Issue #864: Support <amp-carousel> in 'Gallery' widget.

There's an existing handler to create 'amp-carousel' elements:
class AMP_Gallery_Embed_Handler.
So override the 'Gallery' widget class.
And use that in render_media().
Otherwise, that function is copied from the parent.
The parent calls gallery_shortcode() at the end of render_media().
And that function doesn't have a filter for the markup.

includes/class-amp-autoloader.php

@@ -86,6 +86,7 @@ class AMP_Autoloader {
 		'AMP_WP_Utils'                                => 'includes/utils/class-amp-wp-utils',
 		'AMP_Widget_Archives'                         => 'includes/widgets/class-amp-widget-archives',
 		'AMP_Widget_Categories'                       => 'includes/widgets/class-amp-widget-categories',
+		'AMP_Widget_Media_Gallery'                    => 'includes/widgets/class-amp-widget-media-gallery',
 		'AMP_Widget_Recent_Comments'                  => 'includes/widgets/class-amp-widget-recent-comments',
 		'WPCOM_AMP_Polldaddy_Embed'                   => 'wpcom/class-amp-polldaddy-embed',
 		'AMP_Test_Stub_Sanitizer'                     => 'tests/stubs',

includes/widgets/class-amp-widget-media-gallery.php

@@ -0,0 +1,51 @@
+<?php
+/**
+ * Class AMP_Widget_Media_Gallery
+ *
+ * @since 0.7.0
+ * @package AMP
+ */
+
+/**
+ * Class AMP_Widget_Media_Gallery
+ *
+ * @since 0.7.0
+ * @package AMP
+ */
+class AMP_Widget_Media_Gallery extends WP_Widget_Media_Gallery {
+
+	/**
+	 * Renders the markup of the widget.
+	 *
+	 * Mainly copied from WP_Widget_Media_Gallery::render_media().
+	 * But instead of calling shortcode_gallery(),
+	 * It uses this plugin's embed handler for galleries.
+	 *
+	 * @since 0.7.0
+	 *
+	 * @param array $instance Data for widget.
+	 * @return void
+	 */
+	public function render_media( $instance ) {
+		if ( ! is_amp_endpoint() ) {
+			parent::render_media( $instance );
+			return;
+		}
+
+		$instance       = array_merge( wp_list_pluck( $this->get_instance_schema(), 'default' ), $instance );
+		$shortcode_atts = array_merge(
+			$instance,
+			array(
+				'link' => $instance['link_type'],
+			)
+		);
+
+		if ( isset( $instance['orderby_random'] ) && ( true === $instance['orderby_random'] ) ) {
+			$shortcode_atts['orderby'] = 'rand';
+		}
+
+		$handler = new AMP_Gallery_Embed_Handler();
+		echo $handler->shortcode( $shortcode_atts ); // WPCS: XSS ok.
+	}
+
+}

tests/test-class-amp-widget-media-gallery.php

@@ -0,0 +1,74 @@
+<?php
+/**
+ * Tests for class AMP_Widget_Media_Gallery.
+ *
+ * @package AMP
+ */
+
+/**
+ * Tests for class AMP_Widget_Media_Gallery.
+ *
+ * @package AMP
+ */
+class Test_AMP_Widget_Media_Gallery extends WP_UnitTestCase {
+
+	/**
+	 * Instance of the widget.
+	 *
+	 * @var object
+	 */
+	public $instance;
+
+	/**
+	 * Setup.
+	 *
+	 * @inheritdoc
+	 */
+	public function setUp() {
+		parent::setUp();
+		AMP_Theme_Support::register_widgets();
+		$this->instance = new AMP_Widget_Media_Gallery();
+	}
+
+	/**
+	 * Test render_media().
+	 *
+	 * @see AMP_Widget_Media_Gallery::widget().
+	 */
+	public function test_render_media() {
+		$first_test_image = '/tmp/test-image.jpg';
+		copy( DIR_TESTDATA . '/images/test-image.jpg', $first_test_image );
+		$first_attachment_id = self::factory()->attachment->create_object( array(
+			'file'           => $first_test_image,
+			'post_parent'    => 0,
+			'post_mime_type' => 'image/jpeg',
+			'post_title'     => 'Test Image',
+		) );
+		wp_update_attachment_metadata( $first_attachment_id, wp_generate_attachment_metadata( $first_attachment_id, $first_test_image ) );
+		$ids[] = $first_attachment_id;
+
+		$second_test_image = '/tmp/test-image.jpg';
+		copy( DIR_TESTDATA . '/images/test-image.jpg', $second_test_image );
+		$second_attachment_id = self::factory()->attachment->create_object( array(
+			'file'           => $second_test_image,
+			'post_parent'    => 0,
+			'post_mime_type' => 'image/jpeg',
+			'post_title'     => 'Test Image',
+		) );
+		wp_update_attachment_metadata( $second_attachment_id, wp_generate_attachment_metadata( $second_attachment_id, $second_test_image ) );
+		$ids[]    = $second_attachment_id;
+		$instance = array(
+			'title' => 'Test Gallery Widget',
+			'ids'   => $ids,
+		);
+
+		ob_start();
+		$this->instance->render_media( $instance );
+		$output = ob_get_clean();
+
+		$this->assertContains( 'amp-carousel', $output );
+		$this->assertContains( $first_test_image, $output );
+		$this->assertContains( $second_test_image, $output );
+	}
+
+}