API idea

worker.ex

defmodule Alarm.Worker do
  @moduledoc false
  use GenServer

  # Pick ports that work on both the GrovePi+ and GrovePi Zero
  @button_pin 14 # Port A0
  @buzzer_pin 3  # Port D3

  def start_link() do
    GenServer.start_link(__MODULE__, [])
  end

  def init([]) do
    with
     #GrovePi is the application so no need to start
     {:ok, _} -> GrovePi.Button.attach(@button_pin),
     {:ok, buzzer_pi} -> GrovePi.Buzzer.attach(@buzzer_pin),
     :ok -> GrovePi.listen_for({:pressed, @button_pin}),
     do
       {:ok, :ok}
     end
  end

  def handle_info({:pressed, @button_pin}, state) do
    IO.puts("Alert!!!!")

    # Sound the alarm, but only for a second
    GrovePi.Buzzer.buzz(@buzzer_pin, 1000)

    {:noreply, state}
  end
end

I was thinking that each attach call would start its own little process tree, but that that tree would be under a GrovePi supervisor. The reason I was thinking that is that I'm looking at the GrovePi as an independent thing that has a bunch of attachments. The outside application code is a configuration, and then it registers for events that come from the GrovePi itself. We could also add a detach to the API that will end that little process tree.

I actually just noticed that this module no longer needs state at this point.