From c8bf3341049b6b49df6af1d4324f24081e0b16d9 Mon Sep 17 00:00:00 2001
From: John Hawthorn <john@hawthorn.email>
Date: Mon, 1 Apr 2019 12:22:57 -0700
Subject: Only clear template caches in dev after changes (#35629)

---
 actionview/CHANGELOG.md                         |  8 ++++
 actionview/lib/action_view.rb                   |  1 +
 actionview/lib/action_view/cache_expiry.rb      | 49 +++++++++++++++++++++++++
 actionview/lib/action_view/digestor.rb          |  6 ---
 actionview/lib/action_view/railtie.rb           |  2 +-
 actionview/lib/action_view/template/resolver.rb |  2 +
 6 files changed, 61 insertions(+), 7 deletions(-)
 create mode 100644 actionview/lib/action_view/cache_expiry.rb

diff --git a/actionview/CHANGELOG.md b/actionview/CHANGELOG.md
index 17a5782f9f..0d1833d1de 100644
--- a/actionview/CHANGELOG.md
+++ b/actionview/CHANGELOG.md
@@ -1,3 +1,11 @@
+*   Only clear ActionView cache in development on file changes
+
+    To speed up development mode, view caches are only cleared when files in
+    the view paths have changed. Applications which have implemented custom
+    ActionView::Resolver subclasses may need to add their own cache clearing.
+
+    *John Hawthorn*
+
 ## Rails 6.0.0.beta3 (March 11, 2019) ##
 
 *   Only accept formats from registered mime types
diff --git a/actionview/lib/action_view.rb b/actionview/lib/action_view.rb
index 8cb4648a67..f398ba0ff0 100644
--- a/actionview/lib/action_view.rb
+++ b/actionview/lib/action_view.rb
@@ -81,6 +81,7 @@ module ActionView
     end
   end
 
+  autoload :CacheExpiry
   autoload :TestCase
 
   def self.eager_load!
diff --git a/actionview/lib/action_view/cache_expiry.rb b/actionview/lib/action_view/cache_expiry.rb
new file mode 100644
index 0000000000..820afc093d
--- /dev/null
+++ b/actionview/lib/action_view/cache_expiry.rb
@@ -0,0 +1,49 @@
+# frozen_string_literal: true
+
+module ActionView
+  class CacheExpiry
+    class Executor
+      def initialize(watcher:)
+        @cache_expiry = CacheExpiry.new(watcher: watcher)
+      end
+
+      def before(target)
+        @cache_expiry.clear_cache_if_necessary
+      end
+    end
+
+    def initialize(watcher:)
+      @watched_dirs = nil
+      @watcher_class = watcher
+      @watcher = nil
+    end
+
+    def clear_cache_if_necessary
+      watched_dirs = dirs_to_watch
+      if watched_dirs != @watched_dirs
+        @watched_dirs = watched_dirs
+        @watcher = @watcher_class.new([], watched_dirs) do
+          clear_cache
+        end
+        @watcher.execute
+      else
+        @watcher.execute_if_updated
+      end
+    end
+
+    def clear_cache
+      ActionView::LookupContext::DetailsKey.clear
+    end
+
+    private
+
+      def dirs_to_watch
+        fs_paths = all_view_paths.grep(FileSystemResolver)
+        fs_paths.map(&:path).sort.uniq
+      end
+
+      def all_view_paths
+        ActionView::ViewPaths.all_view_paths.flat_map(&:paths)
+      end
+  end
+end
diff --git a/actionview/lib/action_view/digestor.rb b/actionview/lib/action_view/digestor.rb
index 9fa8d7eab1..97a6da3634 100644
--- a/actionview/lib/action_view/digestor.rb
+++ b/actionview/lib/action_view/digestor.rb
@@ -6,12 +6,6 @@ module ActionView
   class Digestor
     @@digest_mutex = Mutex.new
 
-    module PerExecutionDigestCacheExpiry
-      def self.before(target)
-        ActionView::LookupContext::DetailsKey.clear
-      end
-    end
-
     class << self
       # Supported options:
       #
diff --git a/actionview/lib/action_view/railtie.rb b/actionview/lib/action_view/railtie.rb
index a25e1d3d02..8bd526cdf9 100644
--- a/actionview/lib/action_view/railtie.rb
+++ b/actionview/lib/action_view/railtie.rb
@@ -81,7 +81,7 @@ module ActionView
     initializer "action_view.per_request_digest_cache" do |app|
       ActiveSupport.on_load(:action_view) do
         unless ActionView::Resolver.caching?
-          app.executor.to_run ActionView::Digestor::PerExecutionDigestCacheExpiry
+          app.executor.to_run ActionView::CacheExpiry::Executor.new(watcher: app.config.file_watcher)
         end
       end
     end
diff --git a/actionview/lib/action_view/template/resolver.rb b/actionview/lib/action_view/template/resolver.rb
index 3b4c8a94bc..095e6cc3a1 100644
--- a/actionview/lib/action_view/template/resolver.rb
+++ b/actionview/lib/action_view/template/resolver.rb
@@ -280,6 +280,8 @@ module ActionView
 
   # A resolver that loads files from the filesystem.
   class FileSystemResolver < PathResolver
+    attr_reader :path
+
     def initialize(path, pattern = nil)
       raise ArgumentError, "path already is a Resolver class" if path.is_a?(Resolver)
       super(pattern)
-- 
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