Simple example of using GenEvent to pass on messages

code.exs

# In this example we're running jobs of some kind, and waiting for events if they finish okay or fail.

defmodule JobEvent do
  def start_link do
    {:ok, pid} = GenEvent.start_link(name: __MODULE__)
  end

  defmodule MessageForwarder do
    use GenEvent

    def handle_event(event, [listener]) do
      send listener, event
      {:ok, [listener]}
    end
  end

  def subscribe do
    GenEvent.add_handler(__MODULE__, MessageForwarder, [self])
  end
  
  def unsubscribe do
    GenEvent.remove_handler(__MODULE__, MessageForwarder, [self])
  end  

  def finished(job) do
    notify {:finished, job}
  end

  def failed(job) do
    notify {:failed, job}
  end

  defp notify(event) do
    :ok = GenEvent.notify(__MODULE__, event)
  end
end

# NOTE: we assume JobEvent is started by a supervisor somewhere

setup do
  JobEvent.subscribe
  
  # unsubscribe after each test so that events will work with multiple tests
  on_exit &JobEvent.unsubscribe/0
end

test "..." do
  job = %{...}

  # call some code that eventually calls, JobEvent.finished(job)

  assert_receive {:finished, job}
end