This is a simple example in which we create a rect and swallow it.

Focusing on the relevant parts of the code we go right to the creation of our rectangle. It should be noted that we don't resize or show our rect, that is because when an object is swallowed it's geometry and visibility is controlled by the theme:

   rect = evas_object_rectangle_add(evas);
   evas_object_color_set(rect, 255, 0, 0, 255);
   edje_object_part_swallow(edje_obj, "part_one", rect);

The other bit of code that is relevant to us now is our check that the swallow worked:

   obj = edje_object_part_swallow_get(edje_obj, "part_one");
   if(obj == rect)
      printf("Swallowing worked!\n");

The full source code follows:

#ifdef HAVE_CONFIG_H
# include "config.h"
#else
# define EINA_UNUSED
#endif

#ifndef PACKAGE_DATA_DIR
#define PACKAGE_DATA_DIR "."
#endif

#include <Ecore.h>
#include <Ecore_Evas.h>
#include <Edje.h>

#define WIDTH  (300)
#define HEIGHT (300)

static void
_on_delete(Ecore_Evas *ee EINA_UNUSED)
{
   ecore_main_loop_quit();
}

/* here just to keep our example's window size and background image's
 * size in synchrony */
static void
_on_canvas_resize(Ecore_Evas *ee)
{
   Evas_Object *bg;
   int          w;
   int          h;

   bg = ecore_evas_data_get(ee, "background");
   ecore_evas_geometry_get(ee, NULL, NULL, &w, &h);
   evas_object_resize(bg, w, h);
}

int
main(int argc EINA_UNUSED, char *argv[] EINA_UNUSED)
{
   const char  *edje_file = PACKAGE_DATA_DIR"/swallow.edj";
   Ecore_Evas  *ee;
   Evas        *evas;
   Evas_Object *bg;
   Evas_Object *rect;
   Evas_Object *obj;
   Evas_Object *edje_obj;

   if (!ecore_evas_init())
     return EXIT_FAILURE;

   if (!edje_init())
     goto shutdown_ecore_evas;

   /* this will give you a window with an Evas canvas under the first
    * engine available */
   ee = ecore_evas_new(NULL, 0, 0, WIDTH, HEIGHT, NULL);
   if (!ee) goto shutdown_edje;

   ecore_evas_callback_delete_request_set(ee, _on_delete);
   ecore_evas_callback_resize_set(ee, _on_canvas_resize);
   ecore_evas_title_set(ee, "Edje Swallow Example");

   evas = ecore_evas_get(ee);

   bg = evas_object_rectangle_add(evas);
   evas_object_color_set(bg, 255, 255, 255, 255); /* white bg */
   evas_object_move(bg, 0, 0); /* at canvas' origin */
   evas_object_resize(bg, WIDTH, HEIGHT); /* covers full canvas */
   evas_object_show(bg);
   ecore_evas_data_set(ee, "background", bg);

   edje_obj = edje_object_add(evas);

   edje_object_file_set(edje_obj, edje_file, "example_group");
   evas_object_move(edje_obj, 20, 20);
   evas_object_resize(edje_obj, WIDTH - 40, HEIGHT - 40);
   evas_object_show(edje_obj);

   rect = evas_object_rectangle_add(evas);
   evas_object_color_set(rect, 255, 0, 0, 255);
   edje_object_part_swallow(edje_obj, "part_one", rect);

   obj = edje_object_part_swallow_get(edje_obj, "part_one");
   if(obj == rect)
      printf("Swallowing worked!\n");

   ecore_evas_show(ee);

   ecore_main_loop_begin();

   ecore_evas_free(ee);
   ecore_evas_shutdown();
   edje_shutdown();

   return EXIT_SUCCESS;

 shutdown_edje:
   edje_shutdown();
 shutdown_ecore_evas:
   ecore_evas_shutdown();

   return EXIT_FAILURE;
}

To compile use this command:

 * gcc -o edje-swallow edje-swallow.c -DPACKAGE_BIN_DIR=\"/Where/enlightenment/is/installed/bin\" -DPACKAGE_LIB_DIR=\"/Where/enlightenment/is/installed/lib\"
 * -DPACKAGE_DATA_DIR=\"/Where/enlightenment/is/installed/share\"
 * `pkg-config --cflags --libs evas ecore ecore-evas edje`
 *
 * edje_cc swallow.edc
 *