刷卡时UIPageControl跳过页面？

Question

目前我有一个UIPageController，它似乎在刷卡时跳过屏幕。这很奇怪，因为我在使用VC时将打印行语句放入其中，并且在快速转到同一滑动中的下一个VC之前，它看起来像是移动到正确的VC。 （即转到第2步，然后转到第3步，但在相同的滑动过程中）以下是它的设置方式：

     var maxPages : Int!
     var currentIndex : Int = 0
     var  listOfViews = ["step1", "step2", "step3", "step4"]


//maxPages = listOfViews.count in viewDidLoad
        public func pageViewController(_ pageViewController: UIPageViewController, viewControllerAfter viewController: UIViewController) -> UIViewController? {

        var index : Int = (viewController.view as? BGMWelcomePageView)!.index

        index += 1

        if (index == maxPages)
        {
            index = 0
        }


        return self.getViewControllerWithImage(index)
    }

    public func pageViewController(_ pageViewController: UIPageViewController, viewControllerBefore viewController: UIViewController) -> UIViewController? {

        var index : Int = (viewController.view as? BGMWelcomePageView)!.index

        index -= 1

        if (index < 0)
        {
            index = maxPages - 1
        }

//This is the function called to get the corresponding view controller
   func getViewControllerWithImage(_ index: Int) -> UIViewController
    {

        var vc : UIViewController
        let loopCount = 999//(index < self.maxPages - 1) ? 1 : 99
        switch listOfViews[index] {
        case "step1":
            vc =  self.buildViewControllerWithImage(titleString: "FIRST_SCREEN_TITLE".localized, descriptionString: "FIRST_SCREEN_DESCRIPTION".localized, backgroundImage: "fractal_blue", jsonFile: "Welcome_Step_01_V01", index: index, loopCount: loopCount)
             print("XXXX Current listView: step1 XXXX")
        case "step2":
            vc =  self.buildViewControllerWithImage(titleString: "SECOND_SCREEN_TITLE".localized, descriptionString: "SECOND_SCREEN_DESCRIPTION".localized, backgroundImage: "fractal_blue", jsonFile:
                "Welcome_Step_02_V01", index: index, loopCount: loopCount)
            print("XXXX Current listView: step2 XXXXX")

        case "step3":
            vc =  self.buildViewControllerWithImage(titleString: "THIRD_SCREEN_TITLE".localized, descriptionString: "THIRD_SCREEN_DESCRIPTION".localized, backgroundImage: "fractal_blue", jsonFile: "Welcome_Step_03_V01", index: index, loopCount: loopCount)
            print("XXXXX Current listView: step3 XXXXXX")

        case "step4":
            vc =  self.buildViewControllerWithImage(titleString: "FOURTH_SCREEN_TITLE".localized, descriptionString: "FOURTH_SCREEN_DESCRIPTION".localized, backgroundImage: "fractal_blue", jsonFile: "Welcome_Step_04_V01", index: index, loopCount: loopCount)
            print("XXXXXX Current listView: step4 XXXXXX")

        default:
            vc =  self.buildViewControllerWithImage(titleString: "FIRST_SCREEN_TITLE".localized, descriptionString: "", backgroundImage: "fractal_blue", jsonFile: "Welcome_Step_01_V01", index: index, loopCount: loopCount)
                print("XXXXX Current listView: default XXXXXX")
        }
        self.currentIndex = index

    //print("XXXXX Current Index: \(self.currentIndex) XXXXXXX")
        return vc
    }