Swift:如何使 ScrollView 与 PageControl 一起工作? [英] Swift: how to make a ScrollView work with PageControl?

问题描述

严格遵循

相反，当我向后滑动图像时，它们会以正确的方式显示，每页一个.或者更好的是，当我在幻灯片中获得第 4 张图像时，这个问题就消失了，只有在这一点上，以下所有图像都是正确的，前一个也是正确的.这是一个影响前 2 或 3 张图像的问题.

此外幻灯片甚至不是从用户刚刚点击的 UICollectionViewCell 开始，而是从第一个开始.您可以在这里阅读所有代码:

导入基础导入 UIKit类 PagedScrollViewController: UIViewController, UIScrollViewDelegate {@IBOutlet var scrollView:UIScrollView！@IBOutlet var pageControl:UIPageControl！/* 这将保存所有要显示的图像——每页 1 个.它必须在 prepareforsegue() 方法中从前一个视图控制器中设置:它将是该照片库的下载图像数组 */var pageImages:[UIImage]！/* 数组图像中第一个显示的位置，即用户刚刚点击的位置 */var firstToShow:Int!var currentImageViewForZoom:UIImageView?/* 这将保存 UIImageView 的实例以在其各自的页面上显示每个图像.这是一个可选的数组，因为你会懒惰地加载页面(即当你需要它们时)所以你需要能够处理数组中的 nil 值.*/var pageViews:[UIImageView?] = []覆盖 func viewDidLoad() {super.viewDidLoad()self.scrollView.delegate = 自我self.scrollView.maximumZoomScale = 1.0self.scrollView.zoomScale = 10.0self.pageControl.numberOfPages = self.pageImages.countself.pageControl.currentPage = self.firstToShow对于 _ in 0..<self.pageImages.count {self.pageViews.append(nil)}/* 和以前一样，滚动视图需要知道它的内容大小.由于您需要水平分页滚动视图，因此您将宽度计算为页数乘以滚动视图的宽度.内容的高度与滚动视图的高度相同*/让 pagesScrollViewSize = self.scrollView.frame.sizeself.scrollView.contentSize = CGSize(宽度: pagesScrollViewSize.width * CGFloat(self.pageImages.count),高度:pagesScrollViewSize.height)//您将需要最初显示的一些页面，因此您调用 loadVisiblePages()self.loadVisiblePages()}func viewForZoomingInScrollView(scrollView: UIScrollView) ->界面视图?{返回 self.currentImageViewForZoom}func scrollViewDidScroll(scrollView: UIScrollView) {self.loadVisiblePages()}/*请记住，每个页面都是一个 UIImageView，存储在一个选项数组中.当视图控制器加载时，数组被 nil 填充.此方法将加载每个页面的内容.1 - 如果它超出了您必须显示的范围，则什么也不做2 - 如果 pageView 为 nil，那么您需要创建一个页面.所以首先，为这个页面制定框架.它被计算为与滚动视图相同的大小，位于零 y 偏移处，然后偏移一页的宽度乘以 x(水平)方向的页码.3 - 最后，将 pageViews 数组中的 nil 替换为刚刚创建的视图，因此，如果要求再次加载此页面，您现在不会进入 if 语句，而是什么都不做，因为已经创建了页面的视图*/func loadPage(page: Int) {如果页面 <0 ||页>= self.pageImages.count {//1返回}//2if let _ = self.pageViews[page] {/*什么都不做.视图已经加载*/}别的 {//2var frame = self.scrollView.boundsframe.origin.x = frame.size.width * CGFloat(page)frame.origin.y = 0.0让 newPageView = UIImageView(image: self.pageImages[page])newPageView.contentMode = .ScaleAspectFitnewPageView.frame = 框架self.scrollView.addSubview(newPageView)//3self.pageViews[page] = newPageViewself.currentImageViewForZoom = newPageView}}/*此函数清除先前通过 loadPage() 创建的页面.它首先检查该页面的 pageViews 数组中的对象是否不为零.如果不是，它会从滚动视图中删除该视图，并再次使用 nil 更新 pageViews 数组，以指示该页面不再存在.你问，为什么要打扰延迟加载和清除页面?好吧，在这个例子中，如果你在开始时加载所有页面并没有太大关系，因为只有五个页面并且它们不会大到足以占用太多内存.但想象一下，你有 100 页，每张图片大小为 5MB.如果一次加载所有页面，那将占用 500MB 内存！您的应用程序将很快超过可用内存量并被操作系统杀死.延迟加载意味着您​​在任何给定时间只会在内存中拥有一定数量的页面.*/func purgePage(page: Int) {如果页面 <0 ||页>= self.pageImages.count {//如果它超出了你必须显示的范围，那么什么都不做返回}//从滚动视图中移除一个页面并重置容器数组如果让 pageView = self.pageViews[page] {pageView.removeFromSuperview()self.pageViews[page] = nil}}func loadVisiblePages() {//首先，确定当前可见的页面让 pageWidth = self.scrollView.frame.size.width//floor() 函数将一个十进制数四舍五入到下一个最小整数让 page = Int(floor((self.scrollView.contentOffset.x * 2.0 + pageWidth)/(pageWidth * 2.0)))/***///更新页面控件self.pageControl.currentPage = 页//计算出要加载的页面让 firstPage = page - 1让 lastPage = page + 1//清除第一页之前的所有内容对于变量索引 = 0；指数<第一页;++索引{self.purgePage(索引)}//加载我们范围内的页面对于 firstPage 中的索引...lastPage {self.loadPage(索引)}//清除最后一页之后的所有内容对于 var index = lastPage+1;指数

