Playing with Elixir supervisors

server.ex

defmodule ElxSupPlay.Server do
  use GenServer

  def start_link({parent_sup, workers_count = 8}) do
    GenServer.start_link(__MODULE__, {parent_sup, workers_count}, name: __MODULE__)
  end

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

  def register_worker(pid), do: GenServer.cast(__MODULE__, {:register_worker, pid})

  # Callbacks
  
  def init({parent_sup, workers_count}) do
    GenServer.cast(__MODULE__, {:start_worker_sup, workers_count})
    {:ok, {parent_sup, :none, []}}
  end

  def handle_cast({:start_worker_sup, workers_count}, {parent_sup, :none, []}) do
    {:ok, workers_sup} = Supervisor.start_child(parent_sup, {ElxSupPlay.WorkerSupervisor, nil})
    (1..workers_count)
    |> Enum.each(fn (_) -> Supervisor.start_child(workers_sup, []) end)
    {:noreply, {parent_sup, workers_sup, []}}
  end

  def handle_cast({:register_worker, pid}, {_, _, workers} = state) when is_pid(pid) do
    Process.monitor(pid)
    {:noreply, put_elem(state, 2, [pid | workers])}
  end

  def handle_call(:report, _from, state) do
    {:reply, state, state}
  end

  def handle_info({:DOWN, _ref, :process, pid, _reason}, {psup, workers_sup, workers}) do
    {:noreply, {psup, workers_sup, List.delete(workers, pid)}}
  end
end

worker.ex

defmodule ElxSupPlay.Worker do
  use GenServer

  # API
  def start_link(number) do
    GenServer.start_link(__MODULE__, number)
  end

  # Callbacks
  def init(start_num) do
    ElxSupPlay.Server.register_worker(self())
    {:ok, %{num: start_num}}
  end

  def handle_cast(:inc, %{num: num} = state) do
    {:noreply, %{state | num: num + 1}}
  end

  def handle_cast(:dec, %{num: num} = state) do
    {:noreply, %{state | num: num - 1}}
  end
end

worker_supervisor.ex

defmodule ElxSupPlay.WorkerSupervisor do
  use Supervisor

  # API
  def start_link(_opts) do
    Supervisor.start_link(__MODULE__, :ok)
  end

  # Callbacks
  def init(:ok) do
    children = [{ElxSupPlay.Worker, 0}]

    opts = [
      strategy: :simple_one_for_one,
      max_restarts: 5,
      max_seconds: 5
    ]

    Supervisor.init(children, opts)
  end
end