You can get the number of rows by calling `get-number` which is an
inherited method from list-control<%>. However, you probably want to keep
the data separate from the list box and sort that: the list box always
contains strings, but your data type might be a number and you want to sort
it as such.
If you want to look at a possible implementation, I wrote a wrapper around
list-box% for exactly the purpose of showing SQL query results in a
list-box%. It supports sorting, adding, deleting and updating individual
rows, and you can also reorder and resize columns and this layout is saved
and can be restored:
https://github.com/alex-hhh/ActivityLog2/blob/master/rkt/widgets.rkt#L982
The name of the class is `qresults-list%` and you can find usage examples
of it throughout the application.
Best regards,
Alex.
On Tuesday, March 6, 2018 at 1:37:57 AM UTC+8, James wrote:
>
> I'm working on something which is leading to a cascade of questions so I
> suppose that I should just start by explaining what I am trying to do. I
> have written some code which takes the results of an SQL query and displays
> it as a table in a GUI using list-box%. This works and I am now writing
> the callback function. One of the things I want it to do is sort the table
> by a given column when you click on that column header. As far as I can
> tell, the way to do this is to get the contents of the table, sort it, and
> put it back. The Racket Guide doesn't show any way to just get all the data
> from the list-box% at once but you can get a row by index number. Fine, so
> I could loop through and get all the rows but how do I know how many rows
> there are? There isn't a way to get the number of rows directly but there
> is a way to get the number of visible items in the list-box% so maybe I
> could use that?
>
> The message is:
> (send a-list-box number-of-visible-items)
>
> So, at this point, I have my callback just using this message to report
> back the number of visible items when I click a column header. I have a 4
> x 8 table and it tells me that there are 35 items. Where is this number
> coming from? Could it be a pair of opposite off-by-one errors and it
> thinks I have a 5x7 table?
>
> There are ways to get around the problem. I could, for example, maintain
> a parallel copy of the contents of the table but I want to write the most
> generalizable code possible since I expect to do this kind of thing
> repeatedly in different contexts. It seems more reliable to work with the
> actual data in the list-box%.
>
> The bigger question is whether I am going about what I want to do the best
> way in the first place. Is list-box really what I want to use? Is there a
> better choice? If not, should I work with someone to modify list-box to be
> more accommodating to my use case?
>
> James
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