ios - 为什么我们需要 visibleCells 函数 (UICollectionView)

Question

目前我有包含 4 个单元格的 Collection View 。这是一个水平 collectionView，每个显示一个单元格，因此每次有一个单元格应该显示自己。当我旋转我的设备时，每个屏幕一个单元格模式也会打开。每次我对 collectionView 说要显示可见单元格时，它都会显示不同的数组。 (从纵向到阵列中的水平 2 个单元格，从水平到纵向甚至阵列中的 3 个单元格)。如果这个 visibleCells 数组每次都不同，为什么我们需要它？为什么每次都不同(我的意思是从 portr 到 hor，反之亦然)那是我的代码

   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)
}
 }

Answer 1

阅读这个问题是问为什么 UICollectionView 有一个 visibleCells 函数...

听起来你不需要它。它并非在每个应用程序中都有用。但有时是这样。这是一个示例:

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。并假设您的 UI 包含一个字段，显示每个事件的时间——“20 秒前”、“3 分钟前”等。为了保持这些时间的准确性，您可以设置一个计时器以定期更新值。计时器触发时应该发生什么？

你可以告诉 TableView 重新加载它的数据。这将是有效的，但比必要的工作要多得多。您无需重新加载整个单元格，只需更新单元格中的一个字段即可。但是哪些细胞呢？只有那些可见的。因此，询问 Collection View 哪些单元格可见，并更新这些单元格中的时间字段。

当设备旋转时询问哪些单元格可见可能没有用，因为当您观察时，可见单元格随即发生变化。不过，有人可能会在我没有想到的那种情况下使用它。