Encapsulate stale_when_importmap_changes

Author: dhh

README.md

@@ -226,14 +226,16 @@ Import your module on the specific page. Note: you'll likely want to use a `cont
 
 ## Include a digest of the import map in your ETag
 
-If you're using [ETags](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag) generated by Rails helpers like `stale?` or `fresh_when`, you need to include the digest of the import map into this calculation. Otherwise your application will return [304](https://developer.mozilla.org/en-US/docs/Web/HTTP/Status/304) cache responses even when your JavaScript assets have changed. You can avoid this with something like:
+If you're using [ETags](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag) generated by Rails helpers like `stale?` or `fresh_when`, you need to include the digest of the import map into this calculation. Otherwise your application will return [304](https://developer.mozilla.org/en-US/docs/Web/HTTP/Status/304) cache responses even when your JavaScript assets have changed. You can avoid this using the `stale_when_importmap_changes` method:
 
 ```ruby
 class ApplicationController < ActionController::Base
-  etag { Rails.application.importmap.digest(resolver: helpers) if request.format&.html? }
+  stale_when_importmap_changes
 end
 ```
 
+This will add the digest of the importmap to the etag calculation when the request format is HTML.
+
 
 ## Sweeping the cache in development and test
 

app/controllers/concerns/importmap/freshness.rb

@@ -0,0 +1,5 @@
+module Importmap::Freshness
+  def stale_when_importmap_changes
+    etag { Rails.application.importmap.digest(resolver: helpers) if request.format&.html? }
+  end
+end

lib/importmap/engine.rb

@@ -48,6 +48,13 @@ class Engine < ::Rails::Engine
       end
     end
 
+    initializer "importmap.concerns" do
+      ActiveSupport.on_load(:action_controller_base) do
+        require_relative "../../app/controllers/concerns/importmap/freshness"
+        extend Importmap::Freshness
+      end
+    end
+
     initializer "importmap.helpers" do
       ActiveSupport.on_load(:action_controller_base) do
         helper Importmap::ImportmapTagsHelper