The callback can be of string, symbol, or function object type.
string这样写:
?modifyCallback "test"
...
procedure(test(filedName lastValue changeMode)
...
t | nil | value
)
t的时候callback生效修改有效,nil的时候callback生效修改无效,value的时候是这样定义的:If the function returns some other value, it must be a string. This value replaces the current value of the field.