iOS-UITableView不会滚动到数据源中的最后一行

Question

I'm working on a messaging component of an iOS application. Each message is a row on my UITableView _messageTable I tried the following code in viewDidLoad , viewWillAppear and in viewDidAppear (messages are coming in from a background thread):

dispatch_async(dispatch_get_main_queue(), ^{
    [_messageTable reloadData];
});
dispatch_async(dispatch_get_main_queue(), ^{
    [self scrollToBottomOfTable];
});

-(void)scrollToBottomOfTable {
    NSInteger rowCount = [_messageTable numberOfRowsInSection:0];
    if (rowCount > 0) {
        NSIndexPath *indexPath = [NSIndexPath indexPathForRow: rowCount-1 inSection: 0];
        [_messageTable scrollToRowAtIndexPath: indexPath atScrollPosition: UITableViewScrollPositionBottom animated: YES];
    }
}

- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section {
    return _privateMessages.count;
}

This code works until there are about 20 or so rows. But when there are more messages, then it doesn't quite scroll all the way to the bottom. Is there some sort of limit? I'm ok with a view load delay if it means scrolling all the way to the bottom. Eventually I will implement some sort of message loading cap, but I would like to first understand and solve this issue.

EDIT

This is a table view within a view controller. I'm saving sent and received Message objects in a User object's array property (the person on the other end of the message conversation. app.hero is the user that is logged in, inheriting form the User object). The thing that doesn't make sense is that the code works perfectly for up to around 20 messages. Beyond that, it doesn't scroll all the way, but I can still manually scroll to the bottom as expected. The rest of the cell config code is :

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath {
tableView.estimatedRowHeight = 20.0;
tableView.rowHeight = UITableViewAutomaticDimension;
Message *message = [_privateMessages objectAtIndex:indexPath.row];
static NSString *cellId = @"messageCell";
UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:cellId];
if (cell == nil) {
    cell = [[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:cellId];
    cell.textLabel.numberOfLines=0;
}
return [self configureTableCell:cell forMessage:message];
}

- (UITableViewCell *)configureTableCell:(UITableViewCell *)cell forMessage:(Message *)message {
AppDelegate *app = (AppDelegate *)[[UIApplication sharedApplication] delegate];
if (message.sender == app.hero) {
    cell.backgroundColor = [UIColor whiteColor];
    cell.textLabel.textAlignment = NSTextAlignmentRight;
} else {
    cell.backgroundColor = [UIColor colorWithRed:0 green:1 blue:0.2 alpha:0.3];
    cell.textLabel.backgroundColor = [UIColor colorWithWhite:0 alpha:0];
    cell.textLabel.textAlignment = NSTextAlignmentLeft;
}
cell.textLabel.text = message.content;
return cell;
}

Answer 1

I've determined that this issue has to do with cell height. By increasing tableView.estimatedRowHeight to 44.0, the table view scrolls flawlessly to the bottom. However, there remains an issue with when messages wrap beyond 1 line in a given cell. For each message longer than 1 line, the table scroll comes up a few more pixels short. Removing tableView.estimatedRowHeight altogether seems to result in similar behavior as setting it to say, 44.0. I want to say my original question is answered, but I'm still not sure how to make it scroll perfectly given the likelihood of multi-line cells.

EDIT - SOLUTION

The problem of incorrect scrolling is in fact solved by removing UITableViewAutomaticDimension and manually calculating the height in heightForRowAtIndexPath .

Answer 2

Here is a solution that works well with auto cell height sizing. It's quite a workaround, but this appears to be an Apple bug.

Set _manualScrolling=NO in viewDidLoad .

-(void)scrollViewDidEndDragging:(UIScrollView *)scrollView willDecelerate:(BOOL)decelerate{
    if (!decelerate) {
        _manualScrolling = NO;
    } else {
        _manualScrolling = YES;
    }
}
- (void)scrollViewWillBeginDragging:(UIScrollView *)scrollView {
    _manualScrolling = YES;
}

-(void)reloadTable {
    dispatch_async(dispatch_get_main_queue(), ^{
        [_messageTable reloadData];
    });
    dispatch_async(dispatch_get_main_queue(), ^{
        [self scrollToBottomOfTable];
    });
}

- (void)tableView:(UITableView *)tableView willDisplayCell:(UITableViewCell *)cell forRowAtIndexPath:(NSIndexPath *)indexPath {
    if (!_manualScrolling && ![self scrolledToBottom]) {
        [self scrollToBottomOfTable];
    }
}

-(BOOL)scrolledToBottom {
    NSInteger rowCount = [_messageTable numberOfRowsInSection:0];
    NSArray *visibleIndices = [_messageTable indexPathsForVisibleRows];
    NSIndexPath *lastVisibleIndex = [visibleIndices lastObject];
    NSIndexPath *lastIndex = [NSIndexPath indexPathForRow: rowCount-1 inSection: 0];
    return lastVisibleIndex.row == lastIndex.row;
}

-(void)scrollToBottomOfTable {
    NSInteger rowCount = [_messageTable numberOfRowsInSection:0];
    if (!rowCount > 0) return;
    NSIndexPath *indexPath = [NSIndexPath indexPathForRow: rowCount-1 inSection: 0];
        [_messageTable scrollToRowAtIndexPath: indexPath atScrollPosition: UITableViewScrollPositionBottom animated: YES];
}