计算字符串的height

#define ISIOS7      ([[[UIDevice currentDevice] systemVersion] compare:@"7.0"] != NSOrderedAscending )

-(CGFloat)fn_heightWithString:(NSString *)string font:(UIFont *)font constrainedToWidth:(CGFloat)width

{

    CGSize hSize;

    if(ISIOS7) {

        NSMutableParagraphStyle *paragraphStyle = [[NSMutableParagraphStyle alloc] init];

        paragraphStyle.lineBreakMode = NSLineBreakByWordWrapping;

        NSDictionary *attributes = @{NSFontAttributeName:font, NSParagraphStyleAttributeName:paragraphStyle.copy};

        

        rtSize = [string boundingRectWithSize:CGSizeMake(width, CGFLOAT_MAX) options:NSStringDrawingUsesLineFragmentOrigin attributes:attributes context:nil].size;

        return ceil(hSize.height) + 0.5;

    } else {

        rtSize = [string sizeWithFont:font constrainedToSize:CGSizeMake(width, CGFLOAT_MAX) lineBreakMode:NSLineBreakByWordWrapping];

        return hSize.height;

    }

}