APTR Func(struct Hook *hook, APTR slider, struct TagItem *tags);
The object is the slider gadget. The message is a tag list with GA_ID, SLIDER_Level, SLIDER_Min and SLIDER_Max. The hook should return a type which matches the format specifier. For example, a LONG when SLIDER_LevelFormat contains "%ld" or a CONST_STRPTR when SLIDER_LevelFormat contains "%s".