为啥我们需要 visibleCells 函数（UICollectionView）

Posted 2023-03-16

【中文标题】为啥我们需要 visibleCells 函数（UICollectionView）

【英文标题】：Why do we need visibleCells function (UICollectionView)为什么我们需要 visibleCells 函数（UICollectionView）

【发布时间】：2018-03-19 15:37:37

【问题描述】：

目前我有由 4 个单元格组成的集合视图。这是一个水平collectionView，每个显示一个单元格，所以每次有一个单元格应该显示自己。当我旋转我的设备时，每个屏幕模式下这个单元格也处于打开状态。每次我对我的 collectionView 说显示可见单元格时，它都会显示不同的数组。 （从纵向到横向 2 个阵列单元格，从横向到纵向阵列甚至 3 个阵列单元格）。如果这个 visibleCells 数组每次都不同，为什么我们需要它？以及为什么每次都不一样（我的意思是从波特到霍尔，反之亦然） 这是我的代码

   override func viewDidLoad() 
    super.viewDidLoad()

    view.addSubview(collectionView)

    view.addSubview(pageControl)

    view.addSubview(skipButton)
    view.addSubview(nextButton)
   print( view.constraints)

    nextButton.anchorWithConstantsToTop(nil, left: nil, bottom: nil, right:
        view.rightAnchor, topConstant: 16, leftConstant: 0, bottomConstant: 0, rightConstant: 0)
    nextButtonTopAnchor = nextButton.topAnchor.constraint(equalTo: view.topAnchor, constant: 16)
    nextButtonTopAnchor?.isActive = true
    nextButton.heightAnchor.constraint(equalToConstant: 50).isActive = true
    nextButton.widthAnchor.constraint(equalToConstant: 60).isActive = true
    skipButton.anchorWithConstantsToTop(nil, left: view.leftAnchor, bottom: nil, right: nil, topConstant: 16, leftConstant: 0, bottomConstant: 0, rightConstant: 0)
    skipButtonTopAnchor = skipButton.topAnchor.constraint(equalTo: view.topAnchor, constant: 16)
    skipButtonTopAnchor?.isActive = true
    skipButton.heightAnchor.constraint(equalToConstant: 50).isActive = true
    skipButton.widthAnchor.constraint(equalToConstant: 60).isActive = true

     pageControl.anchorWithConstantsToTop(nil, left: view.leftAnchor, bottom: nil, right: view.rightAnchor, topConstant: 0, leftConstant: 28, bottomConstant: 0, rightConstant: 28)

    print("\(pageControlConstant) L")
    pageControl.heightAnchor.constraint(equalToConstant: 40).isActive = true
    if UIDevice.current.orientation.isPortrait || (UIDevice.current.orientation.isFlat && UIInterfaceOrientationIsPortrait(self.interfaceOrientation) )   pageControlBottomAnchor = pageControl.bottomAnchor.constraint(equalTo: bottomLayoutGuide.topAnchor, constant: 0)
     else if  UIInterfaceOrientationIsPortrait(self.interfaceOrientation) == false  
  pageControlBottomAnchor =     pageControl.bottomAnchor.constraint(equalTo: bottomLayoutGuide.topAnchor, constant: 5)
    
    pageControlBottomAnchor?.isActive = true

    collectionView.anchorToTop(view.topAnchor, left: view.leftAnchor, bottom: view.bottomAnchor, right: view.rightAnchor)
    registerCells()



func scrollViewDidScroll(_ scrollView: UIScrollView) 
    view.endEditing(true)
  
override func willTransition(to newCollection: UITraitCollection, with coordinator: UIViewControllerTransitionCoordinator)


    if pageControl.currentPage != pages.count 

        if UIDevice.current.orientation.isLandscape || (UIDevice.current.orientation.isFlat && UIInterfaceOrientationIsLandscape(self.interfaceOrientation)) 
        self.nextButtonTopAnchor?.constant = 0
        self.skipButtonTopAnchor?.constant = 0
          self.pageControlBottomAnchor?.constant = 5
            self.tabBarController?.tabBar.isHidden = true
         else 
            self.nextButtonTopAnchor?.constant = 16
            self.skipButtonTopAnchor?.constant = 16
             self.pageControlBottomAnchor?.constant = 0
                self.tabBarController?.tabBar.isHidden = false
        
    

    if pageControl.currentPage == pages.count 


        var mySpecialCell = collectionView.cellForItem(at:IndexPath(item: pages.count, section: 0)) as! LoginCell


        if  (UIDevice.current.orientation.isLandscape || (UIDevice.current.orientation.isFlat && UIInterfaceOrientationIsLandscape(UIInterfaceOrientation.landscapeLeft)) ) 
 
            mySpecialCell.someShit = "landSpace"
             else 
            mySpecialCell.someShit = "portrait"

