Custom react hook can not be called in useEffect with empty dependencies - reactjs

Question

Im new to react hook, Im doing a project with new feature "Hooks" of react.
I've faced a problem and I need an explain for it.
As document, to implement "componentDidMount", just pass empty array in dependencies argument.
useEffect(() => {
// some code here
}, []);
And I can call dispatch function to updateState inside this useEffect.
const [flag, setFlag] = useState(false);
useEffect(() => {
setFlag(true);
}, []);
Above code works perfectly without warning or any errors.
Now I have my custom hook, but I can not call my dispatch inside the effect.
const [customFlag, setCustomFlag] = useCustomHook();
useEffect(() => {
setCustomFlag(true);
}, []);
This is my custom hook.
function useCustomHook() {
const [success, setSuccess] = useState(false):
const component = <div>{ success ? "Success" : "Fail" }</div>;
const dispatch = useCallback(success => {
setSuccess(success);
}, []);
return [component, dispatch];
}
With above code, it requires me to put setCustomFlag inside the dependencies array.
I do not understand why. What is different between them?
Thanks for sharing.

Answer 1

Probably, your custom hook returns different instance of setCustomFlag on each call. It means, that useEffect() will always use first value (returned on first render). Try to memoize it by calling useCallback()/useMemo() hooks:
function useCustomHook() {
...
const setCustomFlag = useCallback(/* setCustomFlag body here */, []);
}
It would be nice to have your custom hook source to say more.

Answer 2

The reason is setFlag from useState is a known dependency, its value won't change between render, hence you don't have to declare it as a dependency
React eslint-plugin-react-hooks can't be sure about your custom hook, that's why you need to put that into the dependency list
This is taken from eslint-plugin-react-hooks
// Next we'll define a few helpers that helps us
// tell if some values don't have to be declared as deps.
// Some are known to be static based on Hook calls.
// const [state, setState] = useState() / React.useState()
// ^^^ true for this reference
// const [state, dispatch] = useReducer() / React.useReducer()
// ^^^ true for this reference
// const ref = useRef()
// ^^^ true for this reference
// False for everything else.
function isStaticKnownHookValue(resolved) {
}