Implement dependency resolution for services

includes/Experiments.php

@@ -493,7 +493,7 @@ public function is_experiment_enabled( string $name ): bool {
 	 *
 	 * @return array List of all enabled experiments.
 	 */
-	public function get_enabled_experiments() {
+	public function get_enabled_experiments(): array {
 		$experiments = array_filter(
 			wp_list_pluck( $this->get_experiments(), 'name' ),
 			[ $this, 'is_experiment_enabled' ]

includes/Infrastructure/HasRequirements.php

@@ -0,0 +1,43 @@
+<?php
+/**
+ * Interface HasRequirements.
+ *
+ * @package Google\Web_Stories
+ */
+
+/**
+ * Copyright 2021 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+namespace Google\Web_Stories\Infrastructure;
+
+/**
+ * Something that requires other services to be registered before it can be registered.
+ *
+ * A class marked as having requirements can return the list of services it requires
+ * to be available before it can be registered.
+ *
+ * @since 1.10.0
+ * @internal
+ */
+interface HasRequirements {
+
+	/**
+	 * Get the list of service IDs required for this service to be registered.
+	 *
+	 * @return string[] List of required services.
+	 */
+	public static function get_requirements(): array;
+}

includes/Infrastructure/ServiceBasedPlugin.php

@@ -19,6 +19,8 @@
 
 use Google\Web_Stories\Exception\InvalidService;
 use Google\Web_Stories\Infrastructure\ServiceContainer\LazilyInstantiatedService;
+use function add_action;
+use function apply_filters;
 use function Google\Web_Stories\rewrite_flush;
 
 
@@ -161,7 +163,7 @@ public function deactivate( $network_wide ) {
 	 */
 	public function register() {
 		if ( false !== static::REGISTRATION_ACTION ) {
-			\add_action(
+			add_action(
 				static::REGISTRATION_ACTION,
 				[ $this, 'register_services' ]
 			);
@@ -205,38 +207,89 @@ public function register_services() {
 			 *                                classes need to implement the
 			 *                                Service interface.
 			 */
-			$filtered_services = \apply_filters(
+			$filtered_services = apply_filters(
 				static::HOOK_PREFIX . static::SERVICES_FILTER,
 				$services
 			);
 
 			$services = $this->validate_services( $filtered_services, $services );
 		}
-		foreach ( $services as $id => $class ) {
-			$id    = $this->maybe_resolve( $id );
-			$class = $this->maybe_resolve( $class );
+
+		while ( null !== key( $services ) ) {
+			$id    = $this->maybe_resolve( key( $services ) );
+			$class = $this->maybe_resolve( current( $services ) );
+
+			// Delay registering the service until all requirements are met.
+			if (
+				is_a( $class, HasRequirements::class, true )
+				&&
+				! $this->requirements_are_met( $class, array_keys( $services ) )
+			) {
+				/*
+				 * Move the service to the end of the array.
+				 *
+				 * Note: Unsetting the key advances the internal array pointer to the next array item, so
+				 * the current array item will have to be temporarily stored so that it can be re-added later.
+				 */
+				$delayed_service = [ key( $services ) => current( $services ) ];
+				unset( $services[ key( $services ) ] );
+				$services[ key( $delayed_service ) ] = current( $delayed_service );
+
+				continue;
+			}
+
 			// Allow the services to delay their registration.
 			if ( is_a( $class, Delayed::class, true ) ) {
 				$registration_action = $class::get_registration_action();
+
 				if ( did_action( $registration_action ) ) {
 					$this->register_service( $id, $class );
-
-					continue;
+				} else {
+					add_action(
+						$class::get_registration_action(),
+						function () use ( $id, $class ) {
+							$this->register_service( $id, $class );
+						}
+					);
 				}
 
-				\add_action(
-					$class::get_registration_action(),
-					function () use ( $id, $class ) {
-						$this->register_service( $id, $class );
-					},
-					$class::get_registration_action_priority()
-				);
-
+				next( $services );
 				continue;
 			}
 
