Re: [Gtk2hs-users] Two questions about the treeview in ModelView module

[pierric <w.p...@gm...>]

2008/8/25 Axel Simon <Axe...@en...>

>
> On Aug 22, 2008, at 15:29, pierric wrote:
>
>  Hi, everyone.
>> In my current project, I use gtk2hs to build the GUI. But I meet a few
>> problems. Following two is about the TreeView.
>> 1. TreeView has a function named treeViewGetModel to achieve the TreeModel
>> data. But there seems no way to covert this model into the concrete ones,
>> such as TreeStore. This problem make the program a bit tougher, for I have
>> to store the TreeStore somewhere. So I wonder why not just let the
>> treeViewGetModel return the TreeStore structure?
>>
>
> The problem is that this is not type safe: Firstly you could then extract
> either a TreeStore or a ListStore, furthermore, the parameter of the values
> in the store could be anything. It's a bit like the read :: Read a => String
> -> a  function which can convert a string into anything that has a Read
> instance. There is not way to make this type safe.


I think following the same way as read is a good idea.
TreeModelClass m => treeViewGetModel :: TreeView -> IO (Maybe m)

But it's also not very safe.  If I store a ListStore in treeview, this
signature allows me to extract it as a TreeStore.

>
> You need to keep track of a ListStore, but you don't need to keep track of
> the TreeView once you've inserted it into a widget tree. Storing data in a
> GUI application is always a difficult issue in Haskell. Usually, the whole
> application state can be stored in one (possibly global) IORef that is
> passed to all callbacks that need it.
>
> Axel.
>
> As Richard has suggested, before update a treeStore, I need to first set
the treeview with a empty model, which require the treeView object. This
means that I cannot simply get rid of the treeView object as long as I have
created it.