Add after_routes_loaded hook for Engines to trigger code after appl…

…ication routes have been loaded (rails#46539)

* Add after_routes_loaded hook

This hook can be used to trigger engine behavior that should only happen
after application routes have been loaded (or reloaded).

* Add changelog entry

* Add note about new after_routes_loaded railtie config to guides

[Rafael Mendonça França + Chris Salzberg]

guides/source/configuring.md

@@ -172,6 +172,16 @@ config.after_initialize do
 end
 ```
 
+### `config.after_routes_loaded`
+
+Takes a block which will be run after Rails has finished loading the application routes. This block will also be run whenever routes are reloaded.
+
+```ruby
+config.after_routes_loaded do
+  # Code that does something with Rails.application.routes
+end
+```
+
 #### `config.allow_concurrency`
 
 Controls whether requests should be handled concurrently. This should only

railties/CHANGELOG.md

@@ -1,3 +1,15 @@
+*   Add `after_routes_loaded` hook to `Rails::Railtie::Configuration` for
+    engines to add a hook to be called after application routes have been
+    loaded.
+
+    ```ruby
+    MyEngine.config.after_routes_loaded do
+      # code that must happen after routes have been loaded
+    end
+    ```
+
+    *Chris Salzberg*
+
 *   Send 303 See Other status code back for the destroy action on newly generated
     scaffold controllers.
 

railties/lib/rails/application/finisher.rb

@@ -166,6 +166,7 @@ def self.complete(_state)
           # some sort of reloaders dependency support, to be added.
           require_unload_lock!
           reloader.execute
+          ActiveSupport.run_load_hooks(:after_routes_loaded, self)
         end
       end
 

railties/lib/rails/railtie/configuration.rb

@@ -71,6 +71,11 @@ def after_initialize(&block)
         ActiveSupport.on_load(:after_initialize, yield: true, &block)
       end
 
+      # Called after application routes have been loaded.
+      def after_routes_loaded(&block)
+        ActiveSupport.on_load(:after_routes_loaded, yield: true, &block)
+      end
+
       # Array of callbacks defined by #to_prepare.
       def to_prepare_blocks
         @@to_prepare_blocks ||= []