我想问题可能是 /***/ 的行，这是我从教程中没有理解的内容.感谢您的关注

更新在这里查看类似的帖子，有人建议在 viewDidLayoutSubviews() 中创建子视图，所以这是我刚刚尝试过的:

覆盖 func viewDidLayoutSubviews() {self.loadVisiblePages()}

现在图像已正确显示，前 3 张图像不再有那种奇怪的效果.但是为什么?我是一名初级 iOS 开发人员，但我仍然不知道所有这些要覆盖的方法以及它们的工作顺序.无论如何，仍然存在的另一个问题是图像总是从第一张开始显示，即使点击另一张图像也是如此.例如，看看这个:

即使点击另一个图像，也始终显示第一张图像(左上角).最后，在我的代码中，我实现了具有缩放功能的委托方法，但它也不起作用.

更新 2

当用户点击一个单元格时，这是上一个 UICollectionViewController 中 prepareForSegue() 的代码:

 override func prepareForSegue(segue: UIStoryboardSegue, sender: AnyObject?) {if (segue.identifier == "toSlideShow") {让 pagedScrollViewController:PagedScrollViewController = segue.destinationViewController 为！PagedScrollViewControllerpagedScrollViewController.pageImages = self.imagesDownloadedpagedScrollViewController.firstToShow = self.collectionView?.indexPathsForSelectedItems()![0].row}}

解决方案

您应该将滚动视图的偏移量更新为与过渡后要显示的图像的偏移量相等.self.scrollView.contentOffset = CGPoint(x: pagesScrollViewSize.width * CGFloat(self.firstToShow), y: 0.0) 您可以在 viewDidLayoutSubviews 中执行此操作，使其看起来像:

覆盖 func viewDidLayoutSubviews() {super.viewDidLayoutSubviews()self.scrollView.delegate = 自我self.scrollView.maximumZoomScale = 2.0self.scrollView.zoomScale = 1.0self.scrollView.minimumZoomScale = 0.5self.pageControl.numberOfPages = self.pageImages.countself.pageControl.currentPage = self.firstToShow对于 _ in 0..<self.pageImages.count {self.pageViews.append(nil)}/* 和以前一样，滚动视图需要知道它的内容大小.由于您需要水平分页滚动视图，因此您将宽度计算为页数乘以滚动视图的宽度.内容的高度与滚动视图的高度相同*/让 pagesScrollViewSize = self.scrollView.frame.sizeself.scrollView.contentSize = CGSize(宽度: pagesScrollViewSize.width * CGFloat(self.pageImages.count),高度:pagesScrollViewSize.height)self.scrollView.contentOffset = CGPoint(x: pagesScrollViewSize.width * CGFloat(self.firstToShow), y: 0.0)//您将需要最初显示的一些页面，因此您调用 loadVisiblePages()self.loadVisiblePages()}

您突出显示的行只是向下舍入到最接近的图像索引以确定滚动视图所在的页面.问题是当你的视图控制器显示时没有滚动，所以它总是显示第一张图像.要首先获取要显示的图像，您只需计算图像的偏移量，将滚动视图的偏移量设置为如上所示.

