Re: [Gtk2hs-users] What are column IDs for?

[Axel S. <Axe...@en...>]

Hi Wolfgang,

On Mar 2, 2009, at 14:27, Wolfgang Jeltsch wrote:

> Hello,
>
> as far as I understand, a tree model contains a forest whose nodes  
> are marked
> with data of an arbitrary type. For example, there is the type  
> ListStore a
> where the “a” can be any type. So there is no notion of column in tree
> models. Columns belong to a view and the values shown in a column are
> determined by applying some function to the one-and-only value at the
> respective row in the model.
>
> Now, I don’t understand why treeModelGetValue takes a ColumnId  
> argument. I
> would expect it to just take a tree model and a tree iterator and  
> return the
> one-and-only value at the respective row.
>
> Could someone please explain?
>

Yes, we can!

Indeed, most widgets implement the CellLayout interface which means  
that you can setup the association between the Haskell stores and the  
widget using cellLayoutSetAttributes. This API has no need for columns.

However, there are  few widgets or properties in Gtk+ that still do  
not implement the CellLayout inferface, or don't implement it  
properly. Examples are the IconView widget or if you want to set a  
sorting function. These functions all require a column. Thus, the  
module CustomStore (form which ListStore and TreeStore are derived)  
contains the function customStoreSetColumn to set up a 'fake' column.  
Specifically, a 'ColumnId' is be associated with an extraction  
function using 'customStoreSetColumn'. The numbers of these columns  
can be arbitrary, but since they are stored in a lookup table, they  
should be small, positive numbers. So, you'd declare a constant  
column id, add the extraction function and then pass this column  
number to a widget that depends on column numbers.

The reason the generic TreeModel interface contains a  
treeModelGetValue only for ColumnIds and not for complete rows is  
that it is indeed the generic variant of ListStore and TreeStore: if  
you have a fully typed model (e.g. ListStore String) you can extract  
a row using listStoreGetValue. If you only have the base class of  
type TreeModel, there's no way you can extract a value in a type safe  
way. Thus, the only way to extract a value from a TreeModel using  
treeModelGetValue is to use a ColumnId which carries the type. The  
function treeModelGetValue is not really needed, but I added it for  
symmetry as it sometimes gives cleaner code if you define a sort  
function, for instance.

I hope this helps,
Axel.