Swift：如何通过UIPageViewController实现统一顶部设计及初始页面配置

一、设置初始显示MainSegPageViewController #

1. 打开Storyboard，选中MainSegPageViewController。
2. 在右侧属性面板勾选Is Initial View Controller，或通过菜单栏Editor -> Set as Initial View Controller设置。
3. 确保MainSegPageViewController的视图包含你设计的顶部区域，同时给子控制器（ViewController1/2）的视图添加约束：顶部对齐到顶部区域的底部，左右及底部对齐到父视图边缘，保证顶部设计始终显示，子控制器仅占据下方白色区域。

二、确保初始加载时顶部设计与子控制器正常显示 #

你的MainSegPageViewController代码中，viewDidLoad已默认加载ViewController1，这部分逻辑没问题。如果顶部设计被子控制器视图覆盖，可添加以下代码调整安全区域：

override func viewDidLoad() {
    super.viewDidLoad()
    dataSource = self
    modalPresentationStyle = .formSheet

    // 假设顶部设计高度为60pt，根据实际值修改
    let topBarHeight: CGFloat = 60
    additionalSafeAreaInsets = UIEdgeInsets(top: topBarHeight, left: 0, bottom: 0, right: 0)

    if let firstViewController = subViewcontrollers.first {
        setViewControllers([firstViewController],
                           direction: .forward,
                           animated: true,
                           completion: nil)
    }

    setUpNavigationBar(with: "Pagination vc", isBackNeed: true)
    navigationController?.navigationBar.isHidden = false
}

三、实现点击“STEP 2”切换到ViewController1 #

场景1：按钮在MainSegPageViewController的顶部设计中 #

在MainSegPageViewController类中添加按钮点击事件：

@IBAction func step2ButtonTapped(_ sender: UIButton) {
    // 切换到数组第一个元素（ViewController1）
    setViewControllers([subViewcontrollers[0]], direction: .forward, animated: true, completion: nil)
}

完成后在Storyboard中把“STEP 2”按钮与该IBAction关联。

场景2：按钮在侧边栏或外部控制器中 #

修改侧边栏跳转代码，实例化后直接指定显示ViewController1：

let vc = UIStoryboard.init(name: "Main", bundle: Bundle.main).instantiateViewController(withIdentifier: "MainSegPageViewController") as! MainSegPageViewController
// 跳转时直接设置显示ViewController1，关闭子控制器切换动画避免冲突
vc.setViewControllers([vc.subViewcontrollers[0]], direction: .forward, animated: false, completion: nil)
navigationController?.pushViewController(vc, animated: true)

额外优化：简化UIPageViewControllerDataSource代码 #

原代码中存在冗余判断，可简化为：

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

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

    func presentationCount(for pageViewController: UIPageViewController) -> Int {
        return subViewcontrollers.count
    }

    func presentationIndex(for pageViewController: UIPageViewController) -> Int {
        guard let firstViewController = viewControllers?.first, let index = subViewcontrollers.firstIndex(of: firstViewController) else {
            return 0
        }
        return index
    }
}

内容的提问来源于stack exchange，提问作者Swift