Allow configuring VAT Price Generator class

core/app/models/spree/variant.rb

@@ -439,7 +439,7 @@ def create_stock_items
     end
 
     def build_vat_prices
-      VatPriceGenerator.new(self).run
+      Spree::Config.variant_vat_prices_generator_class.new(self).run
     end
 
     def set_position

core/lib/spree/app_configuration.rb

@@ -292,6 +292,14 @@ def default_pricing_options
 
     class_name_attribute :variant_search_class, default: 'Spree::Core::Search::Variant'
 
+    # Allows implementing custom vat prices generation
+    # @!attribute [rw] variant_vat_prices_generator_class
+    # @see Spree::Variant::VatPriceGenerator
+    # @return [Class] an object that conforms to the API of
+    #   the standard variant vat prices generator class
+    #   Spree::Variant::VatPriceGenerator.
+    class_name_attribute :variant_vat_prices_generator_class, default: 'Spree::Variant::VatPriceGenerator'
+
     # promotion_chooser_class allows extensions to provide their own PromotionChooser
     class_name_attribute :promotion_chooser_class, default: 'Spree::PromotionChooser'
 

guides/source/developers/taxation/value-added-tax.html.md

@@ -24,8 +24,8 @@ consumer_price / (1 + tax_rate) = expected_revenue
 consumer_price - expected_revenue = vat
 ```
 
-Solidus's [Spree::Calculator::DefaultTax][default-tax-calculator] handles 
-sales tax and VAT. If a tax rate is VAT and should be included in the price, it 
+Solidus's [Spree::Calculator::DefaultTax][default-tax-calculator] handles
+sales tax and VAT. If a tax rate is VAT and should be included in the price, it
 calculates all of the line items that share that tax rate on the order:
 
 ```
@@ -40,7 +40,7 @@ end
 ### VAT amounts are stored in `Spree::Adjustment`s
 
 Note that while VAT does not adjust an order's total, Solidus still creates
-`Spree::Adjustment` objects to store tax amount. These objects have an 
+`Spree::Adjustment` objects to store tax amount. These objects have an
 `included` value of `true` so that the tax is not added to the price.
 
 ## Example order with multiple VAT rates
@@ -52,7 +52,7 @@ Our United Kingdom-based company is required to follow these tax regulations:
 
 - Items of clothing should be taxed at a 5% rate.
 - Consumer electronics should be taxed at a 10% rate.
-- We are required to display the VAT paid to the customer. 
+- We are required to display the VAT paid to the customer.
 
 If a customer orders a single clothing item:
 
@@ -91,17 +91,74 @@ If a customer adds a consumer electronics product to the order:
 - The tax calculator calculates the VAT for the clothing items: `37.98 - (37.98
   / (1 + 0.05)) = 1.81`.
 - The tax calculator calculates the VAT for the consumer electronics item:
-  `16.99 - (16.99 / (1 + 0.10)) = 1.54`. 
+  `16.99 - (16.99 / (1 + 0.10)) = 1.54`.
 
 We can now show the display the final included VAT in the price when the
-UK-based customer arrives at the checkout summary page:	
-	
+UK-based customer arrives at the checkout summary page:
+
 ```
   £17.99 – 1 x T-shirt
   £19.99 – 1 x T-shirt
   £16.99 – 1 x Power adapter
    £1.81 – Clothing tax (5%)
    £1.54 – Consumer electronics tax (10%)
-  
+
   £54.97 – TOTAL
 ```
+
+## Customize your VAT generation
+
+If the provided model in core does not fit your business needs, you
+can easily customize the VAT generation logic by defining your own
+VAT generator class. Let's walk into this with an example:
+
+Let's suppose you need to show the same gross price across different
+countries, no matter what the VAT is but still keeping the VAT tax
+to be visible in checkout.
+
+You need to customize the logic used by Solidus to generate the price
+in countries that use VAT (with a TaxRate that has `included_in_price`
+set to `true`).
+
+### Create your own VAT Prices Generator class
+
+You can create your own class that implements this logic. It has to be
+compliant with the interface of the class provided by default in Solidus
+core, which is [Spree::Variant::VatPriceGenerator][Spree::Variant::VatPriceGenerator].
+
+We suggest to inherit from that class and override only the method that
+you need to be different, for example:
+
+```ruby
+# app/spree/variant/custom_vat_price_generator.rb
+
+# frozen_string_literal: true
+
+module Spree
+  class Variant < Spree::Base
+    class CustomVatPriceGenerator < VatPriceGenerator
+      def run
+        # Early return if there is no VAT rates in the current store.
+        return if !variant.tax_category || variant_vat_rates.empty?
+
+        country_isos_requiring_price.each do |country_iso|
+          # Don't re-create the default price
+          next if variant.default_price && variant.default_price.country_iso == country_iso
+
+          foreign_price = find_or_initialize_price_by(country_iso, variant.default_price.currency)
+          foreign_price.amount = variant.default_price.amount
+        end
+      end
+    end
+  end
+end
+```
+
+Now that you have created your custom class, you can easily ask Solidus to
+use it, by setting the following configuration in an initializer:
+
+```ruby
+Spree::Config.variant_vat_prices_generator_class = 'Spree::Variant::CustomVatPriceGenerator'
+```
+
+[Spree::Variant::VatPriceGenerator]: https://github.com/solidusio/solidus/blob/master/core/app/models/spree/variant/vat_price_generator.rb