iOS 7 计算字符串高度

- (float)(NSString *)str font:(UIFont *)font
{
    NSDictionary *attribute = @{NSFontAttributeName:font};
    
    CGSize size = [str boundingRectWithSize:CGSizeMake(_bgView.frame.size.width - 2*MARGIN_TOP, CGFLOAT_MAX) options: NSStringDrawingTruncatesLastVisibleLine | NSStringDrawingUsesLineFragmentOrigin | NSStringDrawingUsesFontLeading attributes:attribute context:nil].size;
    return size.height;
}