Do not build View watcher until the first updated? check

Currently initialization of every Rails::Engine leads to the creation
of a new View watcher when the engine prepends its paths.
This contributes to the time it takes to perform the first cold request
on a lazy loaded application.

This change delays the initialization of the View watcher until the first
`updated?` check is performed.

Co-Authored-By: Gannon McGibbon <[email protected]>

Author: nvasilevski

actionview/lib/action_view/cache_expiry.rb

@@ -10,16 +10,17 @@ def initialize(watcher:, &block)
         @watcher = nil
         @previous_change = false
 
-        rebuild_watcher
-
         ActionView::PathRegistry.file_system_resolver_hooks << method(:rebuild_watcher)
       end
 
       def updated?
+        build_watcher unless @watcher
         @previous_change || @watcher.updated?
       end
 
       def execute
+        return unless @watcher
+
         watcher = nil
         @mutex.synchronize do
           @previous_change = false
@@ -33,7 +34,7 @@ def reload!
           ActionView::LookupContext::DetailsKey.clear
         end
 
-        def rebuild_watcher
+        def build_watcher
           @mutex.synchronize do
             old_watcher = @watcher
 
@@ -51,6 +52,11 @@ def rebuild_watcher
           end
         end
 
+        def rebuild_watcher
+          return unless @watcher
+          build_watcher
+        end
+
         def dirs_to_watch
           all_view_paths.uniq.sort
         end

actionview/lib/action_view/railtie.rb

@@ -116,7 +116,6 @@ class Railtie < Rails::Engine # :nodoc:
         view_reloader = ActionView::CacheExpiry::ViewReloader.new(watcher: app.config.file_watcher)
 
         app.reloaders << view_reloader
-        view_reloader.execute
         app.reloader.to_run do
           require_unload_lock!
           view_reloader.execute