// each object can overwrite following function
void (*_draw_foreground)(void *obj, cairo_t *c);
- // calculate size. return '1' if size changed, '0' otherwise.
+ // update area's content and update size (width/heith).
+ // return '1' if size changed, '0' otherwise.
int (*_resize)(void *obj);
void (*_add_child)(void *obj);
int (*_remove_child)(void *obj);