如何检测最后一个单元格在UICollectionView中是否可见？

Question

我试图检测collectionView中的最后一个单元格是否可见。

var isLastCellVisible: Bool {

    let lastIndexPath = NSIndexPath(forItem: self.messages.count - 1, inSection: 0)
    let visibleIndexPaths = self.collectionView.indexPathsForVisibleItems()

    let lastCellPositionY = self.collectionView.layoutAttributesForItemAtIndexPath(lastIndexPath)!.frame.origin.y
    let bottomInset = self.collectionView.contentInset.bottom // changes when the keyboard is shown / hidden
    let contentHeight = self.collectionView.contentSize.height

    if visibleIndexPaths.contains(lastIndexPath) && (contentHeight - lastCellPositionY) > bottomInset {
        return true
    } else {
        return false
    }
}

什么有效：

如果最后一个单元格可见并且显示键盘以便单元格不被其隐藏，则上面的代码返回true。如果它是隐藏的，则返回false。

但是，当用户向上滚动并且最后一个单元格位于键盘上方时，我无法弄清楚如何返回true。

当lastCellPositionY向上滚动时，contentHeight和collectionView不会改变。相反self.collectionView.bounds.origin.y确实发生了变化，但我不知道如何将lastCellPositionY与它进行比较，因为它们不具有相同的起源，而self.collectionView.bounds.origin.y明显小于lastCellPositionY。

Answer 1

我在用什么

override func collectionView(collectionView: UICollectionView, willDisplayCell cell: UICollectionViewCell, forItemAtIndexPath indexPath: NSIndexPath) {
        if indexPath.row == dataSource.count - 1 {
            // Last cell is visible
        }
    }

Answer 2

我使用这段代码（从这个帖子的提交翻译成Swift：https://github.com/jessesquires/JSQMessagesViewController/issues/1458）

var isLastCellVisible: Bool {

    if self.messages.isEmpty {
        return true
    }

    let lastIndexPath = NSIndexPath(forItem: self.messages.count - 1, inSection: 0)
    var cellFrame = self.collectionView.layoutAttributesForItemAtIndexPath(lastIndexPath)!.frame

    cellFrame.size.height = cellFrame.size.height

    var cellRect = self.collectionView.convertRect(cellFrame, toView: self.collectionView.superview)

    cellRect.origin.y = cellRect.origin.y - cellFrame.size.height - 100 
    // substract 100 to make the "visible" area of a cell bigger

    var visibleRect = CGRectMake(
        self.collectionView.bounds.origin.x,
        self.collectionView.bounds.origin.y,
        self.collectionView.bounds.size.width,
        self.collectionView.bounds.size.height - self.collectionView.contentInset.bottom
    )

    visibleRect = self.collectionView.convertRect(visibleRect, toView: self.collectionView.superview)

    if CGRectContainsRect(visibleRect, cellRect) {
        return true
    }

    return false
}

Answer 3

对于Swift 3：

var isLastCellVisible: Bool {

    if self.posts.isEmpty {
        return true
    }

    let lastIndexPath = IndexPath(item: self.posts.count - 1, section: 1)
    var cellFrame = self.collectionView?.layoutAttributesForItem(at: lastIndexPath)!.frame

    cellFrame?.size.height = (cellFrame?.size.height)!

    var cellRect = self.collectionView?.convert(cellFrame!, to: self.collectionView?.superview)

    cellRect?.origin.y = (cellRect?.origin.y)! - (cellFrame?.size.height)! - 100 
    // substract 100 to make the "visible" area of a cell bigger

    var visibleRect = CGRect(x: (self.collectionView?.bounds.origin.x)!, y: (self.collectionView?.bounds.origin.y)!, width: 
        (self.collectionView?.bounds.size.width)!, height: 
        (self.collectionView?.bounds.size.height)! - (self.collectionView?.contentInset.bottom)!
    )

    visibleRect = (self.collectionView?.convert(visibleRect, to: self.collectionView?.superview))!

    if visibleRect.contains(cellRect!) {
        return true
    }

    return false
}

Answer 4

Swift版本4.2

 func scrollViewDidScroll(_ scrollView: UIScrollView) {

    if (scrollView.bounds.maxY) == scrollView.contentSize.height{
        // Call Your API or hide UIElements
     }
 }