Re: Overriding iface implementations

[Magnus Bergman <magnus bergman observer net>]

On Wed, 25 Feb 2004 10:25:41 -0600
Federico Mena Quintero <federico ximian com> wrote:

> On Wed, 2004-02-25 at 09:03, Owen Taylor wrote:
> 
> > > Namely, the file chooser uses a GtkListStore for the shortcuts
> > > list.  I want to make just the bookmarks part reorderable, so I
> > > need to override::row_draggable on the source side, and
> > > ::row_drop_possible on the destination side.
> > You just add the interface again and set the functions you want
> > and don't set the others. g_type_interface_peek_parent() can
> > be used to chain up.
> 
> That means deriving from GtkListStore and re-adding the interfaces,
> right?
> 
> My question is more along the following lines.  Can I override
> interface methods for a particular instance of a class?  E.g.
> 
> 	GtkTreeDragSourceIface my_methods = {
> 	  my_row_draggable_func,
> 	  NULL,
> 	  NULL
> 	};
> 
> 	list = gtk_list_store_new ();
> 	override_interface (list, GTK_TYPE_TREE_DRAG_SOURCE,
> 	my_methods);

Even thou GLib doesn't directly support it, it is of course possible. I
don't know about interfaces specifically, but it's probably in the lines
of OO in general. So I hope this helps.

First of all you must have a function pointer in each instance, not just
in the class (which is what you get by default). Something like this:

    struct MyWidget{
        ...
        void(*row_draggable_func_pointer)(struct MyWidget*)
    }

Then make a function that calls the per-instance function, perhaps
something like this:

    void dynamic_row_draggable_func(MyWidget* this_widget){
        this_widget->row_draggable_func_pointer(this_widget);
    }

And just then you can override the function per instance:

    widget_instance->row_draggable_func_pointer = my_row_draggable_func;