传内容string和字体大小，就可以计算多少行。

-(CGFloat)getLineNum:(NSString*)str  font:(UIFont*)font labelWidth:(CGFloat)width
{
    if (str.length<1)
    {
        return 0;
    }
    CGFloat oneRowHeight = [@"占位"         sizeWithAttributes:@{NSFontAttributeName:font}].height;
    CGSize textSize = [str boundingRectWithSize:CGSizeMake(width, CGFLOAT_MAX) options:NSStringDrawingUsesLineFragmentOrigin attributes:@{NSFontAttributeName:font} context:nil].size;
    CGFloat rows = textSize.height / oneRowHeight;
    return rows;
}