Output admin notice if persistent object caching is not present

includes/utils/class-amp-validation-utils.php

@@ -214,6 +214,7 @@ public static function init() {
 		add_filter( 'bulk_actions-edit-' . self::POST_TYPE_SLUG, array( __CLASS__, 'add_bulk_action' ), 10, 2 );
 		add_filter( 'handle_bulk_actions-edit-' . self::POST_TYPE_SLUG, array( __CLASS__, 'handle_bulk_action' ), 10, 3 );
 		add_action( 'admin_notices', array( __CLASS__, 'remaining_error_notice' ) );
+		add_action( 'admin_notices', array( __CLASS__, 'persistent_caching_object_notice' ) );
 		add_action( 'post_action_' . self::RECHECK_ACTION, array( __CLASS__, 'handle_inline_recheck' ) );
 		add_action( 'admin_menu', array( __CLASS__, 'remove_publish_meta_box' ) );
 		add_action( 'admin_menu', array( __CLASS__, 'add_admin_menu_validation_status_count' ) );
@@ -1791,4 +1792,23 @@ public static function get_recheck_link( $post, $redirect_url, $recheck_url = nu
 		);
 	}
 
+	/**
+	 * Outputs an admin notice if persistent object cache is not present.
+	 *
+	 * @return void
+	 */
+	public static function persistent_caching_object_notice() {
+		$screen = get_current_screen();
+		if ( 'toplevel_page_amp-options' === $screen->id ) {
+			if ( ! wp_using_ext_object_cache() ) {
+				?>
+				<div class="notice notice-warning">
+					<p><?php esc_html_e( 'The AMP plugin performs at its best when persistent object cache is enabled.', 'amp' ); ?></p>
+					<p><a href="<?php echo esc_url( 'https://codex.wordpress.org/Class_Reference/WP_Object_Cache#Persistent_Caching' ); ?>">More details</a></p>
+				</div>
+				<?php
+			}
+		}
+	}
+
 }

tests/test-class-amp-validation-utils.php

@@ -114,6 +114,7 @@ public function test_init() {
 		$this->assertEquals( 10, has_filter( 'bulk_actions-edit-' . AMP_Validation_Utils::POST_TYPE_SLUG, self::TESTED_CLASS . '::add_bulk_action' ) );
 		$this->assertEquals( 10, has_filter( 'handle_bulk_actions-edit-' . AMP_Validation_Utils::POST_TYPE_SLUG, self::TESTED_CLASS . '::handle_bulk_action' ) );
 		$this->assertEquals( 10, has_action( 'admin_notices', self::TESTED_CLASS . '::remaining_error_notice' ) );
+		$this->assertEquals( 10, has_action( 'admin_notices', self::TESTED_CLASS . '::persistent_caching_object_notice' ) );
 		$this->assertEquals( 10, has_action( 'admin_menu', self::TESTED_CLASS . '::remove_publish_meta_box' ) );
 		$this->assertEquals( 10, has_action( 'add_meta_boxes', self::TESTED_CLASS . '::add_meta_boxes' ) );
 	}
@@ -1359,4 +1360,42 @@ public function get_mock_errors() {
 		);
 	}
 
+	/**
+	 * Test for persistent_caching_object_notice()
+	 *
+	 * @covers AMP_Validation_Utils::persistent_caching_object_notice()
+	 */
+	public function test_persistent_caching_object_notice() {
+		set_current_screen( 'toplevel_page_amp-options' );
+		$needle = 'The AMP plugin performs at its best when persistent object cache is enabled.';
+
+		$using = null;
+		wp_using_ext_object_cache( $using );
+		ob_start();
+		AMP_Validation_Utils::persistent_caching_object_notice();
+		$this->assertContains( $needle, ob_get_clean() );
+
+		$using = true;
+		wp_using_ext_object_cache( $using );
+		ob_start();
+		AMP_Validation_Utils::persistent_caching_object_notice();
+		$this->assertNotContains( $needle, ob_get_clean() );
+
+		set_current_screen( 'edit.php' );
+
+		$using = null;
+		wp_using_ext_object_cache( $using );
+		ob_start();
+		AMP_Validation_Utils::persistent_caching_object_notice();
+		$this->assertNotContains( $needle, ob_get_clean() );
+
+		$using = true;
+		wp_using_ext_object_cache( $using );
+		ob_start();
+		AMP_Validation_Utils::persistent_caching_object_notice();
+		$this->assertNotContains( $needle, ob_get_clean() );
+
+		unset( $GLOBALS['current_screen'] );
+	}
+
 }