How to implement a resetable countdown timer with a GenServer in Elixir or Erlang

Question

How do we implement a reset-able countdown timer with a GenServer?

1) perform a task after fixed amount of time, say every 60 seconds

2) have a way to reset the countdown back to 60 seconds before the timer elapses

I have looked at How to perform actions periodically with Erlang's gen_server? but it doesn't quite cover the aspect of resting the timer before the countdown elapses.

Thanks.

Answer 1

How can I schedule code to run every few hours in Elixir or Phoenix framework? defines how to do a periodic job.

To implement a cancel using that as a base you could do the following:

defmodule MyApp.Periodically do
  use GenServer

  def start_link(_name \\ nil) do
    GenServer.start_link(__MODULE__, %{}, name: __MODULE__)
  end

  def reset_timer() do
    GenServer.call(__MODULE__, :reset_timer)
  end

  def init(state) do
    timer = Process.send_after(self(), :work, 60_000)
    {:ok, %{timer: timer}}
  end

  def handle_call(:reset_timer, _from, %{timer: timer}) do
    :timer.cancel(timer)
    timer = Process.send_after(self(), :work, 60_000)
    {:reply, :ok, %{timer: timer}}
  end

  def handle_info(:work, state) do
    # Do the work you desire here

    # Start the timer again
    timer = Process.send_after(self(), :work,60_000)

    {:noreply, %{timer: timer}}
  end

  # So that unhanded messages don't error
  def handle_info(_, state) do
    {:ok, state}
  end
end

This maintains a reference to the timer, allowing it to be cancelled. Every time the :work message is received, a new timer is created and stored in the state of the GenServer.