导航栏在更改为内联之前作为大标题短暂闪烁

Question

我在我的 SwiftUI 应用程序中使用了一个 UITabController，它运行良好，除了一个问题。

当我有一个使用大标题的 NavigationView 时，它会在我滚动时变为内联，正如预期的那样。到目前为止一切顺利。

当我切换标签，然后再次切换回来时，它应该记住滚动位置，因此是内联的。相反，它在移动到内联之前会作为一个大标题非常短暂地闪烁。

这似乎只发生在设备上。

Screen recording of the issue

有人可以帮我解决问题吗？

struct ContentView: View {
    var body: some View {
        CustomTabView([
            Tab(view: FirstView(),
                barItem: UITabBarItem(title: "First", image: UIImage(systemName: "1.circle"), selectedImage: nil)),
            Tab(view: SecondView(),
                barItem: UITabBarItem(title: "Second", image: UIImage(systemName: "2.circle"), selectedImage: nil))
        ])
    }
}

struct FirstView: View {
    var body: some View {
        NavigationView {
            List {
                ForEach( 0...40, id: \.self ) {
                    Text("\($0)")
                        .padding()
                }
            }
            .navigationTitle("First View")
        }
    }
}

struct SecondView: View {
    var body: some View {
        NavigationView {
            List {
                ForEach( 41...80, id: \.self ) {
                    Text("\($0)")
                        .padding()
                }
            }
            .navigationTitle("Second View")
        }
    }
}

struct TabBarController: UIViewControllerRepresentable {
    var controllers: [UIViewController]

    func makeUIViewController(context: Context) -> UITabBarController {
        let tabBarController = UITabBarController()
        tabBarController.viewControllers = controllers
        tabBarController.delegate = context.coordinator
        return tabBarController
    }

    func updateUIViewController(_ uiViewController: UITabBarController, context: Context) {}
    
    func makeCoordinator() -> Coordinator {
        Coordinator()
    }
    
    class Coordinator: NSObject, UITabBarControllerDelegate {
        func tabBarController(_ tabBarController: UITabBarController, shouldSelect viewController: UIViewController) -> Bool {
            guard let fromView = tabBarController.selectedViewController?.view, let toView = viewController.view else {
                return false
            }
            if fromView != toView {
                fromView.superview!.addSubview(toView)
            }
            return true
        }
    }
}

struct CustomTabView: View {
    var viewControllers: [UIHostingController<AnyView>]

    init(_ tabs: [Tab]) {
        self.viewControllers = tabs.map {
            let host = UIHostingController(rootView: $0.view)
            host.tabBarItem = $0.barItem
            return host
        }
    }

    var body: some View {
        TabBarController(controllers: viewControllers)
            .edgesIgnoringSafeArea(.all)
    }
}

struct Tab {
    var view: AnyView
    var barItem: UITabBarItem

    init<V: View>(view: V, barItem: UITabBarItem) {
        self.view = AnyView(view)
        self.barItem = barItem
    }
}

Answer 1

我找到了解决方法/修复。

我需要使用自定义 UIHostingController，找到导航控制器，并在 viewWillAppear 函数中添加一行：

public class MyHostingController: UIHostingController<AnyView> {
    override open func viewWillAppear(_ animated: Bool) {
        if let navigationController = navigationController() {
            navigationController.view.setNeedsUpdateConstraints()
        }
        super.viewWillAppear(animated)
    }
}

private extension UIViewController {
    func navigationController() -> UINavigationController? {
        var controller: UINavigationController?
        
        if let navigationController = self as? UINavigationController {
            return navigationController
        }
        
        children.forEach {
            if let navigationController = $0 as? UINavigationController {
                controller = navigationController
            } else {
                controller = $0.navigationController()
            }
        }
        
        return controller
    }
}

然后很明显用 MyHostingController 替换了我原始代码中 UIHostingController 的两种用法。

最重要的一行是：

navigationController.view.setNeedsUpdateConstraints()

希望这对发现类似问题的其他人有所帮助。