腾讯sdk学到了..

+ (CGFloat)heightForContent:(MyMsgTextModel *)content withWidth:(CGFloat)width
{
    CGSize contentSize;
    if (SYSTEM_VERSION_GREATER_THAN_OR_EQUAL_TO(@"7.0")) {
        NSMutableParagraphStyle *paragraphStyle = [[NSMutableParagraphStyle alloc]init];
        paragraphStyle.lineBreakMode = NSLineBreakByWordWrapping;
//        NSDictionary *attributes = @{NSFontAttributeName:CELL_CONTENT_FONT_SIZE, NSParagraphStyleAttributeName:paragraphStyle.copy};
        NSDictionary *attributes = @{NSFontAttributeName:CELL_CONTENT_FONT_SIZE};
        
        contentSize = [content.textMsg boundingRectWithSize:CGSizeMake(width, CGFLOAT_MAX) options:NSStringDrawingUsesLineFragmentOrigin attributes:attributes context:nil].size;
    }
    else{
        contentSize = [content.textMsg sizeWithFont:CELL_CONTENT_FONT_SIZE
                              constrainedToSize:CGSizeMake(width, CGFLOAT_MAX)
                                  lineBreakMode:NSLineBreakByWordWrapping];
    }
    

    return contentSize.height;
}