boundingRectWithSize 动态计算文字的高度

通过设置字符串的属性计算实际的面积

方法如下：

-(CGSize)sizeForString:(NSString *)string{

    
    NSMutableAttributedString *attrStr =[[NSMutableAttributedString alloc]initWithString:string];
    NSRange allRange = [string rangeOfString:string];
    
    [attrStr addAttribute:NSFontAttributeName
                    value:[UIFont systemFontOfSize:16]
                    range:allRange];
    
    
    NSStringDrawingOptions options =  NSStringDrawingUsesLineFragmentOrigin | NSStringDrawingUsesFontLeading;
    
    CGSize textSize = [attrStr boundingRectWithSize:CGSizeMake(size_w, size_h)
                                        options:options
                                        context:nil].size;
    return textSize;
}