React multiple contexts

Another solution is to create a separate context providing the other contexts:

import React, { createContext, memo, useContext } from "react";
import isEqual from "react-fast-compare";

export const MultiContext = createContext(null);
MultiContext.displayName = "MultiContext";

export const MultiContextProvider = memo(
  function({ map, children }) {
    const contextMap = {};
    for (const i in map) {
      contextMap[i] = useContext(map[i]);
    }

    return (
      <MultiContext.Provider value={contextMap}>
        {children}
      </MultiContext.Provider>
    );
  },
  (prevProps, nextProps) => isEqual(prevProps.children, nextProps.children)
);

MultiContextProvider.displayName = "MultiContextProvider";

Example usage:

class DemoConsumer extends React.Component {
  static contextType = MultiContext;

  render() {
    return JSON.stringify({
      someValue: this.context.SomeContext.someValue,
      otherValue: this.context.OtherContext.otherValue,
    });
  }
}

function App() {
  return (
    <MultiContextProvider map={{ SomeContext, OtherContext }}>
      <MultiContextDemoClassConsumer />
    </MultiContextProvider>
  );
}

Demo:

const {
  createContext,
  memo,
  useContext,
  useState,
  useEffect,
} = React;

const MultiContext = createContext(null);
MultiContext.displayName = "MultiContext";

const MultiContextProvider = memo(
  function({ map, children }) {
    console.log("render provider");
    const contextMap = {};
    for (const i in map) {
      contextMap[i] = useContext(map[i]);
    }

    return (
      <MultiContext.Provider value={contextMap}>
        {children}
      </MultiContext.Provider>
    );
  },
  (prevProps, nextProps) => isEqual(prevProps.children, nextProps.children)
);
MultiContextProvider.displayName = "MultiContextProvider";

const initialMinutes = new Date().getMinutes();
const MinutesContext = createContext(initialMinutes);
MinutesContext.displayName = "MinutesContext";

const IncrementContext = createContext(0);
IncrementContext.displayName = "IncrementContext";

class MultiContextDemoClassConsumer extends React.Component {
  static contextType = MultiContext;

  render() {
    return JSON.stringify(this.context);
  }
}

const multiContextMap = { MinutesContext, IncrementContext };
function App() {
  const forceUpdate = useForceUpdate();

  const [minutes, setMinutes] = useState(initialMinutes);
  useEffect(() => {
    const timeoutId = setInterval(() => {
      // console.log('set minutes')
      setMinutes(new Date().getMinutes());
    }, 1000);
    return () => {
      clearInterval(timeoutId);
    };
  }, [setMinutes]);

  const [increment, setIncrement] = useState(0);

  console.log("render app");

  return (
    <MinutesContext.Provider value={minutes}>
      <IncrementContext.Provider value={increment}>
        <MultiContextProvider map={multiContextMap}>
          <MultiContextDemoClassConsumer />
        </MultiContextProvider>
        <button onClick={() => setIncrement(i => i + 1)}>Increment</button>
        <button onClick={forceUpdate}>Force Update</button>
      </IncrementContext.Provider>
    </MinutesContext.Provider>
  );
}

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

<script type="module">
  import React from 'https://dev.jspm.io/react@16';
  import ReactDOM from 'https://dev.jspm.io/react-dom@16';
  import useForceUpdate from 'https://dev.jspm.io/[email protected]';
  import isEqual from 'https://dev.jspm.io/[email protected]';
  window.React = React;
  window.ReactDOM = ReactDOM;
  window.useForceUpdate = useForceUpdate.default;
  window.isEqual = isEqual;
</script>
<div id="root"></div>

You can still use function-as-a-child consumer nodes with the 16.3 Context API, which is what the React documentation suggests doing:

// Theme context, default to light theme
const ThemeContext = React.createContext('light');

// Signed-in user context
const UserContext = React.createContext({
  name: 'Guest',
});

class App extends React.Component {
  render() {
    const {signedInUser, theme} = this.props;

    // App component that provides initial context values
    return (
      <ThemeContext.Provider value={theme}>
        <UserContext.Provider value={signedInUser}>
          <Layout />
        </UserContext.Provider>
      </ThemeContext.Provider>
    );
  }
}

function Layout() {
  return (
    <div>
      <Sidebar />
      <Content />
    </div>
  );
}

// A component may consume multiple contexts
function Content() {
  return (
    <ThemeContext.Consumer>
      {theme => (
        <UserContext.Consumer>
          {user => (
            <ProfilePage user={user} theme={theme} />
          )}
        </UserContext.Consumer>
      )}
    </ThemeContext.Consumer>
  );
}

To use functions in context in your component you'd typically wrap your component in a HOC so the context is passed in as props:

export const withThemeContext = Component => (
  props => (
    <ThemeContext.Consumer>
      {context => <Component themeContext={context} {...props} />}
    </ThemeContext.Consumer>
  )
)

const YourComponent = ({ themeContext, ...props }) => {
  themeContext.someFunction()
  return (<div>Hi Mom!</div>)
}

export default withThemeContext(YourComponent)

If you're running React 16.8+ you can also use hooks to do this more cleanly without using HOCs:

import React, { useContext } from "react"

const YourComponent = props => {
  const theme = useContext(ThemeContext)
  const user = useContext(UserContext)
}

Or, if you consume these contexts a lot, you can even make a custom hook to simplify further:

const useTheme = () => useContext(ThemeContext)
const useUser = () => useContext(UserContext)

const YourComponent = props => {
  const theme = useTheme()
  const user = useUser()
}

You could also simply merge all your contexts into a single one:

const AppContext = React.createContext({
  user: { name: 'Guest' },
  theme: 'light',
})

ChildComponent.contextType = AppContext;

Done. You simply need to merge the new values if you have a different context in some parts of you app (like a different theme or user).