Issue #864: Support 'Categories' widget dropdown.

Props @westonruter for the details of how to do this.
Mainly copy WP_Widget_Categories::widget() into the subclass widget().
Add an 'id' attribute to the <form>.
And use the 'id' in the dropdown <select>.
This dropdown now redirects to the category pages, as expected.
Also, bootstrap the widget support class.

amp.php

@@ -71,6 +71,7 @@ function amp_after_setup_theme() {
 	}
 
 	add_action( 'init', 'amp_init' );
+	add_action( 'init', 'amp_add_widget_support' );
 	add_action( 'admin_init', 'AMP_Options_Manager::register_settings' );
 	add_filter( 'amp_post_template_analytics', 'amp_add_custom_analytics' );
 	add_action( 'wp_loaded', 'amp_post_meta_box' );

includes/admin/functions.php

@@ -136,6 +136,16 @@ function amp_add_custom_analytics( $analytics ) {
 	return $analytics;
 }
 
+/**
+ * Bootstraps AMP widget support class, which registers AMP-compliant widgets.
+ *
+ * @since 0.7
+ */
+function amp_add_widget_support() {
+	$amp_widgets = new AMP_Widgets();
+	$amp_widgets->init();
+}
+
 /**
  * Bootstrap AMP post meta box.
  *

includes/widgets/class-amp-widget-categories.php

@@ -12,20 +12,89 @@
  */
 class AMP_Widget_Categories extends WP_Widget_Categories {
 
+	/**
+	 * Adds an 'on' attribute to the category dropdown's <select>.
+	 *
+	 * @param string $dropdown The markup of the category dropdown.
+	 * @return string $dropdown The filtered markup of the dropdown.
+	 */
+	public function modify_select( $dropdown ) {
+		$new_select = sprintf( '<select on="change:widget-categories-dropdown-%d.submit"', esc_attr( $this->number ) );
+		return str_replace( '<select', $new_select, $dropdown );
+	}
+
 	/**
 	 * Echoes the markup of the widget.
 	 *
-	 * @todo filter $output, to strip the <script> tag.
-	 * @see https://github.com/Automattic/amp-wp/issues/864
+	 * Mainly copied from WP_Widget_Categories::widget()
+	 * There's now an id for the <form>.
+	 * And the dropdown is now filtered with 'wp_dropdown_cats.'
+	 * This enables adding an 'on' attribute, with the id of the form.
+	 * So changing the dropdown value will redirect to the category page, with valid AMP.
+	 *
 	 * @param array $args Widget display data.
 	 * @param array $instance Data for widget.
 	 * @return void.
 	 */
 	public function widget( $args, $instance ) {
-		ob_start();
-		parent::widget( $args, $instance );
-		$output = ob_get_clean();
-		echo AMP_Theme_Support::filter_the_content( $output ); // WPCS: XSS ok.
+		static $first_dropdown = true;
+		$title                 = ! empty( $instance['title'] ) ? $instance['title'] : __( 'Categories' ); // phpcs:ignore.
+		/** This filter is documented in wp-includes/widgets/class-wp-widget-pages.php */
+		$title = apply_filters( 'widget_title', $title, $instance, $this->id_base );
+		$c     = ! empty( $instance['count'] ) ? '1' : '0';
+		$h     = ! empty( $instance['hierarchical'] ) ? '1' : '0';
+		$d     = ! empty( $instance['dropdown'] ) ? '1' : '0';
+		echo wp_kses_post( $args['before_widget'] );
+		if ( $title ) {
+			echo wp_kses_post( $args['before_title'] . $title . $args['after_title'] );
+		}
+		$cat_args = array(
+			'orderby'      => 'name',
+			'show_count'   => $c,
+			'hierarchical' => $h,
+		);
+		if ( $d ) :
+			echo sprintf( '<form action="%s" method="get" id="widget-categories-dropdown-%d">', esc_url( home_url() ), esc_attr( $this->number ) );
+			add_filter( 'wp_dropdown_cats', array( $this, 'modify_select' ) );
+			$dropdown_id    = ( $first_dropdown ) ? 'cat' : "{$this->id_base}-dropdown-{$this->number}";
+			$first_dropdown = false;
+			echo '<label class="screen-reader-text" for="' . esc_attr( $dropdown_id ) . '">' . esc_html( $title ) . '</label>';
+			$cat_args['show_option_none'] = __( 'Select Category' ); // phpcs:ignore.
+			$cat_args['id']               = $dropdown_id;
+			/**
+			 * Filters the arguments for the Categories widget drop-down.
+			 *
+			 * @since 2.8.0
+			 * @since 4.9.0 Added the `$instance` parameter.
+			 *
+			 * @see wp_dropdown_categories()
+			 *
+			 * @param array $cat_args An array of Categories widget drop-down arguments.
+			 * @param array $instance Array of settings for the current widget.
+			 */
+			wp_dropdown_categories( apply_filters( 'widget_categories_dropdown_args', $cat_args, $instance ) );
+			remove_filter( 'wp_dropdown_cats', array( $this, 'modify_select' ) );
+			echo '</form>';
+		else :
+			?>
+			<ul>
+			<?php
+			$cat_args['title_li'] = '';
+			/**
+			 * Filters the arguments for the Categories widget.
+			 *
+			 * @since 2.8.0
+			 * @since 4.9.0 Added the `$instance` parameter.
+			 *
+			 * @param array $cat_args An array of Categories widget options.
+			 * @param array $instance Array of settings for the current widget.
+			 */
+			wp_list_categories( apply_filters( 'widget_categories_args', $cat_args, $instance ) );
+			?>
+			</ul>
+		<?php
+		endif;
+		echo wp_kses_post( $args['after_widget'] );
 	}
 
 }

tests/test-class-amp-widget-categories.php

@@ -48,6 +48,20 @@ public function test_construct() {
 		$this->assertEquals( 'A list or dropdown of categories.', $amp_categories->widget_options['description'] );
 	}
 
+	/**
+	 * Test modify_select().
+	 *
+	 * @see AMP_Widget_Categories::modify_select().
+	 */
+	public function test_modify_select() {
+		$categories             = wp_dropdown_categories( array(
+			'echo' => 0,
+		) );
+		$number                 = 3;
+		$this->instance->number = $number;
+		$this->assertContains( strval( $number ), $this->instance->modify_select( $categories ) );
+	}
+
 	/**
 	 * Test widget().
 	 *