use FileSystem to get fs events

lib/mix/tasks/site.build.ex

@@ -2,12 +2,13 @@ defmodule Mix.Tasks.Site.Build do
   @moduledoc "Build and output the site as HTML"
   @shortdoc "build the site"
   use Mix.Task
+  require Logger
 
   alias SloaneSH.Format
 
   @impl Mix.Task
   def run(_args) do
     {micro, :ok} = :timer.tc(&SloaneSH.build/0)
-    IO.puts("built site in #{Format.time(micro)}")
+    Logger.info("Built site in #{Format.time(micro)}")
   end
 end

lib/mix/tasks/site.watch.ex

@@ -2,10 +2,11 @@ defmodule Mix.Tasks.Site.Watch do
   @moduledoc "Build and output the site as HTML watching for changes"
   @shortdoc "build the site and watch for changes"
   use Mix.Task
+  require Logger
 
   @impl Mix.Task
   def run(_args) do
-    IO.puts("Starting site.watch...")
+    Logger.info("Starting site.watch...")
 
     {:ok, pid} = SloaneSH.watch()
 
@@ -14,7 +15,7 @@ defmodule Mix.Tasks.Site.Watch do
 
       receive do
         {:DOWN, ^ref, _, _, _} ->
-          IO.puts("site.watch terminated")
+          Logger.info("site.watch terminated")
           :ok
       end
     end

lib/sloane_sh.ex

@@ -8,8 +8,6 @@ defmodule SloaneSH do
   end
 
   def watch(_opts \\ []) do
-    Task.start_link(fn ->
-      Process.sleep(5_000)
-    end)
+    SloaneSH.Watch.start_link()
   end
 end

lib/sloane_sh/watch.ex

@@ -0,0 +1,30 @@
+defmodule SloaneSH.Watch do
+  use GenServer
+  require Logger
+
+  def start_link(init_arg \\ [], opts \\ []) do
+    GenServer.start_link(__MODULE__, init_arg, opts)
+  end
+
+  @impl GenServer
+  def init([]) do
+    dirs = [Path.join(:code.priv_dir(:sloane_sh), "site")]
+    Logger.info("Watching #{inspect(dirs)} for changes")
+    {:ok, pid} = FileSystem.start_link(dirs: dirs)
+    FileSystem.subscribe(pid)
+
+    {:ok, pid}
+  end
+
+  @impl GenServer
+  def handle_info({:file_event, pid, {path, events}}, pid) do
+    Logger.info("File event: #{inspect(path)} #{inspect(events)}")
+    {:noreply, pid}
+  end
+
+  @impl GenServer
+  def handle_info({:file_event, pid, :stop}, pid) do
+    Logger.warning("File watcher stopped")
+    {:stop, :watcher_stopped, pid}
+  end
+end