How to dynamically make new channel topics?

Channels are just names. In the Docs you will see that example:

channel "room:*", HelloWeb.RoomChannel

Then there is a join function:

defmodule HelloWeb.RoomChannel do
  use Phoenix.Channel

  def join("room:lobby", _message, socket) do
    {:ok, socket}
  end
  def join("room:" <> _private_room_id, _params, _socket) do
    {:error, %{reason: "unauthorized"}}
  end
end

(in this example the private rooms are always refused since the docs talks about implementing the authorization later on).

As long as your join function accepts _private_room_id as a room name (i.e. it returns ok), then your channel exists, basically.

Thanks I tried it and it worked.

Hello Lud,

Can you give more information on whats on in the foolowing function :

def join("room:" <> _private_room_id, _params, _socket) do
    {:error, %{reason: "unauthorized"}}
  end

AS i understand, “room:” <> _private_room_id is concatenation, how elixir transform a concatenation in param ?

Hello and welcome,

In that case, it’s a pattern match… and _private_room_id will be assigned to what follow “room:”

To complete @kokolegorille 's response, it means that if your channel is set to “room:*” in your socket module, private_room_id will be bound to whatever you javascript client sends for this *.

It can also be simplified like this… maybe it’s more explicit.

iex> "koko:" <> id = "koko:12345"
"koko:12345"
iex> id
"12345"

Woow, ok, thanks for your explanations @kokolegorille and @lud ! this pattern matching feature is sooo great !!