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

[Ben Johnson <ben blarg net>]

On Sat, Jan 29, 2005 at 07:47:08PM +0100, Jean Br?fort wrote:
> Le samedi 29 janvier 2005 ?? 10:10 -0800, Ben Johnson a ??crit :
> > 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);
> 		}
> 	}
> }
> 
> 
> 

excellent!  I think I'm understanding this well enough now.  I have a
couple more questions...

Is GPrintable a completely new type that you defined?  I haven't been
able to find it anywhere else.

There is one thing that's confusing me.  From your definitions earlier I
gathered that an object of GnomeCanvasGroupExtClass is by definition
also a G_TYPE_PRINTABLE.  In the code you just posted...

> 		if (GNOME_IS_CANVAS_GROUP_EXT(item))
> 			gnome_canvas_group_ext_print (G_PRINTABLE (item), pc);
> 		else if (G_IS_PRINTABLE (item))
> 		{

I'm assuming that GNOME_IS_CANVAS_GROUP_EXT(item) tests to see if item
is an object of the GnomeCanvasGroupExtClass class and G_IS_PRINTABLE()
tests to see if item is a G_TYPE_PRINTABLE.  If one is true, isn't the
other?

I'm looking at the GTK_TYPE_EDITABLE interface now as it is used in
GtkEntryClass definition, which is helping a lot too.  I see that the
GtkEditableClass is a lot like any other class but its member functions
have no definition...  so if thing declared itself to be GTK_TYPE_EDITABLE 
but didn't set those function pointers it should probably cause segfaults.

This is very cool.  Thank again.

- Ben