Re: how are interfaces created? (talking about the Java-like "interface")

[Jean Bréfort <jean brefort normalesup org>]

Le samedi 29 janvier 2005 �0:10 -0800, Ben Johnson a �it :
> On Sat, Jan 29, 2005 at 08:27:55AM +0100, Jean Br?fort wrote:
> > Le vendredi 28 janvier 2005 ?? 15:22 -0800, Ben Johnson a ??crit :
> > > I want to understand how interfaces are defined and used/implemented.
> ...
> > 
> > It's a GInterface object. When creating a new class of object, you
> > derive it from an existing class and you can add one or several
> > interfaces to enrich your class.
> > Here is a sample code I used to add printing support to the gnome canvas
> > (this code is used by GChemPaint).
> > 
> > GType
> > gnome_canvas_group_ext_get_type (void)
> > {
> > 	static GType group_ext_type;
> > 
> > 	if (!group_ext_type) {
> > 		static const GTypeInfo object_info = {
> > 			sizeof (GnomeCanvasGroupExtClass),
> > 			(GBaseInitFunc) NULL,
> > 			(GBaseFinalizeFunc) NULL,
> > 			(GClassInitFunc) gnome_canvas_group_ext_class_init,
> > 			(GClassFinalizeFunc) NULL,
> > 			NULL,			/* class_data */
> > 			sizeof (GnomeCanvasGroupExt),
> > 			0,			/* n_preallocs */
> > 			(GInstanceInitFunc) gnome_canvas_group_ext_init,
> > 			NULL			/* value_table */
> > 		};
> > 
> > 		static const GInterfaceInfo print_info = {
> > 			(GInterfaceInitFunc) gnome_canvas_group_print_init,
> > 			NULL, NULL
> > 		};
> > 
> > 		group_ext_type = g_type_register_static (GNOME_TYPE_CANVAS_GROUP_EXT,
> > "GnomeCanvasGroupExt",
> > 						    &object_info, 0);
> > 
> > 		g_type_add_interface_static (group_ext_type, G_TYPE_PRINTABLE,
> > &print_info);
> > 	}
> > 
> > 	return group_ext_type;
> > }
> > 
> > Hope this helps,
> 
> It does help *very* much.  Thank you.  I hope you don't answering a
> couple questions.
> 
> Once an object of GnomeCanvasGroupExtClass is instantiated, can it be
> cast as something else having to do with the print_info (Printable?)?
> How is that done?  And, when the cast is performed, what functionality
> is exposed?

Yes, you can cast it either to a GPrintable or a GnomeCanvasExtGroup. As
a sample, I reproduce the function which prints the group:

void gnome_canvas_group_ext_print (GPrintable *printable,
GnomePrintContext *pc)
{
	GList *list;
	double affine[6];
	GnomeCanvasItem *item;
	g_return_if_fail (GNOME_IS_CANVAS_GROUP_EXT (printable));
	for (list = GNOME_CANVAS_GROUP (printable) ->item_list; list; list =
list->next) {
		item = GNOME_CANVAS_ITEM (list->data);
		if (!(item->object.flags & GNOME_CANVAS_ITEM_VISIBLE))
			continue;
		if (GNOME_IS_CANVAS_GROUP_EXT(item))
			gnome_canvas_group_ext_print (G_PRINTABLE (item), pc);
		else if (G_IS_PRINTABLE (item))
		{
			gnome_canvas_item_i2w_affine (item, affine);
			gnome_print_gsave(pc);
			gnome_print_concat(pc, affine);
			g_printable_print (G_PRINTABLE (item), pc); 
			gnome_print_grestore(pc);
		}
	}
}