计算字符串总高度

+ (CGFloat)heightOfComment:(NSString *)comment {
    UILabel *label = PrototypeCell.commentLabel;

    // NOTE: The height of the comment should always be at least the height of
    //       one line of text.
    if (comment.length == 0)
        comment = @" ";

    return [comment sizeWithFont:label.font
               constrainedToSize:label.frame.size
                   lineBreakMode:label.lineBreakMode].height;    
}