 			$this->register_service( $id, $class );
+
+			next( $services );
+		}
+	}
+
+	/**
+	 * Determine if the requirements for a service to be registered are met.
+	 *
+	 * @since 1.10.0
+	 *
+	 * @param class-string|HasRequirements $class       Service with requirements.
+	 * @param string[]                     $service_ids List of service IDs to be registered.
+	 *
+	 * @throws InvalidService If the required service is not recognized.
+	 *
+	 * @return bool Whether the requirements for the service has been met.
+	 */
+	protected function requirements_are_met( $class, $service_ids ): bool {
+		$requirements = $class::get_requirements();
+
+		foreach ( $requirements as $requirement ) {
+			// Bail if it requires a service that is not recognized.
+			if ( ! in_array( $requirement, $service_ids, true ) ) {
+				throw InvalidService::from_service_id( $requirement );
+			}
+
+			if ( ! $this->get_container()->has( $requirement ) ) {
+				return false;
+			}
 		}
+
+		return true;
 	}
 
 	/**
@@ -396,7 +449,7 @@ function () use ( $class ) {
 	 * @param Injector $injector Injector instance to configure.
 	 * @return Injector Configured injector instance.
 	 */
-	protected function configure_injector( Injector $injector ) {
+	protected function configure_injector( Injector $injector ): Injector {
 		$bindings         = $this->get_bindings();
 		$shared_instances = $this->get_shared_instances();
 		$arguments        = $this->get_arguments();
@@ -412,7 +465,7 @@ protected function configure_injector( Injector $injector ) {
 			 *                                implementation bindings. Both
 			 *                                should be FQCNs.
 			 */
-			$bindings = (array) \apply_filters(
+			$bindings = (array) apply_filters(
 				static::HOOK_PREFIX . static::BINDINGS_FILTER,
 				$bindings
 			);
@@ -428,7 +481,7 @@ protected function configure_injector( Injector $injector ) {
 			 *                                array maps argument names to
 			 *                                values.
 			 */
-			$arguments = (array) \apply_filters(
+			$arguments = (array) apply_filters(
 				static::HOOK_PREFIX . static::ARGUMENTS_FILTER,
 				$arguments
 			);
@@ -442,7 +495,7 @@ protected function configure_injector( Injector $injector ) {
 			 * @param array<string> $shared_instances Array of FQCNs to turn
 			 *                                        into shared objects.
 			 */
-			$shared_instances = (array) \apply_filters(
+			$shared_instances = (array) apply_filters(
 				static::HOOK_PREFIX . static::SHARED_INSTANCES_FILTER,
 				$shared_instances
 			);
@@ -456,7 +509,7 @@ protected function configure_injector( Injector $injector ) {
 			 * @param array<string> $delegations Associative array of class =>
 			 *                                   callable mappings.
 			 */
-			$delegations = (array) \apply_filters(
+			$delegations = (array) apply_filters(
 				static::HOOK_PREFIX . static::DELEGATIONS_FILTER,
 				$delegations
 			);

tests/phpunit/includes/Fixture/DummyServiceWithRequirements.php

@@ -0,0 +1,33 @@
+<?php
+/**
+ * Copyright 2021 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+namespace Google\Web_Stories\Tests\Fixture;
+
+use Google\Web_Stories\Infrastructure\HasRequirements;
+use Google\Web_Stories\Infrastructure\Service;
+
+class DummyServiceWithRequirements implements Service, HasRequirements {
+
+	/**
+	 * Get the list of service IDs required for this service to be registered.
+	 *
+	 * @return string[] List of required services.
+	 */
+	public static function get_requirements(): array {
+		return [ 'service_a' ];
+	}
+}

tests/phpunit/tests/Experiments.php

@@ -84,11 +84,14 @@ public function test_add_menu_page() {
 	 * @covers ::initialize_settings
 	 */
 	public function test_initialize_settings() {
+		global $wp_settings_fields, $wp_settings_sections;
+
 		$experiments = new \Google\Web_Stories\Experiments();
 		$experiments->initialize_settings();
 
-		$options = get_registered_settings();
-		$this->assertArrayHasKey( \Google\Web_Stories\Settings::SETTING_NAME_EXPERIMENTS, $options );
+		$this->assertArrayHasKey( \Google\Web_Stories\Experiments::PAGE_NAME, $wp_settings_fields );
+		$this->assertArrayHasKey( \Google\Web_Stories\Experiments::PAGE_NAME, $wp_settings_sections );
+		$this->assertArrayHasKey( 'web_stories_experiments_section', $wp_settings_sections[ \Google\Web_Stories\Experiments::PAGE_NAME ] );
 	}
 
 	/**