获取NSAttributedString显示html后的高度
UILabel* label = [[UILabel alloc]initWithFrame:CGRectMake(0, 0, 320, 500)];
label.font = [UIFont systemFontOfSize:20];
label.numberOfLines = 0;
label.backgroundColor = [UIColor greenColor];
[self.view addSubview:label];
NSString *html = @"Wow!Now iOS can create NSAttributedString from HTMLs!"; NSDictionary*options=@{NSDocumentTypeDocumentAttribute:NSHTMLTextDocumentType};
NSMutableAttributedString *attrString = [[NSMutableAttributedString alloc] initWithData:[html dataUsingEncoding:NSUTF8StringEncoding] options:options documentAttributes:nil error:nil];
[attrString addAttribute:NSForegroundColorAttributeName value:[UIColor redColor] range:NSMakeRange(0, attrString.length)];
[attrString addAttribute:NSFontAttributeName value:[UIFont systemFontOfSize:14] range:NSMakeRange(0, attrString.length)];
label.attributedText = attrString;
CGRect rect = [attrString boundingRectWithSize:CGSizeMake(320, 10000) options:NSStringDrawingUsesLineFragmentOrigin | NSStringDrawingUsesFontLeading context:nil];
label.frame = CGRectMake(0,20, 320, rect.size.height);
