That is because you are broadcasting to the id topic, which is good for pretty much only killing the channel entirely and all topics built on it, this is not what you want.
Instead you need to put something like MyServer.Endpoint.subscribe("UserChannelBlah:#{socket.assigns.user_id}") in your UserChannel module, say in the join function as an example, and you need to listen for that message as your handle_info is already doing fine (though matching on the message event is useful).
Thanks.
I still don’t understand how Test.Web.Endpoint.broadcast <socket id>, event, msg works though. Is there only a predefined set of events that I can send to it, like "disconnect"?
Here by socket id I mean the value returned by Phoenix.Socket — Phoenix v1.8.8
Nope, can send anything and everything you want. The event just gets put into the %Broadcast{:event} field, nothing special there at all. 
If you are broadcasting to the topic set in your id method on the UserSocket, then you’d handle it in the UserSocket module itself if it allows you to override handle_info (I’m not sure it does for safety reasons, but I’ve not tried).
You can sure, it just won’t do anything unless other broadcast messages use them. Remember that the channels get a ‘copy’ of the assigns, so changing it in the channel will do absolutely nothing to theirs.
If you are broadcasting to the topic set in your id method on the UserSocket, then you’d handle it in the UserSocket module itself if it allows you to override handle_info (I’m not sure it does for safety reasons, but I’ve not tried).
I’ve tried adding
def handle_info(event, msg, socket) do
IO.inspect([event, msg, socket])
{:noreply, socket}
end
def handle_info(event, socket) do
IO.inspect([event, socket])
{:noreply, socket}
end
to UserSocket module but still no IO.inspect got called.
And it doesn’t seem to not conform to the genserver behaviour anywhere in the code phoenix/lib/phoenix/socket.ex at main · phoenixframework/phoenix · GitHub
Thought it might not, Sockets are for handling sockets only so that makes sense, keep it to channels then.
I’m not sure why you would want to receive messages in the socket to be honest, I’ve never thought up a need other than disconnecting them…