无法读取上下文提供程序中 useReducer 钩子更新的状态

Question

I am using useReducer hook to manage my state, but it seems like I have a problem with reading updated state in my context provider.

My context provider is responsible to fetch some remote data and update the state based on responses:

import React, { useEffect } from 'react';
import useAppState from './useAppState';


export const AppContext = React.createContext();

const AppContextProvider = props => {
  const [state, dispatch] = useAppState();

  const initialFunction = () => {
    fetch('/some_path')
      .then(res => {
        dispatch({ type: 'UPDATE_STATE', res });
      });
  };

  const otherFunction = () => {
    fetch('/other_path')
      .then(res => {
        // why is `state.stateUpdated` here still 'false'????
        dispatch({ type: 'DO_SOMETHING_ELSE', res });
      });
    }
  };

  const actions = { initialFunction, otherFunction };

  useEffect(() => {
    initialFunction();
    setInterval(otherFunction, 30000);
  }, []);

  return (
    <AppContext.Provider value={{ state, actions }}>
      {props.children}
    </AppContext.Provider>
  )
};

export default AppContextProvider;

and useAppState.js is very simple as:

import { useReducer } from 'react';


const useAppState = () => {
  const reducer = (state, action) => {
    switch (action.type) {
      case 'UPDATE_STATE':
        return {
          ...state,
          stateUpdated: true,
        };
      case 'DO_SOMETHING_ELSE':
        return {
          ...state,
          // whatever else
        };
      default:
        throw new Error();
    }
  };


  const initialState = { stateUpdated: false };

  return useReducer(reducer, initialState);
};


export default useAppState;

The question is, as stated in the comment above, why is state.stateUpdated in context provider's otherFunction still false and how could I access state with latest changes in the same function?

Answer 1

state will never change in that function

The reason state will never change in that function is that state is only updated on re-render. Therefore, if you want to access state you have two options:

1. useRef to see a future value of state (you'll have to modify your reducer to make this work)

const updatedState = useRef(initialState);
const reducer = (state, action) => {
  let result;
  // Do your switch but don't return, just modify result

  updatedState.current = result;
  return result;
};

return [...useReducer(reducer, initialState), updatedState];

2. You could reset your setInterval after every state change so that it would see the most up-to-date state. However, this means that your interval could get interrupted a lot.

const otherFunction = useCallback(() => {
  fetch('/other_path')
    .then(res => {
      // why is `state.stateUpdated` here still 'false'????
      dispatch({ type: 'DO_SOMETHING_ELSE', res });
    });
  }
}, [state.stateUpdated]);

useEffect(() => {
  const id = setInterval(otherFunction, 30000);
  return () => clearInterval(id);
}, [otherFunction]);