pvanhoof@tinc:/tmp/vtest$ /opt/vala/bin/valac thing.vala implementer.vala interface.vala --pkg=gee-1.0
implementer.c: In function ‘implementer_interface_interface_init’:
implementer.c:44: error: ‘implementer_real_get_keywords’ undeclared (first use in this function)
implementer.c:44: error: (Each undeclared identifier is reported only once
implementer.c:44: error: for each function it appears in.)
error: cc exited with status 256
Compilation failed: 1 error(s), 0 warning(s)
pvanhoof@tinc:/tmp/vtest$ /opt/vala/bin/valac -C thing.vala implementer.vala interface.vala --pkg=gee-1.0
pvanhoof@tinc:/tmp/vtest$ cat implementer.c 

#include "implementer.h"
#include <gee/collection.h>




enum  {
	IMPLEMENTER_DUMMY_PROPERTY
};
static gpointer implementer_parent_class = NULL;
static InterfaceIface* implementer_interface_parent_iface = NULL;



void implementer_add_keyword (Implementer* self, const char* keyword) {
	g_return_if_fail (IS_IMPLEMENTER (self));
	g_return_if_fail (keyword != NULL);
	gee_collection_add (GEE_COLLECTION (interface_get_keywords (INTERFACE (self))), keyword);
}


void implementer_remove_keyword (Implementer* self, const char* keyword) {
	g_return_if_fail (IS_IMPLEMENTER (self));
	g_return_if_fail (keyword != NULL);
	gee_collection_remove (GEE_COLLECTION (interface_get_keywords (INTERFACE (self))), keyword);
}


Implementer* implementer_new (void) {
	Implementer * self;
	self = g_object_newv (TYPE_IMPLEMENTER, 0, NULL);
	return self;
}


static void implementer_class_init (ImplementerClass * klass) {
	implementer_parent_class = g_type_class_peek_parent (klass);
}


static void implementer_interface_interface_init (InterfaceIface * iface) {
	implementer_interface_parent_iface = g_type_interface_peek_parent (iface);
	iface->get_keywords = implementer_real_get_keywords;
}


static void implementer_instance_init (Implementer * self) {
}


GType implementer_get_type (void) {
	static GType implementer_type_id = 0;
	if (implementer_type_id == 0) {
		static const GTypeInfo g_define_type_info = { sizeof (ImplementerClass), (GBaseInitFunc) NULL, (GBaseFinalizeFunc) NULL, (GClassInitFunc) implementer_class_init, (GClassFinalizeFunc) NULL, NULL, sizeof (Implementer), 0, (GInstanceInitFunc) implementer_instance_init };
		static const GInterfaceInfo interface_info = { (GInterfaceInitFunc) implementer_interface_interface_init, (GInterfaceFinalizeFunc) NULL, NULL};
		implementer_type_id = g_type_register_static (TYPE_THING, "Implementer", &g_define_type_info, 0);
		g_type_add_interface_static (implementer_type_id, TYPE_INTERFACE, &interface_info);
	}
	return implementer_type_id;
}




pvanhoof@tinc:/tmp/vtest$ cat *vala
public class Implementer : Thing, Interface {
	public void add_keyword (string keyword) {
		this.keywords.add (keyword);
	}

	public void remove_keyword (string keyword) {
		this.keywords.remove (keyword);
	}
}

public interface Interface {
	public abstract Gee.ArrayList<string> keywords { get; }
}

public class Thing : GLib.Object {

	private Gee.ArrayList<string> mkeywords = null;
	public Gee.ArrayList<string> keywords { 
		get {
			if (mkeywords == null)
				mkeywords = new Gee.ArrayList<string> (GLib.str_equal);
			return mkeywords;
		}
	}

	static void main (string [] args) {
	}
}

pvanhoof@tinc:/tmp/vtest$