动态计算字符串大小

- (CGSize)getframeAndStr:(NSString*)str andwidth:(int)width
{
    CGSize lasize = [str sizeWithFont:[UIFont systemFontOfSize:15] constrainedToSize:CGSizeMake(width, 300) lineBreakMode:NSLineBreakByWordWrapping];
    return lasize;

}

width： 自己定义需要现实的字符串宽度

300:   自己设定的字符串最大高度

    lasize.height  高度

    lasize.width   宽度

ios 7以后

- (float) heightForString:(NSString *)value andWidth:(float)width{

        NSAttributedString *attrStr = [[NSAttributedString alloc] initWithString:value];
        NSRange range = NSMakeRange(0, attrStr.length);
    
        NSDictionary *dic = [attrStr attributesAtIndex:0 effectiveRange:&range];
    
        CGSize sizeToFit = [value boundingRectWithSize:CGSizeMake(width, 300)
                                               options:NSStringDrawingUsesLineFragmentOrigin | NSStringDrawingUsesFontLeading
                                            attributes:dic
                                               context:nil].size;
        return sizeToFit.height;
}

- (float) heightForString:(NSString *)value andWidth:(float)width  andFont:(float)font{
    
    UIFont * fontR = [UIFont systemFontOfSize:font];
    NSDictionary *attribute = @{NSFontAttributeName: fontR};
    CGSize sizeToFit = [value boundingRectWithSize:CGSizeMake(width, 300)
                                           options:NSStringDrawingUsesLineFragmentOrigin | NSStringDrawingUsesFontLeading
                                        attributes:attribute
                                           context:nil].size;
    return sizeToFit.height;
}