Question

目前我的收藏视图包含4个单元格。这是一个水平集合视图，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，每个屏幕模式的这一个单元格也打开。每当我说我的collectionView显示可见的单元格时，它会显示不同的数组。（从纵向到水平2个单元格，从水平到纵向甚至是3个单元格的数组）。如果这个visibleCells数组每次都不同，为什么我们需要呢？为什么它每次都不同（我的意思是从portr到hor，反之亦然）多数民众赞成我的代码

   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

将问题视为询问为什么UICollectionView具有visibleCells功能......

听起来你不需要它。它在每个应用程序中都没用。但有时它是。这是一个例子：

假设您的单元格显示的数据代表了一段时间内的事件 - 也许是时间轴中的Twitter帖子。并假设您的UI包含一个字段，显示每个事件的年龄 - “20s ago”，“3m ago”等。为了保持这些时间准确，您可以设置计时器以定期更新值。计时器启动时会发生什么？

您可以告诉表视图重新加载其数据。这将是有效的，但它的工作量远远超过必要的。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。但是哪个细胞？只有那些可见的。因此，请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。

当设备旋转时，询问哪些单元格可见是没有用的，因为正如您所观察到的那样，可见单元格会发生变化。不过，在我没想过的那种情况下，有人可能会使用它。

为什么我们需要visibleCells函数（UICollectionView）

1 个答案: