使用 UITableViewAutomaticDimension 将 UITableViewCell 更新到位后生涩滚动 [英] Jerky Scrolling After Updating UITableViewCell in place with UITableViewAutomaticDimension

问题描述

我正在构建一个应用程序，该应用程序具有用户提交的帖子的提要视图.这个视图有一个带有自定义 UITableViewCell 实现的 UITableView.在这个单元格内，我有另一个 UITableView 用于显示注释.要点是这样的:

I am building an app that has a feed view for user-submitted posts. This view has a UITableView with a custom UITableViewCell implementation. Inside this cell, I have another UITableView for displaying comments. The gist is something like this:

Feed TableView
  PostCell
    Comments (TableView)
      CommentCell
  PostCell
    Comments (TableView)
      CommentCell
      CommentCell
      CommentCell
      CommentCell
      CommentCell

初始提要将下载 3 条评论供预览，但如果有更多评论，或者用户添加或删除评论，我想更新提要内的 PostCell通过在 PostCell 内的评论表中添加或删除 CommentCells 来实现表格视图.我目前正在使用以下帮助程序来完成该任务:

The initial feed will download with 3 comments for previewing, but if there are more comments, or if the user adds or deletes a comment, I want to update the PostCell in place inside of the feed table view by adding or removing CommentCells to the comments table inside of the PostCell. I am currently using the following helper to accomplish that:

// (PostCell.swift) Handle showing/hiding comments
func animateAddOrDeleteComments(startRow: Int, endRow: Int, operation: CellOperation) {
  let table = self.superview?.superview as UITableView

  // "table" is outer feed table
  // self is the PostCell that is updating it's comments
  // self.comments is UITableView for displaying comments inside of the PostCell
  table.beginUpdates()
  self.comments.beginUpdates()

  // This function handles inserting/removing/reloading a range of comments
  // so we build out an array of index paths for each row that needs updating
  var indexPaths = [NSIndexPath]()
  for var index = startRow; index <= endRow; index++ {
    indexPaths.append(NSIndexPath(forRow: index, inSection: 0))
  }

  switch operation {
  case .INSERT:
    self.comments.insertRowsAtIndexPaths(indexPaths, withRowAnimation: UITableViewRowAnimation.None)
  case .DELETE:
    self.comments.deleteRowsAtIndexPaths(indexPaths, withRowAnimation: UITableViewRowAnimation.None)
  case .RELOAD:
    self.comments.reloadRowsAtIndexPaths(indexPaths, withRowAnimation: UITableViewRowAnimation.None)
  }

  self.comments.endUpdates()
  table.endUpdates()

  // trigger a call to updateConstraints so that we can update the height constraint 
  // of the comments table to fit all of the comments
  self.setNeedsUpdateConstraints()
}

override func updateConstraints() {
  super.updateConstraints()
  self.commentsHeight.constant = self.comments.sizeThatFits(UILayoutFittingCompressedSize).height
}

这样就可以很好地完成更新了.该帖子会按预期在 PostCell 内添加或删除评论就地更新.我在提要表中使用自动调整大小 PostCells.PostCell 的评论表展开以显示所有评论，但动画有点生涩，并且表格在单元格更新动画发生时上下滚动十几个像素左右.

This accomplishes the update just fine. The post is updated in place with comments added or removed inside of the PostCell as expected. I am using auto sizing PostCells in the feed table. The comments table of the PostCell expands to show all of the comments, but the animation is a bit jerky and the table sort of scrolls up and down a dozen pixels or so while the cell update animation takes place.

调整大小时的跳跃有点烦人，但我的主要问题是后来出现的.现在，如果我在提要中向下滚动，滚动会像以前一样平滑，但是如果我在添加评论后刚刚调整大小的单元格上方向上滚动，提要会在到达提要顶部之前向后跳几次.我为 Feed 设置 iOS8 自动调整大小的单元格，如下所示:

The jumping during resizing is a bit annoying, but my main issue comes afterwards. Now if I scroll down in the feed, the scrolling is smooth as before, but if I scroll up above the cell I just resized after adding comments, the feed will jump backwards a few times before it reaches the top of the feed. I setup iOS8 auto sizing cells for the Feed like this:

// (FeedController.swift)
// tableView is the feed table containing PostCells
self.tableView.rowHeight = UITableViewAutomaticDimension
self.tableView.estimatedRowHeight = 560

如果我删除 estimatedRowHeight，表格只会在单元格高度发生变化时滚动到顶部.我现在对此感到很困惑，作为一名新的 iOS 开发者，可以使用您可能拥有的任何技巧.

If I remove the estimatedRowHeight, the table just scrolls to the top anytime a cell height changes. I'm feeling pretty stuck on this now and as a new iOS developer, could use any tips you might have.

推荐答案

这是我找到的解决此类问题的最佳方案(滚动问题 + reloadRows + iOS 8 UITableViewAutomaticDimension)；

Here is the best solution I found to solve this kind of problem (scrolling problem + reloadRows + iOS 8 UITableViewAutomaticDimension);

它包括将每个高度保存在字典中并更新它们(在字典中)，因为 tableView 将显示单元格.

It consists by keeping every heights in a dictionary and updating them (in the dictionary) as the tableView will display the cell.

然后您将在 - (CGFloat)tableView:(UITableView *)tableView EstimatedHeightForRowAtIndexPath:(NSIndexPath *)indexPath 方法中返回保存的高度.

You will then return the saved height in - (CGFloat)tableView:(UITableView *)tableView estimatedHeightForRowAtIndexPath:(NSIndexPath *)indexPath method.

你应该实现这样的东西:

You should implement something like this :

目标 C

- (void)viewDidLoad {
    [super viewDidLoad];

    self.heightAtIndexPath = [NSMutableDictionary new];
    self.tableView.rowHeight = UITableViewAutomaticDimension;
}

- (CGFloat)tableView:(UITableView *)tableView estimatedHeightForRowAtIndexPath:(NSIndexPath *)indexPath {
    NSNumber *height = [self.heightAtIndexPath objectForKey:indexPath];
    if(height) {
        return height.floatValue;
    } else {
        return UITableViewAutomaticDimension;
    }
}

- (void)tableView:(UITableView *)tableView willDisplayCell:(UITableViewCell *)cell forRowAtIndexPath:(NSIndexPath *)indexPath {
    NSNumber *height = @(cell.frame.size.height);
    [self.heightAtIndexPath setObject:height forKey:indexPath];
}

Swift 3

@IBOutlet var tableView : UITableView?
var heightAtIndexPath = NSMutableDictionary()

override func viewDidLoad() {
    super.viewDidLoad()

    tableView?.rowHeight = UITableViewAutomaticDimension
}

func tableView(_ tableView: UITableView, estimatedHeightForRowAt indexPath: IndexPath) -> CGFloat {
    if let height = heightAtIndexPath.object(forKey: indexPath) as? NSNumber {
        return CGFloat(height.floatValue)
    } else {
        return UITableViewAutomaticDimension
    }
}

func tableView(_ tableView: UITableView, willDisplay cell: UITableViewCell, forRowAt indexPath: IndexPath) {
    let height = NSNumber(value: Float(cell.frame.size.height))
    heightAtIndexPath.setObject(height, forKey: indexPath as NSCopying)
}

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