为什么在 Swift 中 CollectionView 函数会被调用两次？

Question

I am having trouble with the function being called 2 times. Because of this, the value in PageView is not set properly because the index value is not correct. How can you control this correctly?

ViewController.swift

class ViewController: UIViewController {

    @IBOutlet weak var collectionView: UICollectionView!
    @IBOutlet weak var pageView: UIPageControl!
    @IBOutlet var guideView: UIView!
    var step : Int! = 0
    var beforeStep : Int! = 0

    var imageArray = [UIImage(named: "swipe1"),
                      UIImage(named: "swipe2"),
                      UIImage(named: "swipe3"),
                      UIImage(named: "swipe4"),
                      UIImage(named: "swipe5")]


    override func viewDidLoad() {
        super.viewDidLoad()
        pageView.numberOfPages = imageArray.count
        pageView.currentPage = 0
        pageView.currentPageIndicatorTintColor = UIColor.color(.dacColor)
        pageView.pageIndicatorTintColor = UIColor.gray
        view.backgroundColor = UIColor(red: CGFloat(252.0/255.0), green: CGFloat(251.0/255.0), blue: CGFloat(245.0/255.0), alpha: CGFloat(1.0))
        collectionView.backgroundColor = UIColor(red: CGFloat(252.0/255.0), green: CGFloat(251.0/255.0), blue: CGFloat(245.0/255.0), alpha: CGFloat(1.0))
    }
}

extension ViewController: UICollectionViewDelegate, UICollectionViewDataSource {
    func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
        return imageArray.count
    }

    func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "cell", for: indexPath)
        if let vc = cell.viewWithTag(111) as? UIImageView {
            vc.image = imageArray[indexPath.row]
        }
        if let ab = guideView.viewWithTag(222) as? UIPageControl {
            ab.currentPage = indexPath.row - 1
        }
        return cell
    }
}

extension ViewController: UICollectionViewDelegateFlowLayout {
    func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize {
        let size = collectionView.frame.size
        return CGSize(width: size.width, height: size.height)
    }

    func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, minimumLineSpacingForSectionAt section: Int) -> CGFloat {
        return 0
    }

    func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, minimumInteritemSpacingForSectionAt section: Int) -> CGFloat {
        return 0
    }
}

problematic function part

func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "cell", for: indexPath)
        if let vc = cell.viewWithTag(111) as? UIImageView {
            vc.image = imageArray[indexPath.row]
        }
        if let ab = guideView.viewWithTag(222) as? UIPageControl {
            ab.currentPage = indexPath.row - 1
        }
        return cell
    }

CollectionView in StoryBoard

View List in Storyboard

I want the PageController to be marked to match the location of the picture.

Has anyone solved the same problem as me?

Answer 1

collectionView(_:cellForItemAt:) is a collection view delegate method which tends to get called more than once, and there are a lot of factors that trigger the call like:

1. The number of visible cells you've got, depending on the height of the collection

2. When scrolling through cells to reuse them

Or triggers to change/refresh the collection like:

3. reloadData() , reloadItems(at:) or reloadSections(_:)

4. Perhaps a layoutIfNeeded() call from the view that contains this collection view.

and more...

The point is, the function getting called more than once is pretty normal. Your answer probably lies in "What triggers the call?"

EDIT :

To update your pageControl according to the index of the collection:

func scrollViewDidEndDecelerating(_ scrollView: UIScrollView) {
   let visibleRect = CGRect(origin: yourCollectionView.contentOffset, size: yourCollectionView.bounds.size)
   let midPointOfVisibleRect = CGPoint(x: visibleRect.midX, y: visibleRect.midY)
   if let visibleIndexPath = yourCollectionView.indexPathForItem(at: midPointOfVisibleRect) {
            yourPageControl.currentPage = visibleIndexPath.row
   }
}

If you're using a UIPageViewController just for the page indication, a UIPageControl should do. You might also have to set yourCollectionView.isPagingEnabled = true as that would be more appropriate for a page control.

Answer 2

In collection view go to layout and select custom:

class CustomCollectionViewFlowLayout: UICollectionViewFlowLayout
{
override func targetContentOffset(forProposedContentOffset proposedContentOffset: CGPoint, withScrollingVelocity velocity: CGPoint) -> CGPoint
{
    if let collectionViewBounds = self.collectionView?.bounds
    {
        let halfWidthOfVC = collectionViewBounds.size.width * 0.5
        let proposedContentOffsetCenterX = proposedContentOffset.x + halfWidthOfVC
        if let attributesForVisibleCells = self.layoutAttributesForElements(in: collectionViewBounds)
        {
            var candidateAttribute : UICollectionViewLayoutAttributes?
            for attributes in attributesForVisibleCells
            {
                let candAttr : UICollectionViewLayoutAttributes? = candidateAttribute
                if candAttr != nil
                {
                    let a = attributes.center.x - proposedContentOffsetCenterX
                    let b = candAttr!.center.x - proposedContentOffsetCenterX
                    if fabs(a) < fabs(b)
                    {
                        candidateAttribute = attributes
                    }
                }
                else
                {
                    candidateAttribute = attributes
                    continue
                }
            }

            if candidateAttribute != nil
            {
                return CGPoint(x: candidateAttribute!.center.x - halfWidthOfVC, y: proposedContentOffset.y);
            }
        }
    }
    return CGPoint.zero
}
}

and then in delegate:

 func scrollViewDidEndDecelerating(_ scrollView: UIScrollView) {
    var visibleRect = CGRect()

    visibleRect.origin = Collections.contentOffset
    visibleRect.size = Collections.bounds.size

    let visiblePoint = CGPoint(x: visibleRect.midX, y: visibleRect.midY)

    guard let indexPath = Collections.indexPathForItem(at: visiblePoint) else { return }


    print(indexPath.item)

}