Allow stand-alone Spree Checkout to run with existing Spree Frontend

app/helpers/spree/store_helper.rb

@@ -2,6 +2,14 @@ module Spree
   module StoreHelper
     include LocaleHelper
 
+    def spree_frontend_path_to_checkout
+      if Spree::Frontend::Engine.checkout_available?
+        spree.checkout_root_path
+      else
+        spree.checkout_path
+      end
+    end
+
     def store_country_iso(store = nil)
       store ||= current_store if defined?(current_store)
 

app/views/spree/orders/edit.html.erb

@@ -82,7 +82,7 @@
             </div>
 
             <div class="d-flex flex-column text-center shopping-cart-buttons" data-hook="cart_buttons">
-              <%= link_to Spree.t('cart_page.checkout'), spree.checkout_path, class: 'btn btn-primary text-uppercase font-weight-bold cart-checkout-btn shopping-cart-buttons-checkout', id: 'checkout-link', method: :get %>
+              <%= link_to Spree.t('cart_page.checkout'), spree_frontend_path_to_checkout, class: 'btn btn-primary text-uppercase font-weight-bold cart-checkout-btn shopping-cart-buttons-checkout', id: 'checkout-link', method: :get %>
               <span class="text-uppercase shopping-cart-buttons-or d-lg-none"><%= Spree.t(:or) %></span>
               <%= link_to Spree.t(:continue_shopping), spree.products_path, class: 'btn btn-outline-primary text-uppercase font-weight-bold d-lg-none shopping-cart-buttons-continue' %>
             </div>

app/views/spree/shared/_product_added_modal.html.erb

@@ -47,7 +47,7 @@
         <div class="container-fluid">
           <div class="row pb-4 justify-content-center">
             <div class="col-12 col-lg-8">
-              <%= link_to spree.checkout_path, class: 'btn btn-primary w-100 font-weight-bold text-uppercase product-added-modal-button', method: :get do %>
+              <%= link_to spree_frontend_path_to_checkout, class: 'btn btn-primary w-100 font-weight-bold text-uppercase product-added-modal-button', method: :get do %>
                 <%= Spree.t(:checkout) %>
               <% end %>
             </div>

config/routes.rb

@@ -9,11 +9,12 @@
     get '/t/*id', to: 'taxons#show', as: :nested_taxons
     get '/product_carousel/:id', to: 'taxons#product_carousel'
 
-    # non-restful checkout stuff
-    patch '/checkout/update/:state', to: 'checkout#update', as: :update_checkout
-    get '/checkout/:state', to: 'checkout#edit', as: :checkout_state
-    get '/checkout', to: 'checkout#edit', as: :checkout
-
+    unless Spree::Frontend::Engine.checkout_available?
+      # non-restful checkout stuff
+      patch '/checkout/update/:state', to: 'checkout#update', as: :update_checkout
+      get '/checkout/:state', to: 'checkout#edit', as: :checkout_state
+      get '/checkout', to: 'checkout#edit', as: :checkout
+    end
     resources :orders, except: [:index, :new, :create, :destroy]
 
     resources :addresses, except: [:index, :show]

lib/spree/frontend/engine.rb

@@ -19,6 +19,10 @@ class Engine < ::Rails::Engine
       initializer 'spree.frontend.environment', before: :load_config_initializers do |_app|
         Spree::Frontend::Config = Spree::Frontend::Configuration.new
       end
+
+      def self.checkout_available?
+        @@checkout_available ||= ::Rails::Engine.subclasses.map(&:instance).map{ |e| e.class.to_s }.include?('Spree::Checkout::Engine')
+      end
     end
   end
 end