Add broadcast room `include` functionality - opposite of `exclude`

[ZachHaber]

Is your feature request related to a problem? Please describe.
I'm trying to emit events to a collection of rooms, but I want to make sure that one of those rooms is always the case.

In my app I have rooms for which roles each socket is able to manage as well as a room for if the socket is currently active in an app. I'd like to be able to send the message to the subset of sockets that are currently active and able to manage the values.

Describe the solution you'd like
I was thinking that we should be able to have include rooms that are always included - akin to the current exclude rooms.

For example, io.include('active1').to(['r1','r2','r3']).emit() would emit specifically to the sockets with both active1 and one of the rooms (r1, r2, r3).

If you were to do something like io.include(['active1','test']).to(['r1','r2','r3']).emit() we'd need to figure out whether both 'active' and 'test' should be applied or if they should be ored together. Similarly if io.include('active1').include('test').to(['r1','r2','r3']).emit() should be the same as the previous or different.

I think that .include(['active1','test']) should be either of those two rooms, while .include('active1').include('test) should require both of the rooms on the same socket to pass.

Describe alternatives you've considered
Alternatively, in .to and .in, there could be an object property to indicate whether the newly added values should be anded or ored in. The same example as above could be: io.to(['r1','r2','r3']).to({type:'and', rooms: ['active1']})

Without having the full boolean logic in one single call, it could be confusing which pieces should be the same and which should be different.

---

Another alternative (for my use case), would be to keep track of which apps the socket is not in and maintain an "inactive" rooms and then use exclude. Which isn't ideal due to the need to keep track of a negative value - and would get far too complicated to keep track of if I wanted to include two rooms instead of just one.

Additional context

[darrachequesne]

Hi! Thanks for raising this.

This sounds like a fairly specific use case, which can be implemented with serverSideEmit(). What do you think?

Reference: https://socket.io/docs/v4/server-api/#serverserversideemiteventname-args

[ZachHaber]

I am unable to solve this with server side emit currently due, in part, to the fact that I invite have one server currently. But I also don't follow where server side emit plays into this. Could you explain that one a little more?
To me the secret side emit is meant for communicating between servers, while what I'm trying to do here is adjust which clients I'm communicating with.

[darrachequesne]

What I had in mind is simply looping over the sockets:

for (const socket of io.of("/").sockets) {
  if (socket.rooms.has("mandatory") && (socket.rooms.has("optional1") || socket.rooms.has("optional2"))) {
    socket.emit("some-event");
  }
}

And with multiple servers:

io.serverSideEmit("broadcast", "hello", {
  andRooms: ["mandatory"],
  orRooms: ["optional1", "optional2"]
});

io.on("broadcast", (content, { andRooms, orRooms }) => {
  for (const socket of io.of("/").sockets) {
    if (andRooms.every(room => socket.rooms.has(room)) && orRooms.some(room => socket.rooms.has(room))) {
      socket.emit("some-event", content);
    }
  }
});

This is a bit less efficient than using io.to().emit(), since the packet is encoded for each socket and not once.

[ZachHaber]

Yeah, that would be doable. I could also improve the performance and prevent the need to re-encode the packet for each socket individually by just building up a list of the socket ids and sending the emit using io.to(socketIds).emit() since each socket is in a room with their I'd by default.

It might be nice to have it in the library, but it does seem doable without it being baked in using the method you suggested. If you don't think this is worth implementing, then feel free to close the issue!