GimpConstraintFunc

When you have have a drawable menu created for you, GIMP makes use of a constraint function to ask you if what should be listed in the menu. This way you can filter out drawables that are the wrong size, image, type, etc. From gimpmenu.h:

typedef gint (*GimpConstraintFunc) (gint32   image_id,
                                    gint32   drawable_id,
                                    gpointer data);
GtkWidget* gimp_drawable_menu_new (GimpConstraintFunc constraint,
                                   GimpMenuCallback   callback,
                                   gpointer           data,
                                   gint32             active_drawable);
	

For every image, _menu_new calls the constraint function with the corresponding image ID and a drawable ID of -1. If the constraint function likes that image and returns TRUE, it's then called and asked about each drawable in the image, one at a time. If the function returns TRUE again, the drawable is added to the menu.