Re: [wxhaskell-users] virtual list control

[Jeremy O'D. <jer...@gm...>]

Hi Fabian,

On 12 May 2012 16:12, Fabian Binz <fab...@ya...> wrote:

>   Hi Jeremy, ****
>
> I don't know if you continued working on this, but I forked wxHaskell on
> GitHub and tried to implement it myself:
> https://github.com/FabianBinz/wxHaskell
>
I have been doing some work, but I don't have a lot of time right now, so
it is going more slowly than I would like.

> For testing purposes, I only implemented the OnGetItemText and
> SetItemCount method and it seems to work.
>
> To implement the "virtual" list control, I defined a new class
> wxVirtualListCtrl in wxc:****
>
>
> https://github.com/FabianBinz/wxHaskell/blob/master/wxc/src/include/virtuallistctrl_impl.h
> ****
>
> Since it needs to invoke a callback function, I created the typedef
> OnGetItemTextCallback. To make wxdirect accept this new type, I extended
> the parser (patomtype) in ParseC.hs. This solution is very ad-hoc and I
> think because wxc is supposed to be a language agnostic C wrapper, we
> should maybe add another macro to wcx_types.hs (something like TCallback),
> which is then recognized by wxdirect.****
>
> So, while this all works pretty well, there is unfortunately a memory leak
> in Graphics.UI.WXCore.VirtualListCtrl, because of my use of newCString. At
> the moment I don't know how to fix it and would be glad if someone could
> give me some advice.
>

You need to use a finalizer - this is what I am looking into. There is an
overview at
http://www.haskell.org/haskellwiki/GHC/Using_the_FFI#Calling_Haskell_from_C,
but it could be clearer! Basic idea is that you free the CString when the
GC has no more references to the closure in which it is used.

I am using the event handler code as a model, since this already allows
callbacks from C to Haskell, and handles reference counting. Just haven't
finished yet.

I will take a look at your fork and see if you are further along than me,
and take code accordingly.

Best regards
Jeremy