iOS融云聊天列表滚动到最下方

之前用的

[self.conversationMessageCollectionView scrollToBottomAnimated:NO];

这个有问题，消息不满一页的时候会把列表整体往下拉，要换成下面这种

NSUInteger finalRow = MAX(0, [self.conversationMessageCollectionView numberOfItemsInSection:0] - 1);
if (0 == finalRow) {
    return;
}
NSIndexPath *finalIndexPath = [NSIndexPath indexPathForItem:finalRow inSection:0];
[self.conversationMessageCollectionView scrollToItemAtIndexPath:finalIndexPath atScrollPosition:UICollectionViewScrollPositionBottom animated:NO];