UIPageViewController - 检测到滚动到下一个视图控制器（几乎可以工作）的一半以更改按钮颜色？

我有一个类，它是 UIPageViewController 的子类，其中包含 4 个控制器，我试图弄清楚当我滚动到第二个视图控制器的一半时如何更改按钮的颜色

仅第一个控制器上的按钮颜色应该不同

这里的代码几乎可以工作，但仅对于第三个控制器，行为不正确，这意味着解决方案不正确。

如果有人可以提供帮助，我将非常感激。谢谢

    public func scrollViewDidScroll(_ scrollView: UIScrollView) {
        let point = scrollView.contentOffset
        let width = scrollView.frame.width
        let percentComplete = abs(point.x - width) / width
        let page = Int(round(percentComplete))

        if percentComplete >= 0.5 {
            if page == 1, currentIndex == 1 {
                nextButton.backgroundColor = .red
            } else {
                nextButton.backgroundColor = .blue
            }
            print ("percentComplete: ", percentComplete,
                   "page: ", page,
                   "currentIndex: ",
                   currentIndex, "point: ", point.x)
        }
    }

即使你说你想使用UIPageViewController，这是一个使用示例实现UIScrollView您可能会发现更容易管理：

class PagedScrollViewController: UIViewController, UIScrollViewDelegate {
    
    let scrollView: UIScrollView = {
        let v = UIScrollView()
        v.isPagingEnabled = true
        v.bounces = false
        return v
    }()
    
    let pageControl: UIPageControl = {
        let v = UIPageControl()
        return v
    }()
    
    let stack: UIStackView = {
        let v = UIStackView()
        v.axis = .horizontal
        v.distribution = .fillEqually
        return v
    }()
    
    var pages: [UIViewController] = []
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        scrollView.translatesAutoresizingMaskIntoConstraints = false
        pageControl.translatesAutoresizingMaskIntoConstraints = false
        stack.translatesAutoresizingMaskIntoConstraints = false
        
        scrollView.addSubview(stack)
        view.addSubview(scrollView)
        view.addSubview(pageControl)
        
        let g = view.safeAreaLayoutGuide
        let svCLG = scrollView.contentLayoutGuide
        let svFLG = scrollView.frameLayoutGuide
        
        NSLayoutConstraint.activate([
            
            scrollView.topAnchor.constraint(equalTo: g.topAnchor, constant: 20.0),
            scrollView.leadingAnchor.constraint(equalTo: g.leadingAnchor, constant: 20.0),
            scrollView.trailingAnchor.constraint(equalTo: g.trailingAnchor, constant: -20.0),
            scrollView.bottomAnchor.constraint(equalTo: g.bottomAnchor, constant: -80.0),
            
            stack.topAnchor.constraint(equalTo: svCLG.topAnchor, constant: 0.0),
            stack.leadingAnchor.constraint(equalTo: svCLG.leadingAnchor, constant: 0.0),
            stack.trailingAnchor.constraint(equalTo: svCLG.trailingAnchor, constant: 0.0),
            stack.bottomAnchor.constraint(equalTo: svCLG.bottomAnchor, constant: 0.0),
            
            stack.heightAnchor.constraint(equalTo: svFLG.heightAnchor, constant: 0.0),
            
            pageControl.topAnchor.constraint(equalTo: scrollView.bottomAnchor, constant: 8.0),
            pageControl.leadingAnchor.constraint(equalTo: scrollView.leadingAnchor),
            pageControl.trailingAnchor.constraint(equalTo: scrollView.trailingAnchor),

        ])
        
        // if we're loading "page" view controllers from Storyboard
        /*
        if let vc = storyboard?.instantiateViewController(withIdentifier: "psFirst") as? PSFirstViewController {
            pages.append(vc)
        }
        if let vc = storyboard?.instantiateViewController(withIdentifier: "psSecond") as? PSSecondViewController {
            pages.append(vc)
        }
        if let vc = storyboard?.instantiateViewController(withIdentifier: "psThird") as? PSThirdViewController {
            pages.append(vc)
        }
        if let vc = storyboard?.instantiateViewController(withIdentifier: "psFourth") as? PSFourthViewController {
            pages.append(vc)
        }
        pages.forEach { vc in
            self.addChild(vc)
            stack.addArrangedSubview(vc.view)
            vc.view.widthAnchor.constraint(equalTo: scrollView.frameLayoutGuide.widthAnchor, constant: 0.0).isActive = true
            vc.didMove(toParent: self)
        }
        */

