代码

Question

我想知道哪个页面对用户可见，并且在检测到页面发生更改时也会执行某项操作。为此，我使用函数scrollViewDidScroll。由于我没有得到我想要的结果（没有发生），我在stackoverflow上寻找答案并找到了以下问题：Detecting UIScrollView page change，更确切地说，我使用迈克尔瀑布的答案并转换了Objective-C代码快速编写代码。

下面你会找到我在class HomeController: UIViewController, UIScrollViewDelegate {中创建scrollView的代码。此外，Michael Waterfall的函数在函数initiateScrollView()中声明并调用。但是，该功能不会打印任何内容。你知道我在做错什么吗？我看不出我犯了什么错误。滚动时没有任何反应（例如从第1页到第2页（homeView到view2））。该函数只打印before: 0一次，之后没有任何反应。

代码

override func viewDidLoad() {

    super.viewDidLoad()
    view.backgroundColor = .white

    initiateScrollView()

}

func initiateScrollView() {

    //create scrollView with paging enabled
    let scrollView = UIScrollView(frame: view.bounds)
    scrollView.isPagingEnabled = true
    view.addSubview(scrollView)

    //get page size
    let pageSize = view.bounds.size

    //individual views
    let homeView = UIView()
    homeView.backgroundColor = .green

    let view2 = UIView()
    view2.backgroundColor = .blue

    let view3 = UIView()
    view3.backgroundColor = .red

    //array with individual views
    let pagesViews = [homeView, view2, view3]

    //amount of views
    let numberOfPages = pagesViews.count
    print(numberOfPages) //prints '3'

    //add subviews (pages)
    for (pageIndex, page) in pagesViews.enumerated(){

        //add individual pages to scrollView
        page.frame = CGRect(origin: CGPoint(x:0 , y: CGFloat(pageIndex) * pageSize.height), size: pageSize)
        scrollView.addSubview(page)

    }

    func scrollViewDidScroll(scrollView: UIScrollView) {

        var previousPage = 0
        let fractionalPage: Float = Float(scrollView.contentOffset.y / pageSize.height)
        let page = lround(Double(fractionalPage))

        print("before:", page) //prints 'before: 0' once

        if previousPage != page {

            // Page has changed, do your thing!
            // ...
            // Finally, update previous page
            print("before:", page)
            previousPage = page
            print("after:", page)

        } //never prints anything

    }

    scrollViewDidScroll(scrollView: scrollView)

    //define size of scrollView
    scrollView.contentSize = CGSize(width: pageSize.width, height: pageSize.height * CGFloat(numberOfPages))
}

Answer 1

您忘记将viewController设置为滚动视图的委托，如下所示：

func initiateScrollView() {

//create scrollView with paging enabled
let scrollView = UIScrollView(frame: view.bounds)
scrollView.isPagingEnabled = true
scrollView.delegate = self
view.addSubview(scrollView)
(...) }

我建议您使用scrollViewDidEndScrollingAnimation(_ scrollView: UIScrollView)功能，而不是scrollViewDidScroll(...)

Answer 2

这是因为您尚未设置scrollview的delegate属性。在launchScrollView方法中添加scrollView.delegate = self。

在启用分页的情况下检测scrollView中y偏移的变化

代码

2 个答案: