向上滚动

Question

我有UICollectionView聊天视图 - 消息显示在底部最新。当用户向上滚动时，它会加载先前的消息并更新集合视图。我试图在添加新数据时保持UICollectionView的内容偏移量，但我无法使其工作。

这就是我现在所拥有的：

// First find the top most visible cell.
if let topCellIndexPath = collectionView.indexPathsForVisibleItems.sorted().first,
   let topCell = collectionView.cellForItem(at: topCellIndexPath),
   let topCellLayout = collectionView.layoutAttributesForItem(at: topCellIndexPath) {

    // Save the y position of the top cell.
    let previousTopCellY = topCellLayout.frame.origin.y

    // Perform updates on the UICollectionView without animation (ignore the fact it says adapter)
    adapter.performUpdates(animated: false) { [weak self] completed in
        if let strongSelf = self,
           let topCellNewIndexPath = strongSelf.collectionView.indexPath(for: topCell),
           let newTopCellLayout = strongSelf.collectionView.layoutAttributesForItem(at: topCellNewIndexPath) {

            // Calculate difference between the previous cell y value and the current cell y value
            let delta = previousTopCellY - newTopCellLayout.frame.origin.y

            // Add this to the collection view content offset
            strongSelf.collectionView.contentOffset.y += delta
        }
    }
}

这似乎不起作用，有时无法在更新后获取单元格的indexPath。

修改 根据@ Arkku的答案，这是有效的。虽然有一个小的闪烁。

let previousContentSize = collectionView.contentSize.height
adapter.performUpdates(animated: false) { [weak self] completed in
    if let strongSelf = self {
        let delta = strongSelf.collectionView.contentSize.height - previousContentSize
        strongSelf.collectionView.contentOffset.y += delta
    }
}

Answer 1

正如我之前评论的那样，从delta获取contentSize而不是特定单元格的来源可能更好。基于您自己版本的建议：

let previousContentHeight = collectionView.contentSize.height
adapter.performUpdates(animated: false) { [weak self] completed in
    guard let strongSelf = self else { return }
    let delta = strongSelf.collectionView.contentSize.height - previousContentHeight
    strongSelf.collectionView.bounds.origin.y += delta
}