How to acess React context from Apollo set Context Http Link

Question

I am trying to access a react context values within the setContext function for my Apollo client. I would like to be able to dynamically update the header for each graphql request with the react context value. But I face an error with no visible error messages in the logs. Is what I am trying to do possible?

import React, { useState, useContext } from "react";
import { render } from "react-dom";

import ApolloClient from "apollo-client";
import { ApolloProvider } from "react-apollo";
import { createHttpLink } from "apollo-link-http";
import { setContext } from "apollo-link-context";
import { InMemoryCache } from "apollo-cache-inmemory";

import Select from "./Select";
import CurrencyContext from "./CurrencyContext";
import ExchangeRates from "./ExchangeRates";

const httpLink = createHttpLink({
  uri: "https://48p1r2roz4.sse.codesandbox.io"
});

const authLink = setContext((_, { headers }) => {

  const token = localStorage.getItem("token");


  const currency = useContext(CurrencyContext); // How to access React context here ?


  return {
    headers: {
      ...headers,
      authorization: token ? `Bearer ${token}` : "",
      currencyContext: currency ? currency : {}
    }
  };
});

const client = new ApolloClient({
  link: authLink.concat(httpLink),
  cache: new InMemoryCache()
});

const currencies = ["USD", "EUR", "BTC"];

const App = () => {
  const [currency, setCurrency] = useState("USD");

  return (
    <ApolloProvider client={client}>
      <CurrencyContext.Provider value={currency}>
        <h2>Provide a Query variable from Context 🚀</h2>
        <Select value={currency} setValue={setCurrency} options={currencies} />
        <ExchangeRates />
      </CurrencyContext.Provider>
    </ApolloProvider>
  );
};

render(<App />, document.getElementById("root"));

Answer 1

You can use useImperativeHandle to access the context values from outside React tree In the context file create a ref

export const ContextRef = React.createRef();

Then inside the Context add

React.useImperativeHandle(ContextRef, () => contextValues);

Finally, you can access the context values with

 ContextRef.current.token

See: https://reactjs.org/docs/hooks-reference.html#useimperativehandle