собственный redux, как написать useSelector?

const createStore = (reducer, initialState) => {
  let currentState = initialState
  let currentReducer = reducer
  var listeners = []
 
  const getState = () => currentState
 
  const dispatch = action => {
    currentState = reducer(currentState, action)
    listeners.forEach(listener => listener())
    return action
  }
 
  const subscribe = listener => listeners.push(listener)
  
 
  return { getState, dispatch, subscribe }
}
 
const Context = React.createContext(null);
 
const useSelector = selector => {
  const ctx = React.useContext(Context)
  
  if (!ctx) {
    return 0
  }
  return selector(ctx.store.getState()) 
}
const useDispatch = (action) => {
  const ctx = React.useContext(Context)
 
  if (!ctx) {
    return () => {}
  }
 
  return ctx.store.dispatch
}
 
const Provider = ({ store, children }) => {
  return <Context.Provider value={{ store }}>{children}</Context.Provider>
}
 
// APP
 
// actions
const UPDATE_COUNTER = 'UPDATE_COUNTER'
const CHANGE_STEP_SIZE = 'CHANGE_STEP_SIZE'
 
// action creators
const updateCounter = value => ({
  type: UPDATE_COUNTER,
  payload: value,
})
 
const changeStepSize = value => ({
  type: CHANGE_STEP_SIZE,
  payload: value,
})
 
 
// reducers
const defaultState = {
  counter: 1,
  stepSize: 1,
}
 
const reducer = (state = defaultState, action) => {
  switch(action.type) {
    case UPDATE_COUNTER:
      return { ...state, counter: state.counter += action.payload }
    case CHANGE_STEP_SIZE:
        return { ...state, stepSize: action.payload }
    default:
      return state
  }
}
 
const Counter = () => {
  const counter = useSelector(state => state.counter)
  const dispatch = useDispatch()
 
  return (
    <div>
      <button onClick={() => dispatch(updateCounter(-1))}>-</button>
      <span> {counter} </span>
      <button onClick={() => dispatch(updateCounter(1))}>+</button>
    </div>
  )
}
 
const Step = () => {
  const stepSize = useSelector(state => state.stepSize, (current, prev) => current === prev)
  const dispatch = useDispatch()
 
  return (
    <div>
      <div>Значение счётчика должно увеличиваться или уменьшаться на заданную величину шага</div>
      <div>Текущая величина шага: {stepSize}</div>
      <input
        type="range"
        min="1"
        max="5"
        value={stepSize}
        onChange={({ target }) => dispatch(changeStepSize(target.value))}
      />
    </div>
  )
}
 
const store = createStore(reducer, defaultState);
 
ReactDOM.render(
  <Provider store={store}>
      <Step />
      <Counter />
  </Provider>,
  document.getElementById('app')
)