static void
multi_device_event_cb (GtkWidget           *widget,
                       GtkDeviceGroup      *group,
                       GtkMultiDeviceEvent *event,
		       gpointer             user_data)
{
  gdouble center_x, center_y, angle;

  if (event->n_events != 2)
    return;

  gdk_events_get_center ((GdkEvent *) event->events[0],
                         (GdkEvent *) event->events[1],
                         &center_x, &center_y);

  gdk_events_get_angle ((GdkEvent *) event->events[0],
                        (GdkEvent *) event->events[1],
                        &angle);
  ...
}

static gboolean
button_cb (GtkWidget *widget,
           GdkEventButton *event,
           gpointer        user_data)
{
  GdkDevice *device;
  MyWidgetPrivate *private_data;

  device = gdk_event_get_device ((GdkEvent *) event);
  private_data = MY_WIDGET (widget)->private;

  if (event->type == GDK_BUTTON_PRESS)
    gtk_device_group_add_device (private_data->group, device);
  else if (event->type == GDK_BUTTON_RELEASE)
    gtk_device_group_remove_device (private_data->group, device);

  return TRUE;
}

static void
my_widget_init (GtkWidget *widget)
{
  MyWidgetPrivate *private_data;

  private_data = MY_WIDGET (widget)->private;
  private_data->group = gtk_widget_create_device_group (widget);

  g_signal_connect (widget, "multidevice-event",
                    G_CALLBACK (multidevice_cb), NULL);

  g_signal_connect (widget, "button-press-event",
                    G_CALLBACK (button_cb), NULL);
  g_signal_connect (widget, "button-release-event",
                    G_CALLBACK (button_cb), NULL);
  ...
}