Trivial mostly representative example of GenEvent usage for blog post

genevent_example.ex

# supervisor.ex
defmodule EventSup do
  use Supervisor

  def start_link, do: Supervisor.start_link(__MODULE__, [], [])

  def init(_) do
    supervise(
      [
        worker(GenEvent, [[name: :event_manager]], [id: :event_manager]),
        worker(EventMonitor, [:event_manager])
      ],
      [strategy: :one_for_one]
    )
  end
end
# event_monitor.ex
defmodule EventMonitor do
  use GenServer

  def start_link(mgr), do: GenServer.start(__MODULE__, [mgr], [])

  def init(mgr) do
    :ok = add_handler(mgr)
    {:ok, mgr}
  end

  def handle_info({:gen_event_EXIT, _handler, _reason}, mgr) do
    :ok = add_handler(mgr)
    {:noreply, mgr}
  end

  def add_handler(mgr) do
    GenEvent.add_mon_handler(mgr, EventHandler, [])
  end
end

# event_handler.ex
defmodule EventHandler do
  use GenEvent
  require Logger

  def init(_), do: {:ok, {}}

  def handle_event(event, state) do
    Logger.info "received event #{inspect event}"

    {:ok, state}
  end
end

# send event
iex>GenEvent.notify(:event_manager, {:event, "stuff"})