向下滚动 collectionView 后，将单元格保持在相同的 position

Question

我有一个聊天，当我向下滚动以获取旧消息时，我希望 collectionView 保持静止并允许用户手动滚动刚刚加载的旧消息，就像在 messenger 中一样。 使用我的 collectionView，它会自动滚动到新项目的顶部。 我在 scrollViewDidEndDragging 中添加了一个方法，但有两个问题：

1. 它的故障意味着它滚动到新数据的顶部，然后滚动回“以前的位置”。
2. 它并没有完全回到以前的 position，加载旧数据之前最旧的消息不在同一个地方，它几乎在中心而不是保持在顶部。 这是我的代码：

 override func scrollViewDidEndDragging(_ scrollView: UIScrollView, willDecelerate decelerate: Bool) {

    let contentOffset = scrollView.contentOffset.y
    if contentOffset <= -40 {

        self.collectionView.refreshControl?.beginRefreshing()
        self.fetchMessages()

        let beforeTableViewContentHeight = collectionView.contentSize.height
        let beforeTableViewOffset = collectionView.contentOffset.y

        DispatchQueue.main.asyncAfter(deadline: .now() + 0.5) {
                        self.collectionView.layer.layoutIfNeeded()
                        let insertCellHeight = beforeTableViewOffset + (self.collectionView.contentSize.height - beforeTableViewContentHeight)
                        let newOffSet = CGPoint(x: 0, y: insertCellHeight)
                        self.collectionView.contentOffset = newOffSet
     }
        }
    }
// MARK: - UICollectionViewDelegateFlowLayout
extension RoomMessageViewController: UICollectionViewDelegateFlowLayout {
    func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, referenceSizeForHeaderInSection section: Int) -> CGSize {
        return CGSize(width: view.frame.width, height: 10)
    }
    func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, insetForSectionAt section: Int) -> UIEdgeInsets {
        return .init(top: 16, left: 0, bottom: 16, right: 0)
    }
    func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize {
        
        let frame = CGRect(x: 0, y: 0, width: view.frame.width, height: 50)
        let estimatedSizeCell = RoomMessageCell(frame: frame)
        estimatedSizeCell.roomMessage = chatMessages[indexPath.section][indexPath.row]
        estimatedSizeCell.layoutIfNeeded()
                
        let targetSize = CGSize(width: view.frame.width, height: 1000)
        let estimatedSize = estimatedSizeCell.systemLayoutSizeFitting(targetSize)
        
        return CGSize(width: view.frame.width, height: estimatedSize.height)
    }
}

Answer 1

这是您更新的代码

var isLoading = false
override func scrollViewDidEndDragging(_ scrollView: UIScrollView, willDecelerate decelerate: Bool) {
    
    let contentOffset = scrollView.contentOffset.y
    if contentOffset <= -40 {
        if self.isLoading { return } // Here is the FLAG can help you to avoid spamming scrolling that trigger history loading
        self.isLoading = true
        
        self.collectionView.refreshControl?.beginRefreshing()
        self.fetchMessages() // update your list then reloadData, end refreshing, set isLoading = false (reset FLAG)
    }
}

您甚至可以使用此 function 进行更多反应触发

func scrollViewDidScroll(scrollView: UIScrollView) {
    let contentOffset = scrollView.contentOffset.y
    if contentOffset <= -40 {
        if self.isLoading { return } // Here is the FLAG can help you to avoid spamming scrolling that trigger history loading
        self.isLoading = true
        
        self.collectionView.refreshControl?.beginRefreshing()
        self.fetchMessages() // update your list then reloadData, end refreshing, set isLoading = false (reset FLAG)
    }
}

获得新数据后，reloadData()，然后现在您可以执行 animation。

self.collectionView?.setContentOffset(.zero, animated: true) // zero because your scroll to the top