问题描述

我觉得这可能是一个常见问题，想知道是否有任何常见的解决方案.

I feel like this might be a common issue and was wondering if there was any common solution to it.

基本上，我的 UITableView 具有每个单元格的动态单元格高度.如果我不在 UITableView 的顶部并且我 tableView.reloadData()，向上滚动会变得跳跃.

Basically, my UITableView has dynamic cell heights for every cell. If I am not at the top of the UITableView and I tableView.reloadData(), scrolling up becomes jumpy.

我相信这是因为我重新加载了数据，当我向上滚动时，UITableView 正在重新计算每个单元格的高度，以使其可见.如何减轻这种情况，或者如何仅将某个 IndexPath 中的数据重新加载到 UITableView 的末尾?

I believe this is due to the fact that because I reloaded data, as I'm scrolling up, the UITableView is recalculating the height for each cell coming into visibility. How do I mitigate that, or how do I only reloadData from a certain IndexPath to the end of the UITableView?

此外，当我设法一直滚动到顶部时，我可以向下滚动然后向上滚动，没有跳跃的问题.这很可能是因为 UITableViewCell 的高度已经计算过了.

Further, when I do manage to scroll all the way to the top, I can scroll back down and then up, no problem with no jumping. This is most likely because the UITableViewCell heights were already calculated.

推荐答案

为了防止跳跃，你应该在单元格加载时保存它们的高度，并在 tableView:estimatedHeightForRowAtIndexPath 中给出准确的值:

To prevent jumping you should save heights of cells when they loads and give exact value in tableView:estimatedHeightForRowAtIndexPath:

斯威夫特:

var cellHeights = [IndexPath: CGFloat]()

func tableView(_ tableView: UITableView, willDisplay cell: UITableViewCell, forRowAt indexPath: IndexPath) {
    cellHeights[indexPath] = cell.frame.size.height
}

func tableView(_ tableView: UITableView, estimatedHeightForRowAt indexPath: IndexPath) -> CGFloat {
    return cellHeights[indexPath] ?? UITableView.automaticDimension
}

目标 C:

// declare cellHeightsDictionary
NSMutableDictionary *cellHeightsDictionary = @{}.mutableCopy;

// declare table dynamic row height and create correct constraints in cells
tableView.rowHeight = UITableViewAutomaticDimension;

// save height
- (void)tableView:(UITableView *)tableView willDisplayCell:(UITableViewCell *)cell forRowAtIndexPath:(NSIndexPath *)indexPath {
    [cellHeightsDictionary setObject:@(cell.frame.size.height) forKey:indexPath];
}

// give exact height value
- (CGFloat)tableView:(UITableView *)tableView estimatedHeightForRowAtIndexPath:(NSIndexPath *)indexPath {
    NSNumber *height = [cellHeightsDictionary objectForKey:indexPath];
    if (height) return height.doubleValue;
    return UITableViewAutomaticDimension;
}

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