Conditional sibling of a PageView (UIPageViewController) breaks paging

问题

I'm using this UIPageViewController implementation in SwiftUI as a starting point, but I want to have a view (eg: "OK" Text or a Button) appear above or below the PageView when currentPage is the last page.

However any time I add an if statement above or below the PageView, as soon as I swipe to the second page, all other pages get removed and it becomes impossible to swipe between pages. If I take out the if logic, everything works fine. Here's the code:

The PageViewController:

struct PageViewController: UIViewControllerRepresentable {
    var controllers: [UIViewController]

    @Binding var currentPage: Int

    func makeCoordinator() -> Coordinator {
        Coordinator(self)
    }

    func makeUIViewController(context: Context) -> UIPageViewController {
        let pageViewController = UIPageViewController(
            transitionStyle: .scroll,
            navigationOrientation: .horizontal)
        pageViewController.dataSource = context.coordinator
        pageViewController.delegate = context.coordinator

        return pageViewController
    }

    func updateUIViewController(_ pageViewController: UIPageViewController, context: Context) {
        var dir = UIPageViewController.NavigationDirection.forward
        if let visibleViewController = pageViewController.viewControllers?.first,
            let nextPage = controllers.firstIndex(of: visibleViewController) {
            if currentPage < nextPage {
                dir = .reverse
            }
        }

        pageViewController.setViewControllers([controllers[currentPage]], direction: dir, animated: true)
    }

    class Coordinator: NSObject, UIPageViewControllerDataSource, UIPageViewControllerDelegate {
        var parent: PageViewController

        init(_ pageViewController: PageViewController) {
            self.parent = pageViewController
        }

        func pageViewController(
            _ pageViewController: UIPageViewController,
            viewControllerBefore viewController: UIViewController) -> UIViewController?
        {
            guard let index = parent.controllers.firstIndex(of: viewController) else {
                return nil
            }
            if index == 0 {
                return nil
            }
            return parent.controllers[index - 1]
        }

        func pageViewController(
            _ pageViewController: UIPageViewController,
            viewControllerAfter viewController: UIViewController) -> UIViewController?
        {
            guard let index = parent.controllers.firstIndex(of: viewController) else {
                return nil
            }
            if index + 1 == parent.controllers.count {
                return nil
            }
            return parent.controllers[index + 1]
        }

        func pageViewController(_ pageViewController: UIPageViewController, didFinishAnimating finished: Bool, previousViewControllers: [UIViewController], transitionCompleted completed: Bool) {
            if completed,
                let visibleViewController = pageViewController.viewControllers?.first,
                let index = parent.controllers.firstIndex(of: visibleViewController)
            {
                parent.currentPage = index
            }
        }
    }
}

The PageView:

struct PageView<Page: View>: View {
    var viewControllers: [UIHostingController<Page>]
    @Binding var currentPage: Int

    init(_ views: [Page], currentPage: Binding<Int>) {
        self._currentPage = currentPage
        self.viewControllers = views.map { UIHostingController(rootView: $0) }
    }

    var body: some View {
        PageViewController(controllers: viewControllers, currentPage: $currentPage).frame(width: UIScreen.main.bounds.width)
    }
}

The main ContentView:

struct TestView: View {
    @State private var curPage: Int = 0

    var body: some View {
        ZStack {
            Color(.secondarySystemBackground)

            VStack {
                if curPage == 9892832 {
                    Text("OK").padding().foregroundColor(.blue).padding(.top, 60)
                }

                PageView([
                    Text("Page 1"),
                    Text("Page 2"),
                    Text("Page 3"),
                    Text("Page 4")
                ], currentPage: $curPage)
            }

        }.edgesIgnoringSafeArea(.all)
    }
}

struct TestView_Previews: PreviewProvider {
    static var previews: some View {
        TestView()
    }
}

I can't tell if this is a SwiftUI bug or a side effect of using UIViewControllerRepresentable or what. Is anyone else seeing this w/ conditional views? Any ideas on a workaround?

回答1:

I don't know what the root cause of this issue is. But I solved it with two nested VStacks. Put your PageView in the inner one and put your if or other Views to the outer VStack.

VStack {
    if curPage == 9892832 {
                Text("OK").padding().foregroundColor(.blue).padding(.top, 60)
    }

    VStack {

        PageView([
                Text("Page 1"),
                Text("Page 2"),
                Text("Page 3"),
                Text("Page 4")
            ], currentPage: $curPage)
    }

}

来源：https://stackoverflow.com/questions/59448446/conditional-sibling-of-a-pageview-uipageviewcontroller-breaks-paging