Strctly following this tutorial, in section Paging with UIScrollView, I have just implemented a ScrollView to use as a slideshow with downloaded photos from a previous UICollectionViewController. When scroll view is loaded, it does not work well because I see these ones:

Instead, when I slide back the images they are displayed in the correct way, one for each page. Or better, this problem disappears when I get the 4-th image in the slideshow, and only at that point all the following ones are correct, and so are the previous too. It is a problem which affects the first 2 or 3 images.

Moreover the slideshow does not even start from the UICollectionViewCell the user has just tapped, but always from the first. You can read all the code here:

import Foundation
import UIKit

class PagedScrollViewController: UIViewController, UIScrollViewDelegate {

@IBOutlet var scrollView: UIScrollView!
@IBOutlet var pageControl: UIPageControl!

/* This will hold all the images to display – 1 per page. 
   It must be set from the previous view controller in prepareforsegue() method: 
   it will be the array of downloaded images for that photo gallery */
var pageImages:[UIImage]!

/* position in array images of the first to be showed, i.e. the one the user has just tapped */
var firstToShow:Int!

var currentImageViewForZoom:UIImageView?

/* This will hold instances of UIImageView to display each image on its respective page. 
   It’s an array of optionals, because you’ll be loading the pages lazily (i.e. as and when you need them) 
   so you need to be able to handle nil values from the array. */
var pageViews:[UIImageView?] = []

override func viewDidLoad() {
    super.viewDidLoad()

    self.scrollView.delegate = self
    self.scrollView.maximumZoomScale = 1.0
    self.scrollView.zoomScale = 10.0

    self.pageControl.numberOfPages = self.pageImages.count
    self.pageControl.currentPage = self.firstToShow

    for _ in 0..<self.pageImages.count {
        self.pageViews.append(nil)
    }

    /* The scroll view, as before, needs to know its content size. 
       Since you want a horizontal paging scroll view, you calculate the width to be the number of pages multiplied by the width of the scroll view. 
       The height of the content is the same as the height of the scroll view
    */
    let pagesScrollViewSize = self.scrollView.frame.size
    self.scrollView.contentSize = CGSize(width: pagesScrollViewSize.width * CGFloat(self.pageImages.count),
        height: pagesScrollViewSize.height)

    // You’re going to need some pages shown initially, so you call loadVisiblePages()
    self.loadVisiblePages()

}

func viewForZoomingInScrollView(scrollView: UIScrollView) -> UIView? {
    return self.currentImageViewForZoom
}

func scrollViewDidScroll(scrollView: UIScrollView) {
    self.loadVisiblePages()
}

/*
Remember each page is a UIImageView stored in an array of optionals. 
When the view controller loads, the array is filled with nil. 
This method will load the content of each page.

1 - If it's outside the range of what you have to display, then do nothing

2 - If pageView is nil, then you need to create a page. So first, work out the frame for this page. 
    It’s calculated as being the same size as the scroll view, positioned at zero y offset, 
    and then offset by the width of a page multiplied by the page number in the x (horizontal) direction.

3 - Finally, you replace the nil in the pageViews array with the view you’ve just created, 
    so that if this page was asked to load again, you would now not go into the if statement and instead do nothing, 
    since the view for the page has already been created

*/
func loadPage(page: Int) {
    if page < 0 || page >= self.pageImages.count {
        //1
        return
    }

    //2
    if let _ = self.pageViews[page] {/*Do nothing. The view is already loaded*/}
    else {
        // 2
        var frame = self.scrollView.bounds
        frame.origin.x = frame.size.width * CGFloat(page)
        frame.origin.y = 0.0

        let newPageView = UIImageView(image: self.pageImages[page])
        newPageView.contentMode = .ScaleAspectFit
        newPageView.frame = frame
        self.scrollView.addSubview(newPageView)

        // 3
        self.pageViews[page] = newPageView
        self.currentImageViewForZoom = newPageView
    }
}

/*
This function purges a page that was previously created via loadPage(). 
It first checks that the object in the pageViews array for this page is not nil. 
If it’s not, it removes the view from the scroll view and updates the pageViews array with nil again to indicate that this page is no longer there.
Why bother lazy loading and purging pages, you ask? 
Well, in this example, it won’t matter too much if you load all the pages at the start, since there are only five and they won’t be large enough to eat up too much memory. 
But imagine you had 100 pages and each image was 5MB in size. That would take up 500MB of memory if you loaded all the pages at once! 
Your app would quickly exceed the amount of memory available and be killed by the operating system. 
Lazy loading means that you’ll only have a certain number of pages in memory at any given time.
*/
func purgePage(page: Int) {
    if page < 0 || page >= self.pageImages.count {
        // If it's outside the range of what you have to display, then do nothing
        return
    }

