ios - 向上滚动时在 UICollectionView 中保持滚动位置

Question

我有一个 UICollectionView，带有类似聊天的 View - 消息显示在底部，最新的消息。当用户向上滚动时，它会加载以前的消息并更新 Collection View 。我试图在添加新数据时保持 UICollectionView 的内容偏移量，但我无法让它工作。

这是我目前拥有的:

// 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
        }
    }
}

这个好像不行，更新后有时获取不到cell的indexPath

编辑基于@Arkku 的回答，这是可行的。虽然有一点点闪烁。

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 contentSize 获取 delta 可能比从特定单元格的来源获取更好。根据您自己的版本提出的建议:

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