SwiftUI：UIViewControllerRepresentable UIPageViewController 的性能问题

如何解决SwiftUI：UIViewControllerRepresentable UIPageViewController 的性能问题

由于使用 TabView 和 .tabViewStyle(PageTabViewStyle()) 的纯 SwiftUI PageViewControllers 当前在页面之间滑动时出现严重滞后，因此我尝试使用 UIViewControllerRepresentable 和 UIPageViewController 实现一个 PageViewController，如下所示由 Apple 在 SwiftUI 教程“与 UIKit 交互”(https://developer.apple.com/tutorials/swiftui/interfacing-with-uikit) 中提供。

不幸的是，当用户第一次滑动页面（或第一次点击 TextField）时，PageViewController 实现调用了所有页面的 body，这与 PageViewController 的基本思想相矛盾，后者是一次只加载几页。当处理大量页面和/或复杂页面时，应用程序最终无法使用，因为第一次滑动显然没有反应，或者 - 当尝试使用 TextField 输入文本时 - 可能需要很长时间才能输入字母键盘显示在 TextField 中。

有没有办法避免这种（错误）行为（在 Xcode 12.4 和 iOS 14.4 上测试过）？

为了演示我创建了以下小项目：

内容视图：

import SwiftUI

struct ContentView: View {
    let textArray = ["First","Second","Third","Fourth","Fifth","Sixth","Seventh","Eighth","Ninth","Tenth"]

    var body: some View {
        PageView(pages: textArray.map { PageContentView(text: $0) })
    }
}

PageContentView（带有展示行为的打印语句）：

import SwiftUI

struct PageContentView: View {
    let text: String
    @State private var keyboardInput = ""
    
    var body: some View {
        print ("PageContentView body for: \(text)")
        return VStack {
            Text(text)
            TextField("Enter Text",text: $keyboardInput)
                .textFieldStyle(RoundedBorderTextFieldStyle())
                .padding()
        }
    }
} 

其余视图均取自教程（删除 PageControl 部分以简化示例）：

页面浏览：

import SwiftUI

struct PageView<Page: View>: View {
    var pages: [Page]
    @State private var currentPage = 0
    
    var body: some View {
        PageViewController(pages: pages,currentPage: $currentPage)
    }
}

页面视图控制器：

import SwiftUI
import UIKit

struct PageViewController<Page: View>: UIViewControllerRepresentable {
    var pages: [Page]
    @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) {
        pageViewController.setViewControllers(
            [context.coordinator.controllers[currentPage]],direction: .forward,animated: true)
    }

    class Coordinator: NSObject,UIPageViewControllerDataSource,UIPageViewControllerDelegate {
        var parent: PageViewController
        var controllers = [UIViewController]()

        init(_ pageViewController: PageViewController) {
            parent = pageViewController
            controllers = parent.pages.map { UIHostingController(rootView: $0) }
        }

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

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

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