    // Remove a page from the scroll view and reset the container array
    if let pageView = self.pageViews[page] {
        pageView.removeFromSuperview()
        self.pageViews[page] = nil
    }
}

func loadVisiblePages() {
    // First, determine which page is currently visible
    let pageWidth = self.scrollView.frame.size.width

    // floor() function will round a decimal number to the next lowest integer
    let page = Int(floor((self.scrollView.contentOffset.x * 2.0 + pageWidth) / (pageWidth * 2.0))) /***/

    // Update the page control
    self.pageControl.currentPage = page

    // Work out which pages you want to load
    let firstPage = page - 1
    let lastPage = page + 1

    // Purge anything before the first page
    for var index = 0; index < firstPage; ++index {
        self.purgePage(index)
    }

    // Load pages in our range
    for index in firstPage...lastPage {
        self.loadPage(index)
    }

    // Purge anything after the last page
    for var index = lastPage+1; index < self.pageImages.count; ++index {
        self.purgePage(index)
    }
}
}

I guess the problem could be the line with /***/ which is something I have not understood from the tutorial. Thank you for your attention

UPDATE Looking here in SO for similar posts, someone advised to create subviews in viewDidLayoutSubviews(), so here's what I have just tried:

override func viewDidLayoutSubviews() {
    self.loadVisiblePages()
}

and now the images are correctly displayed and there's no more that strange effect for the first 3 images. But why? I am a junior iOS developer and I still don't know all those methods to override and the order in which they work. Anyway, the other problem which persists is that images are showed always from the first, even if another image is tapped. For example, have a look at this:

always the first image (left up corner) is displayed even if another one is tapped. And finally, in my code I implemented the delegate method to have zoom but it does not work too.

UPDATE 2

Here's the code of prepareForSegue() from the previous UICollectionViewController when the user taps a cell:

 override func prepareForSegue(segue: UIStoryboardSegue, sender: AnyObject?) {
    if (segue.identifier == "toSlideShow") {
        let pagedScrollViewController:PagedScrollViewController = segue.destinationViewController as! PagedScrollViewController
        pagedScrollViewController.pageImages = self.imagesDownloaded
        pagedScrollViewController.firstToShow = self.collectionView?.indexPathsForSelectedItems()![0].row
    }
}

解决方案

You should update the scroll view's offset to the be equal to the offset of the image you want to be showing after you transition. self.scrollView.contentOffset = CGPoint(x: pagesScrollViewSize.width * CGFloat(self.firstToShow), y: 0.0) You can do this in viewDidLayoutSubviews, which would make it look like:

override func viewDidLayoutSubviews() {
        super.viewDidLayoutSubviews()
        self.scrollView.delegate = self
        self.scrollView.maximumZoomScale = 2.0
        self.scrollView.zoomScale = 1.0
        self.scrollView.minimumZoomScale = 0.5

        self.pageControl.numberOfPages = self.pageImages.count
        self.pageControl.currentPage = self.firstToShow

        for _ in 0..<self.pageImages.count {
            self.pageViews.append(nil)
        }

        /* The scroll view, as before, needs to know its content size.
        Since you want a horizontal paging scroll view, you calculate the width to be the number of pages multiplied by the width of the scroll view.
        The height of the content is the same as the height of the scroll view
        */
        let pagesScrollViewSize = self.scrollView.frame.size
        self.scrollView.contentSize = CGSize(width: pagesScrollViewSize.width * CGFloat(self.pageImages.count),
            height: pagesScrollViewSize.height)

        self.scrollView.contentOffset = CGPoint(x: pagesScrollViewSize.width * CGFloat(self.firstToShow), y: 0.0)

        // You’re going to need some pages shown initially, so you call loadVisiblePages()
        self.loadVisiblePages()

    }

The line you highlighted is just rounding down to the closest image index to determine what page the scroll view is on. The problem is when your view controller is displayed no scrolling has been done so it will always show the first image. To get the image you want to show first, you can just calculate what the offset should be for the image set your scroll view's offset to that as shown above.

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