如何检测最后一个单元格在 UICollectionView 中是否可见？

Question

我正在尝试检测 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了键盘，因此该单元格未被它隐藏，则上述代码return为真。如果它 是隐藏的 它 return 是假的。

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，我不知道如何 return 为真。

当 collectionView 向上滚动时，

lastCellPositionY 和 contentHeight 不会改变。反而 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，因为它们的来源不同，而且 self.collectionView.bounds.origin.y 明显小于 lastCellPositionY。

Answer 1

我使用此代码（翻译为 Swift 来自此线程中的提交：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 2

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 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
     }
 }

Answer 5

一个方便的扩展，带有两个计算属性。

extension UICollectionView {

    var isLastItemFullyVisible: Bool {

        let numberOfItems = numberOfItems(inSection: 0)
        let lastIndexPath = IndexPath(item: numberOfItems - 1, section: 0)

        guard let attrs = collectionViewLayout.layoutAttributesForItem(at: lastIndexPath) 
        else { 
            return false 
        }
        return bounds.contains(attrs.frame)
    }

    // If you don't care if it's fully visible, as long as some of it is visible
    var isLastItemVisible: Bool {
       let numberOfItems = collectionView.numberOfItems(inSection: 0)
       return indexPathsForVisibleItems.contains(where: { [=10=].item == numberOfItems - 1 })
    }
}

有 isLastItemFullyVisible 的替代方法。一个班轮。

var isLastItemFullyVisible: Bool {
   contentSize.width == contentOffset.x + frame.width - contentInset.right
}

Answer 6

Swift 5

public extension IndexPath {

  func isLastRow(at tableView: UITableView) -> Bool {
    return row == (tableView.numberOfRows(inSection: section) - 1)
  }

  func isLastRow(at collectionView: UICollectionView) -> Bool {
    return row == (collectionView.numberOfItems(inSection: section) - 1)
  }
}

对于集合视图

public func collectionView(_ collectionView: UICollectionView, willDisplay cell: UICollectionViewCell, forItemAt indexPath: IndexPath) {
    if indexPath.isLastRow(at: collectionView) {
      ..//
    }
}

对于 TableView

public func tableView(_ tableView: UITableView, willDisplay cell: UITableViewCell, forRowAt indexPath: IndexPath) {
    if indexPath.isLastRow(at: collectionView) {
      ..//
    }
}

如何检测最后一个单元格在 UICollectionView 中是否可见？

How to detect if last cell is visible in UICollectionView?

cocoa-touch

uicollectionview

uicollectionviewlayout

swift