Create a resolver API for picking a checkout flow

modules/checkout/commerce_checkout.services.yml

@@ -1,7 +1,18 @@
 services:
+  commerce_checkout.chain_checkout_flow_resolver:
+    class: Drupal\commerce_checkout\Resolver\ChainCheckoutFlowResolver
+    tags:
+      - { name: service_collector, call: addResolver, tag: commerce_checkout.checkout_flow_resolver }
+
+  commerce_checkout.default_checkout_flow_resolver:
+    class: Drupal\commerce_checkout\Resolver\DefaultCheckoutFlowResolver
+    arguments: ['@entity_type.manager']
+    tags:
+      - { name: commerce_checkout.checkout_flow_resolver, priority: -100 }
+
   commerce_checkout.checkout_order_manager:
     class: Drupal\commerce_checkout\CheckoutOrderManager
-    arguments: ['@entity_type.manager']
+    arguments: ['@commerce_checkout.chain_checkout_flow_resolver']
 
   plugin.manager.commerce_checkout_flow:
     class: Drupal\commerce_checkout\CheckoutFlowManager

modules/checkout/src/CheckoutOrderManager.php

@@ -2,38 +2,37 @@
 
 namespace Drupal\commerce_checkout;
 
+use Drupal\commerce_checkout\Resolver\ChainCheckoutFlowResolverInterface;
 use Drupal\commerce_order\Entity\OrderInterface;
-use Drupal\Core\Entity\EntityTypeManagerInterface;
 
+/**
+ * Manages checkout flows for orders.
+ */
 class CheckoutOrderManager implements CheckoutOrderManagerInterface {
 
   /**
-   * The entity type manager.
+   * The chain checkout flow resolver.
    *
-   * @var \Drupal\Core\Entity\EntityTypeManagerInterface
+   * @var \Drupal\commerce_checkout\Resolver\ChainCheckoutFlowResolverInterface
    */
-  protected $entityTypeManager;
+  protected $chainCheckoutFlowResolver;
 
   /**
    * Constructs a new CheckoutOrderManager object.
    *
-   * @param \Drupal\Core\Entity\EntityTypeManagerInterface $entity_type_manager
-   *   The entity type manager.
+   * @param \Drupal\commerce_checkout\Resolver\ChainCheckoutFlowResolverInterface $chain_checkout_flow_resolver
+   *   The chain checkout flow resolver.
    */
-  public function __construct(EntityTypeManagerInterface $entity_type_manager) {
-    $this->entityTypeManager = $entity_type_manager;
+  public function __construct(ChainCheckoutFlowResolverInterface $chain_checkout_flow_resolver) {
+    $this->chainCheckoutFlowResolver = $chain_checkout_flow_resolver;
   }
 
   /**
    * {@inheritdoc}
    */
   public function getCheckoutFlow(OrderInterface $order) {
     if ($order->checkout_flow->isEmpty()) {
-      /** @var \Drupal\commerce_order\Entity\OrderTypeInterface $order_type */
-      $order_type = $this->entityTypeManager->getStorage('commerce_order_type')->load($order->bundle());
-      $checkout_flow = $order_type->getThirdPartySetting('commerce_checkout', 'checkout_flow', 'default');
-      // @todo Allow other modules to add their own resolving logic.
-      $order->checkout_flow->target_id = $checkout_flow;
+      $order->checkout_flow = $this->chainCheckoutFlowResolver->resolve($order);
       $order->save();
     }
 

modules/checkout/src/Resolver/ChainCheckoutFlowResolver.php

@@ -0,0 +1,58 @@
+<?php
+
+namespace Drupal\commerce_checkout\Resolver;
+
+use Drupal\commerce_checkout\Entity\CheckoutFlowInterface;
+use Drupal\commerce_order\Entity\OrderInterface;
+
+/**
+ * Default implementation of the chain checkout flow resolver.
+ */
+class ChainCheckoutFlowResolver implements ChainCheckoutFlowResolverInterface {
+
+  /**
+   * The resolvers.
+   *
+   * @var \Drupal\commerce_checkout\Resolver\CheckoutFlowResolverInterface[]
+   */
+  protected $resolvers = [];
+
+  /**
+   * Constructs a new ChainCheckoutFlowResolver object.
+   *
+   * @param \Drupal\commerce_checkout\Resolver\CheckoutFlowResolverInterface[] $resolvers
+   *   The resolvers.
+   */
+  public function __construct(array $resolvers = []) {
+    $this->resolvers = $resolvers;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function addResolver(CheckoutFlowResolverInterface $resolver) {
+    $this->resolvers[] = $resolver;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getResolvers() {
+    return $this->resolvers;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function resolve(OrderInterface $order) {
+    foreach ($this->resolvers as $resolver) {
+      $result = $resolver->resolve($order);
+      if ($result instanceof CheckoutFlowInterface) {
+        return $result;
+      }
+    }
+
+    return NULL;
+  }
+
+}

