页面加载后滚动后如何访问UICollectionView中的特定单元格？

Question

我正在尝试建立一个像这样的时间表：

时间轴中的每个单元格代表一个月，因此May是水平collectionView中的最后一个单元格。我想要的是在collectionView加载时自动滚动到可能不可见的位置，然后在单元格上调用特定函数以绘制箭头。我用于尝试执行此操作的代码如下。

    override func viewDidLoad() {
    super.viewDidLoad()
    self.timeline.register(UINib(nibName: "TimelineMonthViewCell", bundle: nil), forCellWithReuseIdentifier: "TimelineMonthViewCell")
    self.collectionView.register(UINib(nibName: "PostCollectionViewCell", bundle: nil), forCellWithReuseIdentifier: "PostCollectionViewCell")
    pageControl.hidesForSinglePage = true
    self.navigationItem.largeTitleDisplayMode = .never

    UserService.posts(for: User.current,tagString: self.selectedTagTitles.first!) { (postStuff) in

        self.posts = postStuff.0
        self.postIds = postStuff.1
        self.timeline.dataSource = self
        self.collectionView.dataSource = self
        self.timeline.delegate = self
        self.collectionView.delegate = self
        self.firstPostDate = self.posts.first?.timeStamp
        self.currentPostDate = self.posts.last?.timeStamp
    }
}

override open func viewDidAppear(_ animated: Bool) {

    super.viewDidAppear(animated)

    let indexToScrollTo = IndexPath(row: self.posts.count - 1, section: 0)
    self.collectionView.scrollToItem(at: indexToScrollTo, at: .left, animated: false)
    let firstPostDate = self.posts.first?.timeStamp
    let diff = self.posts.last?.timeStamp.months(from: (firstPostDate?.startOfMonth())!)
    //self.currentPostMonth = diff
    let monthCellIndexPath = IndexPath(row: diff!, section: 0)
    self.timeline.scrollToItem(at: monthCellIndexPath, at: .centeredHorizontally, animated: false)
    let months = self.posts.last?.timeStamp.months(from: (self.posts.first?.timeStamp.startOfMonth())!)
    print("cells are",self.timeline.indexPathsForVisibleItems)
    print(months,"cell months")
    if let optionalCell = self.timeline.cellForItem(at: IndexPath(row: months!, section: 0)) {
        let cell = optionalCell as! TimelineMonthViewCell
        let day = Calendar.current.component(.day, from: self.currentPostDate!)
        cell.drawArrow(day: day)
    } else {
        print("cell out of range")
    }
}

除了两个问题外，这几乎可行。

首先，初始单元格闪烁，然后自动滚动到最后一个单元格。我认为这是因为我使用的是viewDidAppear而不是viewWillAppear。但是，当我使用viewWillLoad时，应用崩溃，并显示错误：Thread 1: EXC_BAD_ACCESS (code=1, address=0xfffffffffffffff8)

第二，打印的self.timeline.indexPathsForVisibleItems是：[[0，0]，[0，1]，[0，2]，[0，3]，[0，4]]。因此，它查看的是前5个而不是滚动到的最后5个。然后，它找不到该单元格：“单元格超出范围”，因为索引约为28（最后一个单元格）。

那么我如何才能自动滚动到最后一个单元格并正确找到它呢？

Answer 1

不要将这些行设置在当前位置

data-src

在img上方可能会更好，以便在vc加载时利用第一次自动重新加载的优势，也可以在其位置插入

self.timeline.dataSource = self
self.collectionView.dataSource = self
self.timeline.delegate = self
self.collectionView.delegate = self

您可以在其中插入滚动代码

viewDidLoad

或在self.timeline.reloadData() self.collectionView.reloadData() 中，如果您当前的for循环不是异步的

Answer 2

我建议将滚动代码移至viewWillAppear。这样一来，便可以在显示视图之前但在一切都可以查看之后完成。

override open func viewWillAppear(_ animated: Bool) {

    super.viewWillAppear(animated)

    let indexToScrollTo = IndexPath(row: self.posts.count - 1, section: 0)
    self.collectionView.scrollToItem(at: indexToScrollTo, at: .left, animated: false)
    let firstPost = self.posts.first?.timeStamp
    let firstOfFirstMonth = firstPost?.startOfMonth()
    let diff = self.posts.last?.timeStamp.months(from: firstOfFirstMonth!)
    //self.currentPostMonth = diff
    let monthCellIndexPath = IndexPath(row: diff!, section: 0)
    self.timeline.scrollToItem(at: monthCellIndexPath, at: .centeredHorizontally, animated: false)
}