“Existing subscription to channel” while only subscribing once about pusher-js

[pl]

Hi,

It's a bit of a race condition, which is caused by pusher-js using these events internally. You can subscribe to channels before connecting, and when connection is established, pusher-js sends actual subscription requests. Problem here is that you bind to the state_changed event, which is dispatched before the connected event that is used internally, so the flow is following:

1. you create the Pusher instance,
2. it binds to the connected event,
3. you bind the callback to the state_changed,
4. connection is established,
5. state_changed event is dispatched,
6. you subscribe to the channel,
7. connected event is dispatched,
8. pusher-js assumes that all channels on the list haven't had a chance to subscribe, subscribes your channel again,
9. existing subscription error is raised.

While it's a bit of a bug, I would advise against relying on the pusher.connection state machine. Whenever there are problems with connection, you will get state changes, while subscriptions are retried automatically after re-establishing the connection.

You should subscribe once, after creating the Pusher instance (doesn't matter when as long as it's outside of the connection callback loop). You can then bind to pusher:subscription_succeeded on the channel.

Event emitters can cause problems like this, unfortunately.

[dirkbonhomme]

Sounds like a solid explanation. I don't use this flow in real applications for the reasons you mention. Fixing this would be a waste of time it seems :)