modules/checkout/src/Resolver/ChainCheckoutFlowResolverInterface.php

@@ -0,0 +1,29 @@
+<?php
+
+namespace Drupal\commerce_checkout\Resolver;
+
+/**
+ * Runs the added resolvers one by one until one of them returns the checkout flow.
+ *
+ * Each resolver in the chain can be another chain, which is why this interface
+ * extends the checkout flow resolver one.
+ */
+interface ChainCheckoutFlowResolverInterface extends CheckoutFlowResolverInterface {
+
+  /**
+   * Adds a resolver.
+   *
+   * @param \Drupal\commerce_checkout\Resolver\CheckoutFlowResolverInterface $resolver
+   *   The resolver.
+   */
+  public function addResolver(CheckoutFlowResolverInterface $resolver);
+
+  /**
+   * Gets all added resolvers.
+   *
+   * @return \Drupal\commerce_checkout\Resolver\CheckoutFlowResolverInterface[]
+   *   The resolvers.
+   */
+  public function getResolvers();
+
+}

modules/checkout/src/Resolver/CheckoutFlowResolverInterface.php

@@ -0,0 +1,24 @@
+<?php
+
+namespace Drupal\commerce_checkout\Resolver;
+
+use Drupal\commerce_order\Entity\OrderInterface;
+
+/**
+ * Defines the interface for checkout flow resolvers.
+ */
+interface CheckoutFlowResolverInterface {
+
+  /**
+   * Resolves the checkout flow.
+   *
+   * @param \Drupal\commerce_order\Entity\OrderInterface $order
+   *   The order that is being checked out.
+   *
+   * @return \Drupal\commerce_checkout\Entity\CheckoutFlowInterface
+   *   The checkout flow, if resolved. Otherwise NULL, indicating that
+   *   the next resolver in the chain should be called.
+   */
+  public function resolve(OrderInterface $order);
+
+}

modules/checkout/src/Resolver/DefaultCheckoutFlowResolver.php

@@ -0,0 +1,41 @@
+<?php
+
+namespace Drupal\commerce_checkout\Resolver;
+
+use Drupal\commerce_order\Entity\OrderInterface;
+use Drupal\Core\Entity\EntityTypeManagerInterface;
+
+/**
+ * Returns the order type's default checkout flow.
+ */
+class DefaultCheckoutFlowResolver implements CheckoutFlowResolverInterface {
+
+  /**
+   * The entity type manager.
+   *
+   * @var \Drupal\Core\Entity\EntityTypeManagerInterface
+   */
+  protected $entityTypeManager;
+
+  /**
+   * Constructs a new DefaultCheckoutFlowResolver object.
+   *
+   * @param \Drupal\Core\Entity\EntityTypeManagerInterface $entity_type_manager
+   *   The entity type manager.
+   */
+  public function __construct(EntityTypeManagerInterface $entity_type_manager) {
+    $this->entityTypeManager = $entity_type_manager;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function resolve(OrderInterface $order) {
+    /** @var \Drupal\commerce_order\Entity\OrderTypeInterface $order_type */
+    $order_type = $this->entityTypeManager->getStorage('commerce_order_type')->load($order->bundle());
+    $checkout_flow_id = $order_type->getThirdPartySetting('commerce_checkout', 'checkout_flow', 'default');
+    $checkout_flow = $this->entityTypeManager->getStorage('commerce_checkout_flow')->load($checkout_flow_id);
+    return $checkout_flow;
+  }
+
+}

modules/checkout/tests/src/Kernel/ChainCheckoutFlowResolverTest.php

@@ -0,0 +1,69 @@
+<?php
+
+namespace Drupal\Tests\commerce_checkout\Kernel;
+
+use Drupal\commerce_order\Entity\Order;
+use Drupal\Tests\commerce\Kernel\CommerceKernelTestBase;
+
+/**
+ * Tests the chain checkout flow resolver.
+ *
+ * @group commerce
+ */
+class ChainCheckoutFlowResolverTest extends CommerceKernelTestBase {
+
+  /**
+   * Modules to enable.
+   *
+   * @var array
+   */
+  public static $modules = [
+    'path',
+    'options',
+    'entity',
+    'entity_reference_revisions',
+    'views',
+    'address',
+    'profile',
+    'state_machine',
+    'inline_entity_form',
+    'commerce',
+    'commerce_price',
+    'commerce_store',
+    'commerce_product',
+    'commerce_order',
+    'commerce_checkout',
+  ];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp() {
+    parent::setUp();
+    $this->installEntitySchema('commerce_order');
+    $this->installConfig('commerce_order');
+    $this->installConfig('commerce_product');
+    $this->installConfig('commerce_checkout');
+  }
+
+  /**
+   * Tests resolving the checkout flow.
+   */
+  public function testCheckoutFlowResolution() {
+    $user = $this->createUser(['mail' => $this->randomString() . '@example.com']);
+    $order = Order::create([
+      'type' => 'default',
+      'mail' => $user->getEmail(),
+      'uid' => $user->id(),
+      'store_id' => $this->store->id(),
+    ]);
+    $order->save();
+
+    $resolver = $this->container->get('commerce_checkout.chain_checkout_flow_resolver');
+    /** @var \Drupal\commerce_checkout\Entity\CheckoutFlowInterface $checkout_flow */
+    $checkout_flow = $resolver->resolve($order);
+
+    $this->assertEquals('default', $checkout_flow->id());
+  }
+
+}