Adding add_subscriber and add listeners

event_disptcher_2.ex

defmodule Rsed.Subscriber do
  @doc """
  Returns a map between event name and own event handlers

      defmodule Rsed.Test.TestSubscriber2 do
        @behaviour Rsed.Subscriber

        def get_subscriber_events() do
          %{
            test_event_2: :handler_2,
            test_event_3: :handler_3,
            test_event_4: [
              handler_4: 200,
              handler_5: 100
            ]
          }
        end

        def handler_2(event = %Rsed.Event{}), do: nil
        def handler_3(event = %Rsed.Event{}), do: nil
        def handler_4(event = %Rsed.Event{}), do: nil
        def handler_5(event = %Rsed.Event{}), do: nil
      end
  """
  @callback get_subscriber_events() :: map()
end


defmodule Rsed.EventDispatcher do
  ...
  
  @doc """
  Adds an event subscriber

  The subscriber is asked for all the events it is
  interested in and added as a listener for these events.

  """
  @spec add_subscriber(subscriber :: module()) :: term
  def add_subscriber(subscriber) do
    GenServer.call(my_name(), {:add_subscriber, subscriber})
  end

  @doc """
  Adds an event listener that listens on the specified events.
  """
  @spec add_listener(event_name :: String.t(), listener :: {module :: module(), callback: atom()}, priority :: integer()) :: term
  def add_listener(event_name, listener, priority \\ 0) do
    GenServer.call(my_name(), {:add_listener, event_name, listener, priority})
  end
  
  def handle_call({:add_subscriber, subscriber}, _from, listeners) do
    listeners = Rsed.ListenersBag.add_subscriber(listeners, subscriber)
    {:reply, :ok, listeners}
  end

  def handle_call({:add_listener, event_name, callback, priority}, _from, listeners) do
    listeners = Rsed.ListenersBag.add_listener(listeners, event_name, callback, priority)
    {:reply, :ok, listeners}
  end
  
end