🎁 Add listener for handling file set attachment

Why a listener and not a transaction?  In part because the moment I want
to perform the conditional enqueuing is at the point where the
`Hyrax::WorkUploadsHandler` does it's job.  That is when we have:

- the parent work
- the file set
- the original file
- the user

The `Hyrax::WorkUploadsHandler` is most analogous to the behavior in
`Hyrax::Actors::FileSetActor#attach_to_work` and
`Hyrax::Actors::FileSetActor#create_content`.  Fortunately, Hyrax's
transaction and upload handler remove the conditional handling we needed
between uploading a remote file and directly uploading a file.

Related to:

- scientist-softserv/hykuup_knapsack#35
- scientist-softserv/hykuup_knapsack#99
- #312

app/listeners/iiif_print/listener.rb

@@ -0,0 +1,33 @@
+##
+# @see https://github.com/samvera/hyrax/wiki/Hyrax's-Event-Bus-(Hyrax::Publisher)
+# @see https://www.rubydoc.info/gems/hyrax/Hyrax/Publisher
+# @see https://dry-rb.org/gems/dry-events
+module IiifPrint
+  class Listener
+    ##
+    # Responsible for conditionally enqueuing the creation of child works from a PDF.
+    #
+    # @param event [#[]] a hash like construct with keys :user and :file_set
+    # @param service [#conditionally_enqueue]
+    #
+    # @see Hyrax::WorkUploadsHandler
+    def on_file_set_attached(event, service: IiifPrint::SplitPdfs::ChildWorkCreationFromPdfService)
+      user = event[:user]
+      file_set = event[:file_set]
+
+      return false unless file_set
+      return false unless file_set.file_set?
+
+      work = IiifPrint.parent_for(file_set)
+
+      # A short-circuit to avoid fetching the underlying file.
+      return false unless work
+
+      # TODO: Verify that this is the correct thing to be sending off for conditional enquing.  That
+      # will require a more involved integration test.
+      file = file_set.original_file
+
+      service.conditionally_enqueue(file_set: file_set, work: work, file: file, user: user)
+    end
+  end
+end

lib/iiif_print/engine.rb

@@ -46,6 +46,8 @@ class Engine < ::Rails::Engine
         IiifPrint::PluggableDerivativeService
       )
 
+      Hyrax.publisher.subscribe(IiifPrint::Listener.new)
+
       Hyrax::IiifManifestPresenter.prepend(IiifPrint::IiifManifestPresenterBehavior)
       Hyrax::IiifManifestPresenter::Factory.prepend(IiifPrint::IiifManifestPresenterFactoryBehavior)
       Hyrax::ManifestBuilderService.prepend(IiifPrint::ManifestBuilderServiceBehavior)

spec/listeners/iiif_print/listener_spec.rb

@@ -0,0 +1,33 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe IiifPrint::Listener do
+  describe '#on_file_set_attached' do
+    subject { described_class.new.on_file_set_attached(event) }
+    let(:file_set) { double(Hyrax::FileSet, file_set?: true) }
+    let(:user) { double(User) }
+    let(:event) { { user: user, file_set: file_set } }
+
+    before { allow(IiifPrint).to receive(:parent_for).with(file_set).and_return(parent) }
+
+    context 'without a parent work' do
+      let(:parent) { nil }
+
+      it "does not call the service's #conditionally_enqueue method" do
+        expect(IiifPrint::SplitPdfs::ChildWorkCreationFromPdfService).not_to receive(:conditionally_enqueue)
+        subject
+      end
+    end
+
+    context 'with a parent work' do
+      let(:parent) { double(Valkyrie::Resource) }
+
+      it "calls the service's #conditionally_enqueue method" do
+        expect(IiifPrint::SplitPdfs::ChildWorkCreationFromPdfService).to receive(:conditionally_enqueue)
+        expect(file_set).to receive(:original_file).and_return(double)
+        subject
+      end
+    end
+  end
+end