retain the default listeners for future unsubscribing

subscribe the hyrax default listeners in batch. provide a hook for looking up
the `Hyrax.publisher.default_listeners`. this makes it possible to unsubscribe
individual listeners at a later time.

these instances have to live in memory for the life of the Publisher anyway, so
this shouldn't impact when they are freed for garbage collection.

config/initializers/listeners.rb

@@ -1,16 +1,10 @@
 # frozen_string_literal: true
 
-Hyrax.publisher.subscribe(Hyrax::Listeners::AclIndexListener.new)
 Hyrax.publisher.subscribe(Hyrax::Listeners::ActiveFedoraAclIndexListener.new) unless Hyrax.config.disable_wings
-Hyrax.publisher.subscribe(Hyrax::Listeners::BatchNotificationListener.new)
-Hyrax.publisher.subscribe(Hyrax::Listeners::FileMetadataListener.new)
-Hyrax.publisher.subscribe(Hyrax::Listeners::FileSetLifecycleListener.new)
-Hyrax.publisher.subscribe(Hyrax::Listeners::FileSetLifecycleNotificationListener.new)
-Hyrax.publisher.subscribe(Hyrax::Listeners::MemberCleanupListener.new)
-Hyrax.publisher.subscribe(Hyrax::Listeners::MetadataIndexListener.new)
-Hyrax.publisher.subscribe(Hyrax::Listeners::ObjectLifecycleListener.new)
-Hyrax.publisher.subscribe(Hyrax::Listeners::TrophyCleanupListener.new)
-Hyrax.publisher.subscribe(Hyrax::Listeners::WorkflowListener.new)
+
+Hyrax.publisher.default_listeners.each do |listener|
+  Hyrax.publisher.subscribe(listener)
+end
 
 # Publish events from old style Hyrax::Callbacks to trigger the listeners
 # When callbacks are removed and replaced with direct event publication, drop these blocks

lib/hyrax/publisher.rb

@@ -194,5 +194,21 @@ class Publisher
     # @since 3.2.0
     # @macro a_registered_event
     register_event('object.file.uploaded')
+
+    ##
+    # @return Array[Object] the listeners Hyrax subscribes by default.
+    def default_listeners
+      @default_listeners ||=
+        [Hyrax::Listeners::AclIndexListener.new,
+         Hyrax::Listeners::BatchNotificationListener.new,
+         Hyrax::Listeners::FileMetadataListener.new,
+         Hyrax::Listeners::FileSetLifecycleListener.new,
+         Hyrax::Listeners::FileSetLifecycleNotificationListener.new,
+         Hyrax::Listeners::MemberCleanupListener.new,
+         Hyrax::Listeners::MetadataIndexListener.new,
+         Hyrax::Listeners::ObjectLifecycleListener.new,
+         Hyrax::Listeners::TrophyCleanupListener.new,
+         Hyrax::Listeners::WorkflowListener.new].freeze
+    end
   end
 end

spec/hyrax/publisher_spec.rb

@@ -0,0 +1,16 @@
+# frozen_string_literal: true
+
+RSpec.describe Hyrax::Publisher do
+  subject(:publisher) { described_class.instance } # singleton instance
+
+  describe "#default_listeners" do
+    it "returns a collection of listeners" do
+      # listeners can be any Object, so we can't verify they are valid here
+      expect(publisher.default_listeners).to be_a Enumerable
+    end
+
+    it "returns the same collection on successive calls" do
+      expect(publisher.default_listeners).to eql publisher.default_listeners
+    end
+  end
+end