For some reason, I want to broadcast a message when someone leaves a channel or disconnects due to some network issues.

I wrote this code:

defmodule MyAppWeb.SomeChannel do
  use MyAppWeb, :channel

  @impl true
  def terminate(_reason, socket) do
    broadcast(socket, "left", %{})
  end
end

And here’s the test:

test "broadcast when leave", %{socket: socket} do
  IO.inspect self()
  leave(socket)
  assert_broadcast("left", _)
end

When I ran this test, I got the following error with no additional detail:

#PID<0.875.0>
** (EXIT from #PID<0.875.0>) shutdown: :left

I tested my implementation using the Chrome websocket extension “WebSocket King Client”, and it works.

My question is, how to test broadcasting in terminate/2, or is there another way to implement this behavior that can be tested?