        

        collectionView.reloadData()
    
    collectionView.collectionViewLayout.invalidateLayout()
     collectionView.reloadData()
    DispatchQueue.main.async 
        let indexPath = IndexPath(item: self.pageControl.currentPage, section: 0)
        self.collectionView.scrollToItem(at: indexPath, at: .centeredHorizontally, animated: true)

        if (UIDevice.current.orientation.isLandscape || (UIDevice.current.orientation.isFlat && UIInterfaceOrientationIsLandscape(UIInterfaceOrientation.landscapeLeft)) ) 


         else 

        
         self.collectionView.reloadData()
    

 



func scrollViewWillEndDragging(_ scrollView: UIScrollView, withVelocity velocity: CGPoint, targetContentOffset: UnsafeMutablePointer<CGPoint>) 
    let pageNumber = Int(targetContentOffset.pointee.x / view.frame.width)
  print(  view.constraints)
    pageControl.currentPage = Int(pageNumber)
    if pageNumber == pages.count 



        nextButtonTopAnchor?.constant = -40
        skipButtonTopAnchor?.constant = -40
        pageControlBottomAnchor?.constant = 50


        print(pageControlBottomAnchor)
        UIView.animate(withDuration: 0.5, delay: 0, usingSpringWithDamping: 1, initialSpringVelocity: 1, options: .curveEaseOut, animations: 

            self.view.layoutIfNeeded()
        , completion: nil)

     else 

        if UIDevice.current.orientation.isLandscape || (UIDevice.current.orientation.isFlat && UIInterfaceOrientationIsLandscape(self.interfaceOrientation) )  
        pageControlBottomAnchor?.constant = 5
            self.nextButtonTopAnchor?.constant = 0
            self.skipButtonTopAnchor?.constant = 0
         else if (UIDevice.current.orientation.isFlat && UIInterfaceOrientationIsPortrait(self.interfaceOrientation) || !UIDevice.current.orientation.isLandscape ) 
            self.nextButtonTopAnchor?.constant = 16
            self.skipButtonTopAnchor?.constant = 16
            pageControlBottomAnchor?.constant = 0
        



    UIView.animate(withDuration: 0.5, delay: 0, usingSpringWithDamping: 1, initialSpringVelocity: 1, options: .curveEaseOut, animations: 

            self.view.layoutIfNeeded()
        , completion: nil)

    


fileprivate func moveControlConstraintsOffScreen() 

    pageControlBottomAnchor?.constant = 150
    pageControlBottomAnchor?.isActive = true

fileprivate func registerCells() 
      collectionView.register(PageCell.self, forCellWithReuseIdentifier: cellId)
    collectionView.register(LoginCell.self, forCellWithReuseIdentifier: loginCellId)
    collectionView.register(LoginCellLandscape.self, forCellWithReuseIdentifier: loginCellLandscapeId)


func collectionView(_ collectionView: UICollectionView, willDisplay cell: UICollectionViewCell, forItemAt indexPath: IndexPath) 
    print("I will display \(cell) for \(indexPath)")

func collectionView(_ collectionView: UICollectionView, didEndDisplaying cell: UICollectionViewCell, forItemAt indexPath: IndexPath) 
    print("I did end displaying \(cell) for \(indexPath)")


func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int 
    return pages.count + 1


func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell 

           let loginCell = collectionView.dequeueReusableCell(withReuseIdentifier: loginCellId, for: indexPath)
        return loginCell

 



    let cell = collectionView.dequeueReusableCell(withReuseIdentifier: cellId, for: indexPath) as! PageCell

    let page = pages[indexPath.item]
    cell.page = page

    return cell


func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize 
    return CGSize(width: view.frame.width, height: view.frame.height)

 

【参考方案1】：

将问题解读为问为什么UICollectionView 有visibleCells 函数...

听起来你不需要它。它不是在每个应用程序中都有用。但有时确实如此。这是一个例子：

假设您的单元格显示表示随时间变化的事件的数据——也许是时间轴中的 Twitter 帖子。并且假设您的 UI 包含一个显示每个事件发生时间的字段——“20 秒前”、“3 米前”等。为了保持这些时间准确，您可以设置一个计时器以定期更新值。定时器触发时应该发生什么？

你可以告诉表视图重新加载它的数据。这将是有效的，但它比必要的工作要多得多。您不需要重新加载整个单元格，只需更新单元格中的一个字段。但是哪些细胞？只有那些可见的。因此，询问集合视图哪些单元格可见，并更新这些单元格中的时间字段。

询问设备旋转时哪些单元格可见可能没有用，因为当您观察到时，可见单元格会发生变化。不过，在我没想到的情况下，有人可能会用到它。