XState in Solid

How to use the best state machine library in Solid

Posted in XState, Solid JS, Finite State Machines
December 8, 2022 - 5 min read
XState in Solid

The more I play with Solid, the more I find its abstraction incredibly fascinating. I think it might be as revolutionary as React's "just render everything every time and diff the DOMs" approach was back in 2013. In my own words as a total n00b:

When you write your JSX with conditionals and props that are signals, Solid is able to notice exactly which pieces of your UI depend on which signal value, and can rerender only that dependent UI when the signal value changes.

When I wrote my Proof of Concept using an external state management system with Solid, I realized that I had to communicate via signals, i.e. I had to keep the state I wanted to use in the UI in a signal. This is vaguely analogous to how the way to force a React component to rerender is to change some part of its state with setState().

One important difference in Solid, when coming from React, is realizing that your "render function" (the component body itself) is ONLY CALLED ONCE on first render. All UI updates are dependent on signal changes from inside the JSX.

So let's try and plug my favorite framework agnostic state management library into Solid!


The way you use XState is that you create a machine definition of your finite state machine, and then you "boot up an instance" of that machine using interpret(), which gives you a "service". Once you .start() your service, you can .send() events to it, and generally observe it via listeners or grabbing a snapshot of the state of the service at any given time.


The primary way that you can subscribe to one particular slice of the machine's "context" (where it stores data) is via a "selector", very much like Redux.


The way you determine what state the machine is in is with a .matches() function. From the XState docs:

The state.matches(parentStateValue) method determines whether the current state.value is a subset of the given parentStateValue. The method determines if the parent state value “matches” the state value. For example, assuming the current state.value is { red: 'stop' }:

state.matches("red"); // => true
state.matches("red.stop"); // => true
state.matches({ red: "stop" }); // => true
state.matches("green"); // => false


A predicate that tells you if you "can" send a given event – i.e. that the machine is currently listening for that event.

Quoth the docs:

The state.can(event) method determines whether an event will cause a state change if sent to the interpreted machine. The method will return true if the state will change due to the event being sent; otherwise the method will return false.

This is especially useful for visually disabling buttons that don't do anything right now.


A state machine isn't much good if you can't send events to it, so we're going to need to expose the .send() function as well.


I've written a hook function called useMachine() that takes a state machine definition and then exposes the above API to be consumed in Solid. can, and matches use signals internally, and select returns the actual signal to the data selected. Remember that we must use signals if we want Solid to rerender parts of our UI.

Oh, and it's all strongly typed AF.

This is what it looks like to consume:

import { Component, Match, Switch } from "solid-js";
import { useMachine } from "./useMachine";
import { trafficMachine } from "./trafficMachine";
import "./App.css";
export const App: Component = () => {
const { can, matches, select, send } = useMachine(trafficMachine);
const cycles = select((s) => s.context.cycles); // defines a signal on first render
return (
<div class="app">
<div>Cycles: {cycles()}</div>
<button onClick={() => send("Reset")}>Reset</button>
<Match when={matches("Stopped")}>
<button disabled={!can("Start")} onClick={() => send("Start")}>
<Match when={matches("Started")}>
<button disabled={!can("Stop")} onClick={() => send("Stop")}>
<div class="traffic-light">
<div class={`red ${matches("Started.Red") ? "on" : "off"}`} />
<div class={`yellow ${matches("Started.Yellow") ? "on" : "off"}`} />
<div class={`green ${matches("Started.Green") ? "on" : "off"}`} />

State Chart

Just a humble traffic light that counts how many times it's gone through Red.

Traffic Light State Chart


Click "Open Sandbox" to dig into the actual useMachine() code.

If you inspect the DOM, you'll see that only the nodes that need to change are changing.

Traffic Light DOM GIF


This was mostly a learning exercise for me. I'd be happy to contribute this code to initiate an official @xstate/solid library. The Stately people, that maintain XState, know how to contact me.

Thanks for reading, and I hope you learned something.

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© 2023 Erik Rasmussen