ios - 如何优化 UIPageViewController 以具有类似于 tableView.dequeueReusableCell(withIdentifier :for:)?) 的重用机制

Question

到目前为止，我遇到的大多数 UIPageViewController 教程都会在滑动到所需页面时动态创建 UIViewController。

未优化的 UIPageViewController 示例

import UIKit

class ViewController: UIViewController {

    @IBOutlet weak var contentView: UIView!

    let dataSource = ["View Controller One", "View Controller Two", "View Controller Three", "View Controller Four"]

    var currentViewControllerIndex = 0

    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view.
        configurePageViewController()
    }

    func configurePageViewController() {

        guard let pageViewController = storyboard?.instantiateViewController(withIdentifier: String(describing: CustomPageViewController.self)) as? CustomPageViewController else {
            return
        }

        pageViewController.delegate = self
        pageViewController.dataSource = self

        addChild(pageViewController)
        pageViewController.didMove(toParent: self)

        pageViewController.view.translatesAutoresizingMaskIntoConstraints = false

        contentView.addSubview(pageViewController.view)

        let views: [String: Any] = ["pageView": pageViewController.view]

        contentView.addConstraints(NSLayoutConstraint.constraints(withVisualFormat: "H:|-0-[pageView]-0-|", options: NSLayoutConstraint.FormatOptions(rawValue: 0), metrics: nil, views: views))

        contentView.addConstraints(NSLayoutConstraint.constraints(withVisualFormat: "V:|-0-[pageView]-0-|", options: NSLayoutConstraint.FormatOptions(rawValue: 0), metrics: nil, views: views))

        guard let startingViewController = detailViewControllerAt(index: currentViewControllerIndex) else {
            return
        }

        pageViewController.setViewControllers([startingViewController], direction: .forward, animated: true)
    }

    func detailViewControllerAt(index: Int) -> DataViewController? {

        if index >= dataSource.count || dataSource.count == 0 {
            return nil
        }

        guard let detailViewController = storyboard?.instantiateViewController(withIdentifier: String(describing: DataViewController.self)) as? DataViewController else {
            return nil
        }

        detailViewController.index = index
        detailViewController.displayText = dataSource[index]

        return detailViewController
    }
}

extension ViewController: UIPageViewControllerDelegate, UIPageViewControllerDataSource {

    func presentationIndex(for pageViewController: UIPageViewController) -> Int {

        print ("presentationIndex \(currentViewControllerIndex)")

        return currentViewControllerIndex
    }

    func presentationCount(for pageViewController: UIPageViewController) -> Int {

        print ("presentationCount \(dataSource.count)")

        return dataSource.count
    }

    func pageViewController(_ pageViewController: UIPageViewController, viewControllerBefore viewController: UIViewController) -> UIViewController? {
        let dataViewController = viewController as? DataViewController

        guard var currentIndex = dataViewController?.index else {
            return nil
        }

        print ("viewControllerBefore \(currentIndex)")

        if (currentIndex == 0) {
            return nil
        }

        currentIndex -= 1

        currentViewControllerIndex = currentIndex

        return detailViewControllerAt(index: currentIndex)
    }

    func pageViewController(_ pageViewController: UIPageViewController, viewControllerAfter viewController: UIViewController) -> UIViewController? {
        let dataViewController = viewController as? DataViewController

        guard var currentIndex = dataViewController?.index else {
            return nil
        }

        print ("viewControllerBefore \(currentIndex)")

        if currentIndex == dataSource.count-1 {
            return nil
        }

        currentIndex += 1

        currentViewControllerIndex = currentIndex

        return detailViewControllerAt(index: currentIndex)
    }
}

但是，我几乎看不到任何关于如何优化 UIPageViewController 以处理大量页面的示例。

为每个页面重新创建新的 UIViewController 似乎不是最佳解决方案，因为它会很慢。它还占用了不必要的内存空间，因为大部分屏幕外的 UIViewController 是不可见的。

我们怎样才能拥有类似于 tableView.dequeueReusableCell(withIdentifier:for:) 的东西？

我们可以重新使用那些以前创建的、当前不可见的 UIViewController 吗？

非常感谢有关如何正确实现此功能的任何代码示例或教程资源。谢谢。

Answer 1

动态创建 Controller 可能会导致一些故障，所以我通常有一些像 var viewControllers: [UIViewController] 这样的数组来存储我的页面并实现 UIPageViewControllerDataSource

    func pageViewController(_ pageViewController: UIPageViewController, viewControllerBefore viewController: UIViewController) -> UIViewController? {
        guard let index = viewControllers.firstIndex(of: viewController), index > 0 else {
            return nil
        }

        currentViewControllerIndex = index - 1
        return viewControllers[currentViewControllerIndex]
    }

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

        currentViewControllerIndex = index + 1
        return viewControllers[currentViewControllerIndex]
    }

为了减少内存使用，您可以在页面更改时释放 View Controller 的 View :

    var currentViewControllerIndex = 0 {
        didSet {
            let range = currentViewControllerIndex - 1...currentViewControllerIndex + 1

            for (offset, element) in viewControllers.enumerated() {
                if range ~= offset {
                    element.loadViewIfNeeded()
                } else {
                    element.view = nil
                }
            }
        }
    }