gen_reset/lib/gen_reset.ex

defmodule GenReset do
  @moduledoc """
  Documentation for GenServerReset.

  GenServerReset allows GenServers to be reset during testing.
  """

  defmacro __using__(_args) do
    quote do
      def handle_call(:"$gen_reset", _from, state) do
        args = case :erlang.get(:"$gen_reset_initial_args") do
          :undefined -> nil
          rest -> rest
        end
        case init(args) do
          {:ok, state} ->
            {:reply, :ok, state}

          {:ok, state, continue} ->
            {:reply, :ok, state, continue}
        end
      end

      def register_gen_reset(args) do
        GenReset.Tracker.add(self())
        :erlang.put(:"$gen_reset_initial_args", args)
      end
    end
  end
end

defmodule Server do
  use GenServer
  use GenReset

  def init(args) do
    register_gen_reset(args)
    {:ok, args}
  end

  def handle_call(:state, _from, state), do: {:reply, state, state}
  def handle_cast({:set, value}, _state), do: {:noreply, value}
end