checkout_controller_decorator.rb

# app/controllers/spree/checkout_controller_decorator.rb
Spree::CheckoutController.class_eval do
  after_action :after_order_processing

  def after_order_processing
    # This doesn't seem to be called either. Also I would have to check the
    # current state each time this ran for the 'complete' state
    binding.pry
  end
end

order_decorator.rb

# app/models/spree/order_decorator.rb
Spree::Order.class_eval do
  # Adding after_transition to state doesn't work
  state_machine.after_transition to: :complete,
                                 do: :after_order_processing

  def after_order_processing
    binding.pry
  end

  # Aliasing method doesn't work
  def finalize_with_notify_service!
    finalize_without_notify_service!

    binding.pry
  end
  alias_method_chain :finalize!, :notify_service!
end

Turns out I was missing something in engine.rb to load in files with the _decorator.rb suffix. Once I added it, things started working.

module SpreeEngineName
  class Engine < ::Rails::Engine
    ...

    def self.activate
      Dir.glob(File.join(File.dirname(__FILE__), "../../app/**/*_decorator.rb")) do |c|
        Rails.configuration.cache_classes ? require(c) : load(c)
      end
    end

   config.to_prepare &method(:activate).to_proc

   ...
  end
end

Nice!

👍