Add Spree::Config.tax_adjuster extension point

To allow easier customization of tax calculation in extensions or
applications.

CHANGELOG.md

@@ -1,5 +1,10 @@
 ## Solidus 2.1.0 (master, unreleased)
 
+*   Added Spree::Config.tax_adjuster_class
+
+    To allow easier customization of tax calculation in extensions or
+    applications.
+
 *   Remove `is_default` boolean from `Spree::Price` model
 
     This boolean used to mean "the price to be used". With the new
@@ -19,7 +24,7 @@
 *   Removals
 
     * Removed deprecated method `Spree::TaxRate.adjust` (not to be confused with
-      Spree::TaxRate#adjust) in favor of `Spree::Tax::OrderAdjuster`.
+      Spree::TaxRate#adjust) in favor of `Spree::Config.tax_adjuster_class`.
 
       https://github.com/solidusio/solidus/pull/1462
 

core/app/models/spree/app_configuration.rb

@@ -371,6 +371,16 @@ def add_payment_sources_to_wallet_class
       @add_payment_sources_to_wallet_class ||= Spree::Wallet::AddPaymentSourcesToWallet
     end
 
+    # Allows providing your own class for calculating taxes on an order.
+    #
+    # @!attribute [rw] tax_adjuster_class
+    # @return [Class] a class with the same public interfaces as
+    #   Spree::Tax::OrderAdjuster
+    attr_writer :tax_adjuster_class
+    def tax_adjuster_class
+      @tax_adjuster_class ||= Spree::Tax::OrderAdjuster
+    end
+
     def static_model_preferences
       @static_model_preferences ||= Spree::Preferences::StaticModelPreferences.new
     end

core/app/models/spree/order.rb

@@ -350,7 +350,7 @@ def line_item_options_match(line_item, options)
     # include taxes then price adjustments are created instead.
     # @deprecated This now happens during #update!
     def create_tax_charge!
-      Spree::Tax::OrderAdjuster.new(self).adjust!
+      Spree::Config.tax_adjuster_class.new(self).adjust!
     end
     deprecate create_tax_charge!: :update!, deprecator: Spree::Deprecation
 

core/app/models/spree/order_updater.rb

@@ -197,7 +197,7 @@ def update_order_promotions
     end
 
     def update_taxes
-      Spree::Tax::OrderAdjuster.new(order).adjust!
+      Spree::Config.tax_adjuster_class.new(order).adjust!
 
       [*line_items, *shipments].each do |item|
         tax_adjustments = item.adjustments.select(&:tax?)

core/spec/models/spree/order_updater_spec.rb

@@ -298,6 +298,23 @@ def create_adjustment(label, amount)
             }.from(1).to(2)
           end
         end
+
+        context 'with a custom tax_adjuster_class' do
+          let(:custom_adjuster_class) { double }
+          let(:custom_adjuster_instance) { double }
+
+          before do
+            order # generate this first so we can expect it
+            Spree::Config.tax_adjuster_class = custom_adjuster_class
+          end
+
+          it 'uses the configured class' do
+            expect(custom_adjuster_class).to receive(:new).with(order).at_least(:once).and_return(custom_adjuster_instance)
+            expect(custom_adjuster_instance).to receive(:adjust!).at_least(:once)
+
+            order.update!
+          end
+        end
       end
     end