timer.ex

defmodule Timer do
  use GenServer

  @interval 10 * 1_000

  # Public API
  defmodule State do
    defstruct caller: nil,
              timer: nil
  end

  def begin(caller) do
    start_link(caller)
  end

  def start_link(caller) do
    GenServer.start_link(__MODULE__, caller)
  end

  def reset(pid) do
    GenServer.cast(pid, :reset)
  end

  # GenServer API

  def init(caller) do
    time_ref = :erlang.start_timer(@interval, self, :timeout)

    {:ok, {caller, time_ref}}
  end

  def handle_cast(:reset, {caller, timer}) do
    :erlang.cancel_timer(timer)
    time_ref = :erlang.start_timer(@interval, self, :timeout)

    {:noreply, {caller, time_ref}}
  end

  def handle_info({:timeout, _ref, :timeout}, {caller, timer}) do
    :erlang.cancel_timer(timer)
    time_ref = :erlang.start_timer(@interval, self, :timeout)

    Kernel.send(caller, :timeout)

    {:noreply, {caller, time_ref}}
  end
end