如何使用 API 响应设置 useReducer 初始 state？

Question

I'm new to React and trying to build an app with a cart feature where users can add robots to the cart. I'm using Context API to provide cart across the app and useReducer for states. The robots are being fetched from a server and load just fine. But I can't seem to find a way to set the initial state of the reducer using fetched products. The state always returns an empty array for robots. How to solve this?

const CartProvider = ({ children }) => {
    const [robots, setRobots] = useState([]);

    useEffect(() => {
        fetch('http://localhost:8000/api/robots')
            .then(res => res.json())
            .then(data => setRobots(data?.data))
    }, [])

    const initialState = {
        robots: [...robots],
        cart: []
    }
    const [state, dispatch] = useReducer(cartReducer, initialState);

    return (
        <CartContext.Provider value={{ state, dispatch }}>
            {children}
        </CartContext.Provider>
    );
}
export default CartProvider;

Answer 1

const [state, dispatch] = useReducer(cartReducer, initialState);

useReducer will not monitor changes to initialState , it will only consider its initial value. You can just dispatch an action to update the state once the data is fetched.

const initialState = {
    robots: [],
    cart: []
}

const CartProvider = ({ children }) => {
    const [state, dispatch] = useReducer(cartReducer, initialState);

    useEffect(() => {
        fetch('http://localhost:8000/api/robots')
            .then(res => res.json())
            .then(data => {
                // dispatch action to update the reducer state 
            })
    }, [])
    

    return (
        <CartContext.Provider value={{ state, dispatch }}>
            {children}
        </CartContext.Provider>
    );
}
export default CartProvider;

If the nested components don't work well until the state is populated then you can conditionally render the provider:

  return (
    <>
      {state.robots.length && (
        <CartContext.Provider value={{ state, dispatch }}>
          {children}
        </CartContext.Provider>
      )}
    </>
  );

Answer 2

Since the initial state value is fetched asynchronously you'll need to dispatch an action to set the state value.

Example:

const cartReducer = (state, action) => {
  switch(action.type) {
    ... other reducer cases ...

    case 'INITIALIZE_CART': 
      return action.payload;

    default:
      return state;
  }
};

CartProvider

const initialState = {
  robots: [],
  cart: []
};

const CartProvider = ({ children }) => {
  const [robots, setRobots] = useState([]);

  const [state, dispatch] = useReducer(cartReducer, initialState);

  useEffect(() => {
    fetch('http://localhost:8000/api/robots')
      .then(res => res.json())
      .then(data => {
        dispatch({
          type: 'INITIALIZE_CART',
          payload: {
            ...initialState,
            robots: data?.data,
          }
        });
      });
  }, []);

  return (
    <CartContext.Provider value={{ state, dispatch }}>
      {children}
    </CartContext.Provider>
  );
};

It's common to also hold some state on the app initialization status. UI conditionally renders based on this state value.

Example:

const initialState = {
  initialized: false,
  robots: [],
  cart: []
};

...

const cartReducer = (state, action) => {
  switch(action.type) {
    ... other reducer cases ...

    case 'INITIALIZE_CART': 
      return {
        ...action.payload,
        initialized: true,
      };

    default:
      return state;
  }
};