Support class reloading for Event Subscribers

Before this change, editing an instance of Spree::Event::Subscriber
would result in not being reloaded or being subscribed twice.

With this change each includer of Event::Subscriber is registered by its
name and later unsubscribed just before class unload (using old module
instance), then resubscribed after the new code is loaded.

core/lib/spree/core/engine.rb

@@ -44,8 +44,15 @@ class Engine < ::Rails::Engine
         Migrations.new(config, engine_name).check
       end
 
-      initializer 'spree.core.mailer_subscriber' do
-        Spree::MailerSubscriber.subscribe!
+      # Setup Event Subscribers
+      initializer 'spree.core.initialize_subscribers' do |app|
+        app.reloader.to_prepare do
+          Spree::Event.subscribers.each(&:subscribe!)
+        end
+
+        app.reloader.before_class_unload do
+          Spree::Event.subscribers.each(&:unsubscribe!)
+        end
       end
 
       # Load in mailer previews for apps to use in development.

core/lib/spree/event.rb

@@ -98,6 +98,12 @@ def suffix
       Spree::Config.events.suffix
     end
 
+    # @!attribute [r] subscribers
+    #   @return [Array<Spree::Event::Subscriber>] A list of subscribers used to support class reloading for Spree::Event::Subscriber instances
+    def subscribers
+      Spree::Config.events.subscribers
+    end
+
     private
 
     def name_with_suffix(name)

core/lib/spree/event/configuration.rb

@@ -1,8 +1,16 @@
 # frozen_string_literal: true
 
+require 'spree/core/class_constantizer'
+
 module Spree
   module Event
     class Configuration
+      def subscribers
+        @subscribers ||= ::Spree::Core::ClassConstantizer::Set.new.tap do |set|
+          set << 'Spree::MailerSubscriber'
+        end
+      end
+
       attr_writer :adapter, :suffix
 
       def adapter

core/spec/lib/spree/event_spec.rb

@@ -89,4 +89,30 @@
       end
     end
   end
+
+  describe '.subscribers' do
+    let(:subscriber) { instance_double(Module, 'Subscriber') }
+    let(:subscriber_name) { instance_double(String, 'Subscriber name') }
+
+    before do
+      described_class.subscribers.clear
+      allow(subscriber).to receive(:name).and_return(subscriber_name)
+      allow(subscriber_name).to receive(:constantize).and_return(subscriber)
+      allow(subscriber_name).to receive(:to_s).and_return(subscriber_name)
+    end
+
+    it 'accepts the names of constants' do
+      Spree::Config.events.subscribers << subscriber_name
+
+      expect(described_class.subscribers.to_a).to eq([subscriber])
+    end
+
+    it 'discards duplicates' do
+      described_class.subscribers << subscriber_name
+      described_class.subscribers << subscriber_name
+      described_class.subscribers << subscriber_name
+
+      expect(described_class.subscribers.to_a).to eq([subscriber])
+    end
+  end
 end