Callback helpers inteded for widget implementations. 


/* Generic callback sets; shouldn't be used directly by the application but
   through widget specific callback set functions,
   e.g. mbtk_button_callback_click()  */
void mbtk_callback_set_noarg(mbtk_widget_t *w, mbtk_kw_t id, mbtk_event_handled_t (*cb)(mbtk_widget_t *w, mbtk_kw_t id, void *user_data), void *user_data);
void mbtk_callback_set_event(mbtk_widget_t *w, mbtk_kw_t id, mbtk_event_handled_t (*cb)(mbtk_widget_t *w, mbtk_kw_t id, mbtk_event_t *ev, void *user_data), void *user_data);

/* Helper functions to perform a callback on a widget. Calls widget's
   callbeck named in id with the arguments passed (user_data passed on
   callback registration is appended automatically to the arg list). Returns
   what the user callback returns; if user callback is not bound, returns
   MBTK_EVENT_NOT_HANDLED. */
MBTK_INLINE mbtk_event_handled_t mbtk_callback_noarg(mbtk_widget_t *widget, mbtk_kw_t id);
MBTK_INLINE mbtk_event_handled_t mbtk_callback_event(mbtk_widget_t *widget, mbtk_kw_t id, mbtk_event_t *ev);