Is it possible to create topics dynamically in Phoenix

I am new to Phoenix. I was wondering if it is possible to create topics dynamically in Phoenix.
From what I understood till now in Phoenix if a user joins a topic they get all the messages sent on that topic. What if I want add a functionality in which user can create own topic and then send messages on it. Is that possible ?

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Thats totally possible, so in the user_socket.ex file you add a room as "room:name:*". Just add whatever you want after the star when joining the channel and boom your off.

For example I have a team channel defined like :

channel("team:*", TpPhoenixWeb.RequestCounterChannel)

The user then joins a channel named "team:API_KEY_HERE" and only they can see what is broadcast there.

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But can these topics be created on the go. As in without being written in user_socket.ex in the start ?

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You would need a macro that dynamically creates user_socket.ex based on input provided to the macro.

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To be clear, what the macros in user_socket do is route topics that have a particular prefix to a particular channel module. You could make a generic one:

channel "generic:*", GenericChannel

Then you can put literally anything after generic:

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I believe that you can meaningfully accomplish this task without creating a macro as specified by @benwilson512 above. And in general if you can accomplish something with just modules and functions then use that approach. Quoth the Elixir docs: “Macros should only be used as a last resort”.

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Yes, I think that whatever use case the OP has should be covered by using the method provided by @benwilson512.

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How do we join multiple topics in phoenix.js? Looking at the source, I see it supports only 1 topic per channel, while the docs states, “the client may join any number of topics on any number of channels”.

Joining multiple topics can be accomplished by calling socket.join multiple times, this will create multiple channels multiplexed over the same socket. Here’s an example:

let socket = new Socket("/socket", {params: {userToken: "123"}})
socket.connect()

let channel1 = socket.channel("room:123", {token: roomToken})

channel1.join()
  .receive("ok", ({messages}) => console.log("catching up", messages) )
  .receive("error", ({reason}) => console.log("failed join", reason) )
  .receive("timeout", () => console.log("Networking issue. Still waiting..."))

let channel2 = socket.channel("room:456", {token: roomToken})

channel2.join()
  .receive("ok", ({messages}) => console.log("catching up", messages) )
  .receive("error", ({reason}) => console.log("failed join", reason) )
  .receive("timeout", () => console.log("Networking issue. Still waiting..."))

Since joining multiple topics as a client is a different topic than creating dynamic topics as the server, if you still have questions it’s probably better to continue the discussion in a separate post.