defmodule Excellent.Mixfile do
  use Mix.Project

  def project do
  ...
  end

  defp deps do
    [
      { :file_system, "~> 0.1.0", only: :test },
    ]
  end
  ...
end

{:ok, pid} = FileSystem.start_link(dirs: ["/path/to/some/files"])
FileSystem.subscribe(pid)

{:ok, pid} = FileSystem.start_link(dirs: ["/path/to/some/files"], name: :my_monitor_name)
FileSystem.subscribe(:my_monitor_name)

{:file_event, worker_pid, {file_path, events}}

{:file_event, worker_pid, :stop}

defmodule Watcher do
  use GenServer

  def start_link(args) do
    GenServer.start_link(__MODULE__, args)
  end

  def init(args) do
    {:ok, watcher_pid} = FileSystem.start_link(args)
    FileSystem.subscribe(watcher_pid)
    {:ok, %{watcher_pid: watcher_pid}}
  end

  def handle_info({:file_event, ^watcher_pid, {path, events}}, %{watcher_pid: watcher_pid}=state) do
    # YOUR OWN LOGIC FOR PATH AND EVENTS
    {:noreply, state}
  end

  def handle_info({:file_event, ^watcher_pid, :stop}, %{watcher_pid: watcher_pid}=state) do
    # YOUR OWN LOGIC WHEN MONITOR STOP
    {:noreply, state}
  end
end

FileSystem.start_link(dirs: ["/path/to/some/files"], listener_extra_args: "--latency=0.0")