Pass objects with async methods between WebWorkers and the main thread easily

This is a Node.js module available through the npm registry. It can be installed using the npm or yarn command line tools.

npm install magic-portal --save

You load the library in both the main thread and the worker (same library, there's no client/server distinction) and then create a new MagicPortal(channel) where channel is either the worker or the window. Then you call .set and .get on it like it's a Map, except you have to await the .get. The functions on the object HAVE to be async functions that return Promises.

index.html

<script src="https://unpkg.com/magic-portal/dist/index.umd.js"></script>
<script>
;(async () => {

  let worker = new Worker("./worker.js")
  const portal = new MagicPortal(worker)

  portal.set('main', {
    alert: async (msg) => window.alert(msg)
  })

  let adder = await portal.get('adder')

  let result = await adder.add(2, 2)
  console.log('2 + 2 =', result)

})();
</script>

worker.js

importScripts([
  "https://unpkg.com/magic-portal/dist/index.umd.js"
])
;(async () => {

  const portal = new MagicPortal(self)

  portal.set('adder', {
    add: async (a, b) => a + b
  })

  let main = await portal.get('main')

  main.alert('hello from worker')

})();

<script type="module">
import MagicPortal from "https://unpkg.com/magic-portal/dist/index.es6.js"
;(async () => {

  let worker = new Worker("./worker.js", {type: "module"})
  const portal = new MagicPortal(worker)

  portal.set('main', {
    alert: async (msg) => window.alert(msg)
  })

  let adder = await portal.get('adder')

  let result = await adder.add(2, 2)
  console.log('2 + 2 =', result)

})();
</script>

import MagicPortal from "https://unpkg.com/magic-portal/dist/index.es6.js"
;(async () => {

  const portal = new MagicPortal(self)

  portal.set('adder', {
    add: async (a, b) => a + b
  })

  let main = await portal.get('main')

  main.alert('hello from worker')

})();

If you have some methods where you don't care about the return value, you can use the void option to tell MagicPortal you don't need to wait for the result. This will cut the number of postMessage calls used in half, which could be useful if you have very high throughput (like an event emitter).

portal.set('main', {
  add: async (a, b) => a + b,
  alert: async (msg) => window.alert(msg)
}, {
  void: ['alert']
})

Under the hood it uses a very simple postMessage remote procedure call (RPC) that looks like this:

// Announce "I am able to start receiving messages"
{
  type: "MP_INIT"
}
// Announce "I have this object"
{
  type: "MP_SET",
  object: "adder",
  methods: ["adder"]
}
// Method call (request)
{
  type: "MP_CALL",
  object: "adder",
  method: "add",
  args: [2, 2],
  id: 36
}
// Return value (response)
{
  type: "MP_RETURN",
  id: 36,
  result: 4
}
// or Error
{
  type: "MP_RETURN",
  id: 36,
  error: "this is the error message"
}

Both sides of the MagicPortal queue their messages until they have received an MP_INIT message to account for the slight delay in starting up WebWorker threads. Calling .set sends (or queues) an MP_SET message, and when you call .get, it returns a promise that is resolved once a corresponding MP_SET message is received. (Therefore you should try to make all your .set calls before you start awaiting for .get calls to avoid a mutual deadlock where both threads are awaiting.) The object returned by .get has methods that correspond to the function properties of the original object passed to .set. Calling these methods sends an MP_CALL message with the arguments, and the MagicPortal on the other side of the channel will receive the MP_CALL message and call the original method with those arguments. Therefore the function arguments have to be serializable by the structured clone algorithm used by postMessage to send values.

npm install
npm test

None

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MIT