Msw: Unregister self when not activated?

[kentcdodds]

I have MSW working for a project on localhost:3000, then I close that project and open a new one which does not have MSW and that starts up on localhost:3000 (the default for create-react-app based apps) and MSW hangs around, logs in the console, and generally confuses people.

Can we have MSW's service worker unregister itself when it's not activated when the app starts up? I'd be fine if this is opt-in.

[kettanaito]

This is a good point. I've been stumbling upon this myself, with the service worker at place where it shouldn't have been. At the moment it does _deactivate_ the mocking upon window unload, but I see there is no logic to unregister the worker:

https://github.com/open-draft/msw/blob/75d2562065d0315911cbca718477d7ef4df684e8/src/composeMocks.ts#L29-L34

I find it a good idea to unregister the worker, for it's bound to a host (I believe it's not even bound to the port), yet I have one concerns:

• When should the service worker be unregistered?

I'll try to perform that on the same beforeunload window event, for that is a reliable way of knowing that window is being closed. My only worry is that there is no guarantee that the service worker unregister promise will resolve prior to the window actually unloading. That may lead into some unexpected scenarios, I suppose. I'd like to test how it behaves.

Need to keep in mind that there may be multiple clients listening to the same mocking Service Worker. The latter must not unregister when one window closes, but, ideally, when all its clients are closed.

[kentcdodds]

I think that's a good approach.

Another thing that might work is when the service worker starts up, it starts a timer and if it's not activated after a certain amount of time, then it unregisters itself? I can see why this may need to be opt-in though.

[kettanaito]

I gave it a quick manual test and unregistering the service worker on beforeunload seems to wield an expected result.

Timer sounds good, yet I'm afraid it creates a side-effect that is even harder to reason about than sw.unregister(). I can imagine it being subjected to a race condition (what finishes first: page unload or timer's callback?), and can't say from the top of my head if you reload the page with an action in a timer, would that action still execute after the page loads?

[kentcdodds]

Maybe on start up it can post a message to the client and if it doesn't get a response then it can unregister itself?

[kettanaito]

That's a possibility. Still I'd like to delegate registration responsibility to the client-side library, and not the service worker. This way there is a single point of control, treating the service worker as a ready-only artifact, if that makes sense.

[kettanaito]

Released in 0.13.0.
This release comes with a change to mockServiceWorker.js, so please make sure to:

$ npm install msw@latest
$ npx msw init <PUBLIC_DIR>

Service Worker instance should now unregister itself when the last controlled client is closed. That is achieved by keeping a map of controlled clients internally in the worker's scope, and removing a client by ID once it fires a beforeunload window event.

Eventually, I added a self-terminating logic to the worker, despite my previous comment. Service Worker is a reliable context to unregister itself, as it's persistent between clients' sessions, and keeps running even if you close the client. It's enough for the client to signal it's being closed, and there is no race condition for the worker to unregister itself, as the handling of unregistration is not bound to the client.

[kentcdodds]

Tested this out and it works perfectly. Thanks!