iOS 7 计算字符串高度

- (float)width:(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;
}