Many React applications follow the Flux architecture popularised by Redux. This setup can be characterised by a few key ideas:

1. It uses a single object at the top of your app which stores all application state, often called the store.
2. It provides a single dispatch function which can be used to send messages up to the store. Redux calls these actions, but I'll be calling them events - as they're known in XState.
3. How the store responds to these messages from the app are expressed in pure functions - most often in reducers.

This article won't go into depth on whether the Flux architecture is a good idea. David Khourshid's article Redux is half a pattern goes into great detail here. For the purposes of this article, we're going to assume that you like having a global store, and you want to replicate it in XState.

There are many reasons for wanting to do so. XState is second-to-none when it comes to managing complex asynchronous behaviour and modelling difficult problems. Managing this in Redux apps usually involves middleware: either redux-thunk, redux-loop or redux-saga. Choosing XState gives you a first-class way to manage complexity.

A globally available store

To mimic Redux's globally-available store, we're going to use React context. React context can be a tricky tool to work with - if you pass in values which change too often, in can result in re-renders all the way down the tree. That means we need to pass in values which change as little as possible.

Luckily, XState gives us a first-class way to do that.

import React, { createContext } from 'react';
import { useInterpret } from '@xstate/react';
import { authMachine } from './authMachine';
import { ActorRefFrom } from 'xstate';

interface GlobalStateContextType {
  authService: ActorRefFrom<typeof authMachine>;
}

export const GlobalStateContext = createContext(
  // Typed this way to avoid TS errors,
  // looks odd I know
  {} as GlobalStateContextType,
);

export const GlobalStateProvider = (props) => {
  const authService = useInterpret(authMachine);

  return (
    <GlobalStateContext.Provider value={{ authService }}>
      {props.children}
    </GlobalStateContext.Provider>
  );
};

Using useInterpret returns a service, which is a static reference to the running machine which can be subscribed to. This value never changes, so we don't need to worry about wasted re-renders.

Utilising context

Further down the tree, you can subscribe to the service like this:

import React, { useContext } from 'react';
import { GlobalStateContext } from './globalState';
import { useActor } from '@xstate/react';

export const SomeComponent = (props) => {
  const globalServices = useContext(GlobalStateContext);
  const [state] = useActor(globalServices.authService);

  return state.matches('loggedIn') ? 'Logged In' : 'Logged Out';
};

The useActor hook listens for whenever the service changes, and updates the state value.

Improving Performance

There's an issue with the implementation above - this will update the component for any change to the service. Redux offers tools for deriving state using selectors - functions which restrict which parts of the state can result in components re-rendering.

Luckily, XState provides that too.

import React, { useContext } from 'react';
import { GlobalStateContext } from './globalState';
import { useSelector } from '@xstate/react';

const selector = (state) => {
  return state.matches('loggedIn');
};

export const SomeComponent = (props) => {
  const globalServices = useContext(GlobalStateContext);
  const isLoggedIn = useSelector(globalServices.authService, selector);

  return isLoggedIn ? 'Logged In' : 'Logged Out';
};

Now, this component will only re-render when state.matches('loggedIn') returns a different value. This is my recommended approach over useActor for when you want to optimise performance. Dispatching events

For dispatching events to the global store, you can call a service's send function directly.

import React, { useContext } from 'react';
import { GlobalStateContext } from './globalState';

export const SomeComponent = (props) => {
  const globalServices = useContext(GlobalStateContext);

  return (
    <button onClick={() => globalServices.authService.send('LOG_OUT')}>
      Log Out
    </button>
  );
};

Note that you don't need to call useActor for this, it's available right on the context.

Deviations from Flux

Keen-eyed readers may spot that this implementation is slightly different from Flux. For instance - instead of a single global store, one might have several running machines at once: authService, dataCacheService, and globalTimeoutService. Each of them have their own send attributes, too - so you're not calling a global dispatch.

These changes can be worked around. One could create a synthetic send inside the global store which called all the services' send function manually. But personally, I prefer knowing exactly which services my messages are being passed to, and it avoids having to keep events globally namespaced.

Summary

XState can work beautifully as a global store for a React application. It keeps application logic co-located, treats side effects as first-class citizens, and offers good performance with useSelector. You should choose this approach if you're keen on the Flux architecture but feel your app's logic is getting out of hand.