How do web workers work?

May 12, 2020

I've heard some people say I should use web workers, but that always seemed like a pain.

Tonight I'm going to face my fear and use a worker. My goal is pretty simple - click a button and have it add 2 to a count.

Problems:

problem 1: don't know how to create a web worker

problem 2: I use compiled javascript
note - My javascript is passed through Gatsby, so I don't have access to webpack

problem 3: don't know how to talk to a web worker

Step 1: build a web worker

Is there some magic code that I can copy and paste?

Yes - https://github.com/webpack-contrib/worker-loader

Example Worker

// Worker.ts
const ctx: Worker = self as any

// Post data to parent thread
ctx.postMessage({ foo: 'foo' })

// Respond to message from parent thread
ctx.addEventListener('message', event => console.log(event))

Example script that loads a worker (don't worry, you don't have to use typescript - I just want all the help I can get from intellisense)

// WorkerParent.ts
import Worker from 'worker-loader!./Worker'

const worker = new Worker()

worker.postMessage({ a: 1 })
worker.onmessage = event => {}

worker.addEventListener('message', event => {})

Neat, now how do I use it?

Step 2 - configuring workers

Turns out you drop this in your webpack config:

// webpack.config.js
{
  loader: 'worker-loader',
  options: { publicPath: '/workers/' }
}

Step 2a: gatsby makes this harder

Gatsby lets you update the config by updating gatsby-node, so I'm changing it mine to configure the workers like so:

// gatsby-node.js
exports.onCreateWebpackConfig = ({
  actions: { replaceWebpackConfig },
  getConfig,
}) => {
  const config = getConfig()

  config.module.rules.push({
    test: /\.worker\.js$/,
    use: { loader: 'worker-loader', options: { inline: true } },
  })

  config.output.globalObject = 'this'

  replaceWebpackConfig(config)
}

With that in place, I'm able to load the parent, and recieve the dummy message from the sample!

Step 3: send information to and from the worker

Fortunately, it looks like the example files gave me the tools that I need for a simple message. The flow looks like this:

  1. My parent starts up a new worker
  2. The parent posts a message, and binds an "onmessage" event
  3. The child posted a message of {foo: "foo"}
  4. I could access that message under event.message.data in the onmessage listener

So, that answers how to get information back from the worker, but now I need to figure out how to access information while inside the worker.

Do debugger statements work yet?

It turns out that DevTools aren't quite there yet. Well, I know that the event type is a MessageEvent, thanks to typescript, so a quick web search reveals that there's a data payload I can access inside the worker.

Problems Resolved!

Let's wrap up with a simple example.

Here's our new worker:

// Worker.ts
const ctx: Worker = self as any

// Respond to message from parent thread
ctx.addEventListener('message', event => {
  // Read number from data
  let count: number = event.data.count || 0

  // respond with next value
  ctx.postMessage({ next: count + 1 })
})

And the WorkerParent, now a React component:

// WorkerParent.tsx
import React, { useEffect, useState } from "react";
import WebpackWorker from "worker-loader!*";
interface Props {}

type WorkerState = WebpackWorker | undefined;

function WorkerParent(props: Props) {
  const {} = props;
  const [count, setCount] = useState(0);
  const [worker, setWorker] = useState<WorkerState>();

  useEffect(() => {
    console.log("Loading countworker");
    (async () => {
      const countListenerWorker = await import(
        "worker-loader!./countListener.worker.ts"
      );
      const loaded = new countListenerWorker.default();
      console.log("Worker loaded", loaded);
      loaded.onmessage = event => {
        const next = event.data.next;
        setCount(next);
      };
      setWorker(loaded);
    })();
  }, []);

  const handleClick = () => {
    worker?.postMessage({ count });
  };

  return (
    <>
      <h4>Count: {count}</h4>
      <button onClick={handleClick} disabled={!worker}>
        Increase
      </button>
    </>
  );
}

export default WorkerParent;

A note on this - during local development, or a scenario where you're not doing SSR, you can use an ordinary import statement. Since Gatsby renders my code in Node.js though, I need to structure the web worker using the new dynamic import syntax, so that I can conditionally load it only in the browser.

Count: 0

And that about does it!

© Kyle Peacock 2021