在UIPageViewController中加载视图控制器

Question

我使用UIPageViewController滚动浏览3个视图控制器。目标是能够向左和向右滑动以导航它们，我可以使用以下代码进行操作。问题是，在第一次打开应用程序时，导航栏会加载到错误的位置，需要一整秒才能符合约束条件。导航栏的负载高于应有的值。我想知道为什么加载会有这样的延迟。

class ViewController: UIViewController, UIPageViewControllerDataSource {

var myViewControllers = Array(count: 3, repeatedValue:UIViewController())

    override func prepareForSegue(segue: UIStoryboardSegue, sender: AnyObject!) {
        let pvc = segue.destinationViewController as! UIPageViewController

        pvc.dataSource = self

        let storyboard = UIStoryboard(name: "Main", bundle: nil);

        var vc0 = storyboard.instantiateViewControllerWithIdentifier("ViewController1") as! UIViewController
        var vc1 = storyboard.instantiateViewControllerWithIdentifier("ViewController2") as! UIViewController
        var vc2 = storyboard.instantiateViewControllerWithIdentifier("ViewController3") as! UIViewController


        self.myViewControllers = \[vc0, vc1, vc2\]

        pvc.setViewControllers(\[myViewControllers\[1\]\], direction:.Forward, animated:false, completion:nil)

        println("Loaded")
    }

    func pageViewController(pageViewController: UIPageViewController, viewControllerAfterViewController viewController: UIViewController) -> UIViewController? {
        var currentIndex =  find(self.myViewControllers, viewController)!+1
        if currentIndex >= self.myViewControllers.count {
            return nil
        }
        return self.myViewControllers\[currentIndex\]
    }

    func pageViewController(pageViewController: UIPageViewController, viewControllerBeforeViewController viewController: UIViewController) -> UIViewController? {
        var currentIndex =  find(self.myViewControllers, viewController)!-1
        if currentIndex < 0 {
            return nil
        }
        return self.myViewControllers\[currentIndex\]
    }