Make watcher a MFA

lib/phoenix/endpoint.ex

@@ -148,15 +148,24 @@ defmodule Phoenix.Endpoint do
       the "watch" mode of the webpack build tool when the server starts.
       You can configure it to whatever build tool or command you want:
 
-          [node: ["node_modules/webpack/bin/webpack.js", "--mode", "development",
-              "--watch-stdin"]]
+          [
+            node: [
+              "node_modules/webpack/bin/webpack.js",
+              "--mode",
+              "development",
+              "--watch",
+              "--watch-options-stdin"
+            ]
+          ]
+
+      The `:cd` and `:env` options can be given at the end of the list to customize
+      the watcher:
+
+          [node: [..., cd: "assets"]]
 
-      The `:cd` option can be used on a watcher to override the folder from
-      which the watcher will run. By default this will be the project's root:
-      `File.cwd!()`
+      A watcher can also be a module-function-args tuple that will be invoked accordingly:
 
-          [node: ["node_modules/webpack/bin/webpack.js", "--mode", "development",
-              "--watch-stdin", cd: "my_frontend"]]
+          [another: {Mod, :fun, [arg1, arg2]}]
 
     * `:live_reload` - configuration for the live reload option.
       Configuration requires a `:patterns` option which should be a list of

lib/phoenix/endpoint/watcher.ex

@@ -14,9 +14,9 @@ defmodule Phoenix.Endpoint.Watcher do
     Task.start_link(__MODULE__, :watch, [to_string(cmd), args])
   end
 
-  def watch(_cmd, args) when is_function(args, 0) do
+  def watch(_cmd, {mod, fun, args}) do
     try do
-      args.()
+      apply(mod, fun, args)
     catch
       kind, reason ->
         # The function returned a non-zero exit code.

test/phoenix/endpoint/watcher_test.exs

@@ -14,7 +14,7 @@ defmodule Phoenix.Endpoint.WatcherTest do
 
   test "starts watching and writes to stdio with fun" do
     assert capture_io(fn ->
-      {:ok, pid} = Watcher.start_link({"echo", fn -> IO.puts("hello") end})
+      {:ok, pid} = Watcher.start_link({"echo", {IO, :puts, ["hello"]}})
       ref = Process.monitor(pid)
       assert_receive {:DOWN, ^ref, :process, ^pid, :normal}, 1000
     end) == "hello\n"