Re: Mapping of model attributes with unknown values to a corresponding widget display

[Murray Cumming <murrayc murrayc com>]

On Mon, 2011-10-03 at 12:21 +0200, Markus Elfring wrote:
> Hello!
> 
> The class "Gtk::CheckButton" provides the feature to display an intermediate
> status (by the interface "gtk_toggle_button_set_inconsistent"), doesn't it?
> http://developer.gnome.org/gtk3/3.2/GtkToggleButton.html#gtk-toggle-button-set-inconsistent
> http://developer.gnome.org/gtkmm/3.2/classGtk_1_1CheckButton.html#_details
> 
> I would like to know the recommend way to represent unknown values for model
> attributes when it seems that a member function like
> "Gtk::TreeView::append_column" does not support the use of three-valued logic
> directly.
> http://developer.gnome.org/gtkmm/3.2/classGtk_1_1TreeView.html#a0253a5477ce006bf84fae8eb205efc73
> http://en.wikipedia.org/wiki/Three-valued_logic#Representation_of_values

Maybe you mean the Gtk::TreeView::append_column() method overloads that
TreeModelColumn parameters:
http://developer.gnome.org/gtkmm/unstable/classGtk_1_1TreeView.html#a0253a5477ce006bf84fae8eb205efc73

That method's documentation mentions that you can instead create the
TreeView::Column manually and use TreeViewColumn::set_cell_data_func()
with your own callback slot, which you could use to set the
"inconsistent" property:
http://developer.gnome.org/gtkmm/unstable/classGtk_1_1CellRendererToggle.html#a680f8f0b502d82a6c909aafe90284026

> How do you store a "NULL" (SQL) for a model attribute with a data type like
> "bool" and update its visual appearance accordingly?

Obviously C++'s bool type can't represent 3 values. You would need
something else, I suppose. Personally, I use Gnome::Gda::Value, from
libgda, via libgdamm. It's not wonderful, but it works.


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