Move validations to state transitions

app/controllers/split_checkout_controller.rb

@@ -37,10 +37,7 @@ class SplitCheckoutController < ::BaseController
       clear_invalid_payments
       redirect_to_step
     else
-      if @shipping_method_id.blank?
+      flash.now[:error] = I18n.t('split_checkout.errors.global')
-        @order.errors.add(:base, "no_shipping_method_selected")
-      end
-      flash.now[:error] = "#{I18n.t('split_checkout.errors.global')}"
       render :edit
     end
   end
@@ -60,7 +57,6 @@ class SplitCheckoutController < ::BaseController
 
   def update_order
     return unless params[:order]
-    return if @order.state == "address" && params[:shipping_method_id].blank?
 
     @order.update(order_params) && advance_order_state
   end

app/models/spree/order/checkout.rb

@@ -75,7 +75,9 @@ module Spree
 
               before_transition to: :delivery, do: :create_proposed_shipments
               before_transition to: :delivery, do: :ensure_available_shipping_rates
+              before_transition to: :payment, do: :validate_shipping_method!
               before_transition to: :payment, do: :create_tax_charge!
+              before_transition to: :confirmation, do: :validate_payment_method!
 
               after_transition to: :complete, do: :finalize!
               after_transition to: :resumed,  do: :after_resume
@@ -135,6 +137,24 @@ module Spree
               updated_at: Time.zone.now,
             )
           end
+
+          private
+
+          def validate_shipping_method!
+            return unless user && Flipper.enabled?(:split_checkout, user)
+            return if shipping_method.present?
+
+            errors.add :shipping_method, I18n.t('split_checkout.errors.select_a_shipping_method')
+            throw :halt
+          end
+
+          def validate_payment_method!
+            return unless user && Flipper.enabled?(:split_checkout, user)
+            return if payments.any?
+
+            errors.add :payment_method, I18n.t('split_checkout.errors.select_a_payment_method')
+            throw :halt
+          end
         end
       end
     end

config/locales/en.yml

@@ -1660,8 +1660,6 @@ en:
           label: Country
       delivery_address:
           title: Delivery address
-          errors:
-            no_shipping_method_selected: No shipping method selected
       submit: Next - Payment method
       cancel: Back to Edit basket
     step2:
@@ -1697,6 +1695,8 @@ en:
       required: Field cannot be blank
       invalid_number: "Please enter a valid phone number"
       invalid_email: "Please enter a valid email address"
+      select_a_shipping_method: Select a shipping method
+      select_a_payment_method: Select a payment method
   order_paid: PAID
   order_not_paid: NOT PAID
   order_total: Total order