需求：消息如果少于三行，正常显示，如果超过3行，只显示三行，尾部加上展开按钮，点击按钮展开，再次点击收缩

思路

• 通过model来存储是否显示按钮，显示高度
• 按钮回调里修改model中的属性值，刷新某一个cell
• 需要注意的是当前tableView的estimatedRowHeight需要设置0,不然回刷新动画会出现乱跳的情况

计算model

// 计算相关高度
for (MessageMo *msgMo in tmpData) {
    NSMutableDictionary *attributes = [NSMutableDictionary new];
    [attributes setObject:FONT_FT2 forKey:NSFontAttributeName];
    // lable的宽度
    CGFloat width = SCREEN_WIDTH - 30 - 15 - 70 - 30;
    CGFloat textH = [msgMo.messageContent boundingRectWithSize:CGSizeMake(width, MAXFLOAT) options:NSStringDrawingUsesLineFragmentOrigin|NSStringDrawingUsesFontLeading attributes:attributes context:nil].size.height;
                
    CGFloat lineHeight = FONT_FT2.lineHeight;
    NSInteger lineCount = textH / lineHeight;
                
    if (lineCount <= 3) {
        msgMo.showBtn = NO;
        msgMo.btnShowSelect = NO;
        msgMo.cellHeightMax = 119;
        msgMo.cellHeightMin = 119;
    } else {
        msgMo.showBtn = YES;
        msgMo.btnShowSelect = NO;
        msgMo.cellHeightMin = 135;
        msgMo.cellHeightMax = textH + 43 + 10 + 30;
    }
}

// cell赋值
if (_model.showBtn) {
    [self.btnMore mas_makeConstraints:^(MASConstraintMaker *make) {
        make.bottom.equalTo(self.contentView);
        make.right.equalTo(self.labTime);
        make.width.equalTo(42);
        make.height.equalTo(30);
    }];
        
    self.btnMore.selected = _model.btnShowSelect;       
    if (_model.btnShowSelect) {
        self.labMsg.numberOfLines = 0;
    } else {
        self.labMsg.numberOfLines = 3;
    }
}
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// 回调方法
- (void)messageCellDidSelected:(CourseMessageCell *)cell {
    NSIndexPath *indexPath = [_tableView indexPathForCell:cell];
    MessageMo *msgMo = self.data[indexPath.row];
    
    if (msgMo.showBtn) {
        msgMo.btnShowSelect = !msgMo.btnShowSelect;
        [_tableView reloadRowsAtIndexPaths:@[indexPath] withRowAnimation:UITableViewRowAnimationAutomatic];
    }
}

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