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Overview

With Dependencies, you can easily replace parts of Spree core with your custom code. You can replace Services, CanCanCan Abilities (user for Permissions), and API Serializers (used for generating JSON API responses).

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Application (global) customization

This will change every aspect of the application (both APIs, Admin Panel, and Storefront). In your config/initializers/spree.rb file, you can set the following: 
Spree::Dependencies.cart_add_item_service = 'MyAddToCartService'
or 
Spree.dependencies do |dependencies|
  dependencies.cart_add_item_service = 'MyAddToCartService'
end
Now let’s create your custom service. 
mkdir app/services -f && touch app/services/my_add_to_cart_service.rb
And add the following code to it: 
module MyAddToCartService < Spree::Cart::AddItem
  def call(order:, variant:, quantity: nil, public_metadata: {}, private_metadata: {}, options: {})
    ApplicationRecord.transaction do
      run :add_to_line_item
      run Spree::Dependencies.cart_recalculate_service.constantize
      run :update_in_external_system
    end
  end

  private

  def update_in_external_system(new_order_line_item)
    # Your custom logic here
  end
end
This code will:
  1. Inherit from Spree::Cart::AddItem
  2. Override the call method to add your custom logic
  3. Call run :add_to_line_item to add the item to the cart
  4. Call run Spree::Dependencies.cart_recalculate_service.constantize to recalculate the cart
  5. Call run :update_in_external_system to execute your custom logic, eg. updating Order in an external system such as ERP
Values set in the initializer have to be strings, eg. 'MyNewAwesomeAddItemToCart'

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Controller level customization

If you need to replace serializers or Services in a specific API endpoint you can create a code decorator: 
mkdir app/controllers/spree -f && touch app/controllers/spree/cart_controller_decorator.rb
and add the following code to it: 
module Spree
  module CartControllerDecorator
    def resource_serializer
      MyNewAwesomeCartSerializer
    end

    def add_item_service
      MyNewAwesomeAddItemToCart
    end
  end

  CartController.prepend(CartControllerDecorator)
end
This will change the serializer in this API endpoint to MyNewAwesomeCartSerializer and also it will swap the default add_item_service to MyNewAwesomeAddItemToCart. Different API endpoints can have different dependency injection points. You can review their source code to see what you can replace.

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API level customization

Storefront API and Platform API have separate Dependencies injection points so you can easily customize one without touching the other. In your Spree initializer (config/initializers/spree.rb) please add: 
Spree::Api::Dependencies.storefront_cart_serializer = 'MyNewAwesomeCartSerializer'
Spree::Api::Dependencies.storefront_cart_add_item_service = 'MyNewAwesomeAddItemToCart'
This will swap the default Cart serializer and Add Item to Cart service for your custom ones within all Storefront API endpoints that use those classes. You can mix and match both global and API-level customizations: 
Spree::Dependencies.cart_add_item_service = 'MyNewAwesomeAddItemToCart'
Spree::Api::Dependencies.storefront_cart_add_item_service = 'AnotherAddItemToCart'
The second line will have precedence over the first one, and the Storefront API will use AnotherAddItemToCart and the rest of the application will use MyNewAwesomeAddItemToCart

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Default values

By default values can be easily checked by looking at the source code of Dependencies classes: