如何在UIScrollView中分页时一次只加载一个ViewController

import UIKit

class ViewController: UIViewController, UIScrollViewDelegate {
    
    @IBOutlet weak var scrollView: UIScrollView!
    @IBOutlet weak var pageControl: UIPageControl!
    
    var viewControllers: [UIViewController] = []
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        scrollView.delegate = self
        scrollView.isPagingEnabled = true
        
        // 添加需要加载的ViewController到数组中
        let firstViewController = FirstViewController()
        let secondViewController = SecondViewController()
        let thirdViewController = ThirdViewController()
        
        viewControllers = [firstViewController, secondViewController, thirdViewController]
        
        // 设置UIScrollView的contentSize
        scrollView.contentSize = CGSize(width: scrollView.frame.width * CGFloat(viewControllers.count), height: scrollView.frame.height)
        
        // 添加第一个ViewController到UIScrollView中
        addChildViewController(firstViewController)
        firstViewController.view.frame = CGRect(x: 0, y: 0, width: scrollView.frame.width, height: scrollView.frame.height)
        scrollView.addSubview(firstViewController.view)
        firstViewController.didMove(toParentViewController: self)
        
        // 设置UIPageControl的总页数
        pageControl.numberOfPages = viewControllers.count
    }
    
    func scrollViewDidScroll(_ scrollView: UIScrollView) {
        // 计算当前显示的页面索引
        let pageIndex = Int(scrollView.contentOffset.x / scrollView.frame.width)
        
        // 判断是否需要加载新的ViewController
        if pageIndex >= 0 && pageIndex < viewControllers.count {
            let currentViewController = viewControllers[pageIndex]
            
            if currentViewController.parent == nil {
                // 添加新的ViewController到UIScrollView中
                addChildViewController(currentViewController)
                currentViewController.view.frame = CGRect(x: scrollView.frame.width * CGFloat(pageIndex), y: 0, width: scrollView.frame.width, height: scrollView.frame.height)
                scrollView.addSubview(currentViewController.view)
                currentViewController.didMove(toParentViewController: self)
            }
        }
        
        // 更新UIPageControl的当前页码
        pageControl.currentPage = pageIndex
    }
    
    func scrollViewDidEndDecelerating(_ scrollView: UIScrollView) {
        // 根据当前页面索引，设置UIScrollView的contentOffset
        let pageIndex = Int(scrollView.contentOffset.x / scrollView.frame.width)
        scrollView.setContentOffset(CGPoint(x: scrollView.frame.width * CGFloat(pageIndex), y: 0), animated: true)
    }
}