I’ve got a Genserver that writes messages to stdout. I can’t figure out how to test using capture_io. Documentation says By default, capture_io replaces the group_leader (:stdio) for the current process. Perhaps there is a Process.group_leader call that will make this work, but I haven’t been able to find it. Can anyone point me to a solution?

Here’s a failing test script…

#!/usr/bin/env elixir 

ExUnit.start()

defmodule MyGenServer do
  use GenServer

  def start_link(default \\ []) do
    GenServer.start_link(__MODULE__, default, name: __MODULE__)
  end

  def init(initial_state) do
    {:ok, initial_state}
  end

  def handle_call(:sayhi, _caller, state) do
    IO.puts("HI")
    {:reply, :ok, state}
  end
end

defmodule MyGenServerTest do
  use ExUnit.Case

  import ExUnit.CaptureIO

  test "GenServer responds to :sayhi call by writing 'HI' to stdout" do
    {:ok, pid} = MyGenServer.start_link()
    assert capture_io(fn -> GenServer.call(pid, :sayhi) end) == "HI\n"
  end
end