Update Valkyrie index when notified of metadata updates

When an object's metadata is updated, update the Valkyrie-driven index via the
configured indexing adapter.

Note that this is currently a no-op, since we use a null indexing adapter by
default.

app/services/hyrax/listeners/metadata_index_listener.rb

@@ -0,0 +1,23 @@
+# frozen_string_literal: true
+
+module Hyrax
+  module Listeners
+    ##
+    # Reindexes resources when their metadata is updated.
+    #
+    # @note This listener makes no attempt to avoid reindexing when no metadata
+    #   has actually changed, or when real metadata changes won't impact the
+    #   indexed data. We trust that published metadata update events represent
+    #   actual changes to object metadata, and that the indexing adapter
+    #   optimizes reasonably for actual index document contents.
+    class MetadataIndexListener
+      ##
+      # Re-index the resource.
+      #
+      # @param event [Dry::Event]
+      def on_object_metadata_updated(event)
+        Hyrax.index_adapter.save(resource: event[:object])
+      end
+    end
+  end
+end

spec/services/hyrax/listeners/metadata_index_listener_spec.rb

@@ -0,0 +1,24 @@
+# frozen_string_literal: true
+
+RSpec.describe Hyrax::Listeners::MetadataIndexListener do
+  subject(:listener) { described_class.new }
+  let(:data)         { { object: resource } }
+  let(:event)        { Dry::Events::Event.new(event_type, data) }
+  let(:fake_adapter) { FakeIndexingAdapter.new }
+  let(:resource)     { FactoryBot.valkyrie_create(:hyrax_resource) }
+
+  # the listener should always use the currently configured Hyrax Index Adapter
+  before do
+    allow(Hyrax).to receive(:index_adapter).and_return(fake_adapter)
+  end
+
+  describe '#on_object_metadata_updated' do
+    let(:event_type) { :on_object_metadata_updated }
+
+    it 'reindexes the object on the configured adapter' do
+      expect { listener.on_object_metadata_updated(event) }
+        .to change { fake_adapter.saved_resources }
+        .to contain_exactly(resource)
+    end
+  end
+end