计算文本大小

 NSDictionary *dic = [NSDictionary dictionaryWithObjectsAndKeys:[UIFont systemFontOfSize:17], NSFontAttributeName, nil];

    

    CGRect rect = [str boundingRectWithSize:CGSizeMake(300, 100000) options:NSStringDrawingUsesLineFragmentOrigin attributes:dic context:nil];

    NSLog(@"rect = %@", NSStringFromCGRect(rect));

    

    label.frame = CGRectMake(0, 0, label.frame.size.width, rect.size.height);

    [label setText:str];

    [label setBackgroundColor:[UIColor yellowColor]];

    

    sc.contentSize = CGSizeMake(0, label.frame.size.height);