        // for this example,
        //  create 4 view controllers, with background colors
        let colors: [UIColor] = [
            .red, .brown, .blue, .magenta
        ]
        colors.forEach { c in
            let vc = BasePageController()
            vc.view.backgroundColor = c
            self.addChild(vc)
            stack.addArrangedSubview(vc.view)
            vc.view.widthAnchor.constraint(equalTo: scrollView.frameLayoutGuide.widthAnchor, constant: 0.0).isActive = true
            vc.didMove(toParent: self)
            pages.append(vc)
        }
        
        pageControl.numberOfPages = pages.count
        
        scrollView.delegate = self
        
        pageControl.addTarget(self, action: #selector(self.pgControlChange(_:)), for: .valueChanged)
    }
    
    var pgControlScroll: Bool = false
    
    @objc func pgControlChange(_ sender: UIPageControl) {
        pgControlScroll = true
        let w = scrollView.frame.size.width
        guard w != 0 else { return }
        let x = scrollView.contentOffset.x
        let cp = min(Int(round(x / w)), pages.count - 1)
        let np = sender.currentPage
        var r = CGRect.zero
        if np > cp {
            r = CGRect(x: w * CGFloat(np + 1) - 1.0, y: 0, width: 1, height: 1)
        } else {
            r = CGRect(x: w * CGFloat(np), y: 0, width: 1, height: 1)
        }
        scrollView.scrollRectToVisible(r, animated: true)
    }
    func scrollViewWillBeginDragging(_ scrollView: UIScrollView) {
        pgControlScroll = false
    }
    func scrollViewDidScroll(_ scrollView: UIScrollView) {
        let w = scrollView.frame.size.width
        guard w != 0 else { return }
        let x = scrollView.contentOffset.x
        let pg = min(Int(round(x / w)), pages.count - 1)
        let v = stack.arrangedSubviews[pg]
        pageControl.backgroundColor = v.backgroundColor
        if pgControlScroll { return }
        pageControl.currentPage = pg
    }

}

class BasePageController: UIViewController {
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        // add a label at each corner
        for (i, s) in ["top-left", "top-right", "bot-left", "bot-right"].enumerated() {
            let v = UILabel()
            v.backgroundColor = UIColor(white: 0.8, alpha: 1.0)
            v.translatesAutoresizingMaskIntoConstraints = false
            v.text = s
            view.addSubview(v)
            let g = view.safeAreaLayoutGuide
            switch i {
            case 1:
                v.topAnchor.constraint(equalTo: g.topAnchor, constant: 4.0).isActive = true
                v.trailingAnchor.constraint(equalTo: g.trailingAnchor, constant: -4.0).isActive = true
            case 2:
                v.bottomAnchor.constraint(equalTo: g.bottomAnchor, constant: -4.0).isActive = true
                v.leadingAnchor.constraint(equalTo: g.leadingAnchor, constant: 4.0).isActive = true
            case 3:
                v.bottomAnchor.constraint(equalTo: g.bottomAnchor, constant: -4.0).isActive = true
                v.trailingAnchor.constraint(equalTo: g.trailingAnchor, constant: -4.0).isActive = true
            default:
                v.topAnchor.constraint(equalTo: g.topAnchor, constant: 4.0).isActive = true
                v.leadingAnchor.constraint(equalTo: g.leadingAnchor, constant: 4.0).isActive = true
            }
        }
    }
    
}

该代码添加了一个占据大部分屏幕的滚动视图（有一些填充，因此滚动视图的框架很明显），并带有UIPageControl下。

我们添加 4 个视图控制器作为子视图控制器，并将它们的视图添加到UIStackView在滚动视图中。

当您从“一页到另一页”滚动时，页面控件将在您到达下一页/上一页时更新并更改背景颜色。