React createContext issue in Typescript?

问题

So I'm having a very weird issue with React Context + Typescript.

Working example

In the above example, you can see what I'm trying to do actually work. Essentially I'm managing state with the new useContext method, and it works perfectly.

However, when I try to do this on my box, it cannot seem to find the state values being passed through the useReducer.

export function AdminStoreProvider(props: any) {
const [state, dispatch] = useReducer(reducer, initialState);
// state.isAuth is avail here
// state.user is avail here
const value = { state, dispatch };
// value.state.isAuth is avail here
return (
    /* value does not contain state once applied to the value prop */
    <AdminStore.Provider value={value}>{props.children} 
    </AdminStore.Provider>
   );
}

Error message:

Type '{ state: { isAuth: boolean; user: string; }; dispatch: 
Dispatch<Actions>; }' is missing the following properties from type 
'IState': isAuth, user

Keep in mind the code I'm using is exactly what I'm using on my box, I've even downloaded the code from sandbox and tried running it, and it doesn't work.

I'm using VSCode 1.31

I've managed to deduce that if I change how I create my context from:

export const AdminStore = React.createContext(initialState);

to

export const AdminStore = React.createContext(null);

The value property no longer throws that error.

However, now useContext returns an error: state doesn't exist on null. And same if I set defaultState for context to {}.

And of course if I

React.createContext();  

Then TS yells about no defaultValue being provided.

In sandbox, all 3 versions of creating the context object work fine.

Thanks in advance for any advice.

回答1:

It appears defaultValue value for React.createContext is expected to be of type:

interface IContextProps {
  state: IState;
  dispatch: ({type}:{type:string}) => void;
}

Once Context object is created for this type, for example like this:

export const AdminStore = React.createContext({} as IContextProps);

Provider React component should no longer complain about the error.

Here is the list of changes:

admin-store.tsx

import React, { useReducer } from "react";
import { initialState, IState, reducer } from "./reducer";


interface IContextProps {
  state: IState;
  dispatch: ({type}:{type:string}) => void;
}


export const AdminStore = React.createContext({} as IContextProps);

export function AdminStoreProvider(props: any) {
  const [state, dispatch] = useReducer(reducer, initialState);

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

回答2:

I had a fun time with this so I figured I'd share what I came up with.

The SidebarProps represent the context's state. Everything else, besides the reducer actions, can essentially be used as is.

import React, { createContext, Dispatch, Reducer, useContext, useReducer } from 'react';

interface Actions {
    type: string;
    value: any;
}

interface SidebarProps {
    show: boolean;
    content: JSX.Element | null;
}

interface SidebarProviderProps {
    reducer: Reducer<SidebarProps, Actions>;
    initState: SidebarProps;
}

interface InitContextProps {
    state: SidebarProps;
    dispatch: Dispatch<Actions>;
}

export const SidebarContext = createContext({} as InitContextProps);
export const SidebarProvider: React.FC<SidebarProviderProps> = ({ reducer, initState, children }) => {
    const [state, dispatch] = useReducer(reducer, initState);
    const value = { state, dispatch };
    return (
        <SidebarContext.Provider value={value}>
            {children}
        </SidebarContext.Provider>
    );
};
export const useSidebar = () => useContext(SidebarContext);

const SidebarController: React.FC = ({ children }) => {
    const initState: SidebarProps = {
        show: false,
        content: null
    };

    const reducer: Reducer<SidebarProps, Actions> = (state, action) => {
        switch (action.type) {
            case 'setShow':
                return {
                    ...state,
                    show: action.value
                };

            case 'setContent':
                return {
                    ...state,
                    content: action.value
                };

            default:
                return state;
        }
    };

    return (
        <SidebarProvider reducer={reducer} initState={initState}>
            {children}
        </SidebarProvider>
    );
};

export default SidebarController;

来源：https://stackoverflow.com/questions/54577865/react-createcontext-issue-in-typescript