    }
}

extension MyPageViewController: UIPageViewControllerDataSource {
    func pageViewController(_ pageViewController: UIPageViewController, viewControllerBefore viewController: UIViewController) -> UIViewController? {
        // 返回前一个子视图控制器
        guard let index = viewControllers.firstIndex(of: viewController), index > 0 else {
            return nil
        }
        return viewControllers[index - 1]
    }
    
    func pageViewController(_ pageViewController: UIPageViewController, viewControllerAfter viewController: UIViewController) -> UIViewController? {
        // 返回后一个子视图控制器
        guard let index = viewControllers.firstIndex(of: viewController), index < viewControllers.count - 1 else {
            return nil
        }
        return viewControllers[index + 1]
    }
}

extension MyPageViewController: UIPageViewControllerDelegate {
    func pageViewController(_ pageViewController: UIPageViewController, willTransitionTo pendingViewControllers: [UIViewController]) {
        // 在页面切换开始前执行分段操作
        // 这里可以执行一些准备工作，比如更新UI、加载数据等
    }
    
    func pageViewController(_ pageViewController: UIPageViewController, didFinishAnimating finished: Bool, previousViewControllers: [UIViewController], transitionCompleted completed: Bool) {
        // 页面切换完成后执行分段操作
        // 这里可以执行一些后续操作，比如更新UI、保存数据等
    }
}