Dynamically (re)load event subscribers

Until now, adding subscribers to Spree::Event.subscribers was
developers' responsibility.

This commit aims at automating the task by making Rails load
subscriber files located in standard paths both in Solidus core
and in the store app during the Rails initialization processs.

When each file is required, it's now also added to the
`Spree::Event.subscribers` list.

Solidus core event subscribers are always loaded automatically,
while the ones included in the store app can be loaded manually
when the preference Spree::Config.events.autoload_subscribers
is changed to a falsey value.

A similar feature will be added to `solidus_support` in order
to automatically subscribe subscribers from extensions.

core/lib/spree/core/engine.rb

@@ -47,6 +47,7 @@ class Engine < ::Rails::Engine
       # Setup Event Subscribers
       initializer 'spree.core.initialize_subscribers' do |app|
         app.reloader.to_prepare do
+          Spree::Event.require_subscriber_files
           Spree::Event.subscribers.each(&:subscribe!)
         end
 

core/lib/spree/event.rb

@@ -26,6 +26,30 @@ def fire(event_name, opts = {})
       end
     end
 
+    # Loads all Solidus' core and application's event subscribers files.
+    # The latter are loaded automatically only when the preference
+    # Spree::Config.events.autoload_subscribers is set to a truthy value.
+    #
+    # Files must be placed under the directory `app/subscribers` and their
+    # name must end with `_subscriber.rb`.
+    #
+    # Loading the files has the side effect of adding their module to the
+    # list in Spree::Event.subscribers.
+    def require_subscriber_files
+      pattern = "app/subscribers/**/*_subscriber.rb"
+
+      # Load Solidus subscribers
+      # rubocop:disable Rails/DynamicFindBy
+      solidus_core_dir = Gem::Specification.find_by_name('solidus_core').gem_dir
+      # rubocop:enable Rails/DynamicFindBy
+      Dir.glob(File.join(solidus_core_dir, pattern)) { |c| require_dependency(c.to_s) }
+
+      # Load application subscribers, only when the flag is set to true:
+      if Spree::Config.events.autoload_subscribers
+        Rails.root.glob(pattern) { |c| require_dependency(c.to_s) }
+      end
+    end
+
     # Subscribe to an event with the given name. The provided block is executed
     # every time the subscribed event is fired.
     #

core/lib/spree/event/configuration.rb

@@ -6,9 +6,7 @@ module Spree
   module Event
     class Configuration
       def subscribers
-        @subscribers ||= ::Spree::Core::ClassConstantizer::Set.new.tap do |set|
-          set << 'Spree::MailerSubscriber'
-        end
+        @subscribers ||= ::Spree::Core::ClassConstantizer::Set.new
       end
 
       attr_writer :adapter, :suffix, :autoload_subscribers

core/lib/spree/event/subscriber.rb

@@ -26,6 +26,8 @@ def self.included(base)
 
         base.mattr_accessor :event_actions
         base.event_actions = {}
+
+        Spree::Event.subscribers << base.name
       end
 
       # Declares a method name in the including module that can be subscribed/unsubscribed

core/spec/lib/spree/event_spec.rb

@@ -119,6 +119,8 @@
       allow(subscriber_name).to receive(:to_s).and_return(subscriber_name)
     end
 
+    after { described_class.subscribers.clear }
+
     it 'accepts the names of constants' do
       Spree::Config.events.subscribers << subscriber_name
 
@@ -133,4 +135,50 @@
       expect(described_class.subscribers.to_a).to eq([subscriber])
     end
   end
+
+  describe '.require_subscriber_files' do
+    let(:susbcribers_dir) { Rails.root.join('app', 'subscribers', 'spree') }
+
+    def create_subscriber_file(constant_name)
+      FileUtils.mkdir_p(susbcribers_dir)
+      File.open File.join(susbcribers_dir, "#{constant_name.underscore}.rb"), 'w' do |f|
+        f.puts "module Spree::#{constant_name}; include Spree::Event::Subscriber; end"
+      end
+    end
+
+    after { FileUtils.rm_rf(susbcribers_dir) }
+
+    context 'when Spree::Config.events.autoload_subscribers is true (default)' do
+      let(:events_config) { double(autoload_subscribers: true, subscribers: Set.new) }
+
+      before { create_subscriber_file('FooSubscriber') }
+
+      it 'requires subscriber files and loads them into Spree::Event.subscribers' do
+        expect do
+          described_class.require_subscriber_files
+        end.to change { described_class.subscribers.count }.by 1
+
+        expect(defined? Spree::FooSubscriber).to be_truthy
+        expect(described_class.subscribers).to include(Spree::FooSubscriber)
+      end
+    end
+
+    context 'when Spree::Config.autoload_subscribers is false' do
+      let(:events_config) { double(autoload_subscribers: false, subscribers: Set.new) }
+
+      before do
+        stub_spree_preferences(events: events_config)
+        create_subscriber_file('BarSubscriber')
+      end
+
+      it 'does not requires subscriber files' do
+        expect do
+          described_class.require_subscriber_files
+        end.not_to change { described_class.subscribers.count }
+
+        expect(defined? Spree::BarSubscriber).to be_falsey
+        expect(described_class.subscribers).to be_empty
+      end
+    end
+  end
 end