Extensible Widget Visibility

diff --git a/modules/widget-visibility/widget-conditions.php b/modules/widget-visibility/widget-conditions.php
index bcb32dc..6aedfec 100644
--- a/modules/widget-visibility/widget-conditions.php
+++ b/modules/widget-visibility/widget-conditions.php
@@ -17,6 +17,26 @@ class Jetpack_Widget_Conditions {
 			add_action( 'widget_display_callback', array( __CLASS__, 'filter_widget' ) );
 			add_action( 'sidebars_widgets', array( __CLASS__, 'sidebars_widgets' ) );
 		}
+
+		/**
+		 * The widget_visibility_major_conditions filter accepts an array of existing conditions
+		 * and should add new conditions in this format:
+		 *
+		 * $existing_conditions['my_unique_key'] = array(
+		 *     'title' => 'User-facing label for this condition type',
+		 *     'callback' => callable that accepts two arguments: a major condition (e.g., 'my_unique_key')
+		 *                   and optional minor value (e.g., 'one_of_several_values')
+		 */
+		add_filter( 'widget_visibility_major_conditions', array( __CLASS__, 'major_conditions' ) );
+
+		/**
+		 * The widget_visibility_minor_conditions filter accepts two arguments: a major condition (e.g.,
+		 * 'my_unique_key') and an array of exisiting minor values. It should add new minor conditions in
+		 * this format:
+		 *
+		 * $existing_minor_conditions['a_unique_key'] = 'User-facing label for this condition type'
+		 */
+		add_filter( 'widget_visibility_minor_conditions', array( __CLASS__, 'minor_conditions' ), 10, 2 );
 	}
 
 	public static function widget_admin_setup() {
@@ -25,120 +45,278 @@ class Jetpack_Widget_Conditions {
 	}
 
 	/**
-	 * Provided a second level of granularity for widget conditions.
+	 * Register the major condition types that we'll support by default.
 	 */
-	public static function widget_conditions_options_echo( $major = '', $minor = '' ) {
-		switch ( $major ) {
+	public static function major_conditions( $major_conditions ) {
+		return array_merge( $major_conditions, array(
+			'category' => array(
+				'title' => __( 'Category', 'jetpack' ),
+				'callback' => array( __CLASS__, 'condition_callback' ),
+			),
+			'author' => array(
+				'title' => _x( 'Author', 'Noun, as in: "The author of this post is..."', 'jetpack' ),
+				'callback' => array( __CLASS__, 'condition_callback' ),
+			),
+			'tag' => array(
+				'title' => _x( 'Tag', 'Noun, as in: "This post has one tag."', 'jetpack' ),
+				'callback' => array( __CLASS__, 'condition_callback' ),
+			),
+			'date' => array(
+				'title' => _x( 'Date', 'Noun, as in: "This page is a date archive."', 'jetpack' ),
+				'callback' => array( __CLASS__, 'condition_callback' ),
+			),
+			'page' => array(
+				'title' => _x( 'Page', 'Example: The user is looking at a page, not a post.', 'jetpack' ),
+				'callback' => array( __CLASS__, 'condition_callback' ),
+			),
+			'taxonomy' => array(
+				'title' => _x( 'Taxonomy', 'Noun, as in: "This post has one taxonomy."', 'jetpack' ),
+				'callback' => array( __CLASS__, 'condition_callback' ),
+			),
+		) );
+	}
+
+	/**
+	 * Register the possible values for the major conditions that we support.
+	 *
+	 * @param array $minor_conditions An associative array of possible values.
+	 * @param string $major_condition The condition for these values.
+	 * @return array
+	 */
+	public static function minor_conditions( $minor_conditions, $major_condition ) {
+		switch ( $major_condition ) {
 			case 'category':
-				?>
-				<option value=""><?php _e( 'All category pages', 'jetpack' ); ?></option>
-				<?php
+				$minor_conditions[''] = __( 'All category pages', 'jetpack' );
 
 				$categories = get_categories( array( 'number' => 1000, 'orderby' => 'count', 'order' => 'DESC' ) );
 				usort( $categories, array( __CLASS__, 'strcasecmp_name' ) );
-
+		
 				foreach ( $categories as $category ) {
-					?>
-					<option value="<?php echo esc_attr( $category->term_id ); ?>" <?php selected( $category->term_id, $minor ); ?>><?php echo esc_html( $category->name ); ?></option>
-					<?php
+					$minor_conditions[$category->term_id] = $category->name;
 				}
 			break;
 			case 'author':
-				?>
-				<option value=""><?php _e( 'All author pages', 'jetpack' ); ?></option>
-				<?php
-
+				$minor_conditions[''] = __( 'All author pages', 'jetpack' );
+				
 				foreach ( get_users( array( 'orderby' => 'name', 'exclude_admin' => true ) ) as $author ) {
-					?>
-					<option value="<?php echo esc_attr( $author->ID ); ?>" <?php selected( $author->ID, $minor ); ?>><?php echo esc_html( $author->display_name ); ?></option>
-					<?php
+					$minor_conditions[$author->ID] = $author->display_name;
 				}
 			break;
 			case 'tag':
-				?>
-				<option value=""><?php _e( 'All tag pages', 'jetpack' ); ?></option>
-				<?php
-
+				$minor_conditions[''] = __( 'All tag pages', 'jetpack' );
+				
 				$tags = get_tags( array( 'number' => 1000, 'orderby' => 'count', 'order' => 'DESC' ) );
 				usort( $tags, array( __CLASS__, 'strcasecmp_name' ) );
 
 				foreach ( $tags as $tag ) {
-					?>
-					<option value="<?php echo esc_attr($tag->term_id ); ?>" <?php selected( $tag->term_id, $minor ); ?>><?php echo esc_html( $tag->name ); ?></option>
-					<?php
+					$minor_conditions[$tag->term_id] = $tag->name;
 				}
 			break;
 			case 'date':
-				?>
-				<option value="" <?php selected( '', $minor ); ?>><?php _e( 'All date archives', 'jetpack' ); ?></option>
-				<option value="day"<?php selected( 'day', $minor ); ?>><?php _e( 'Daily archives', 'jetpack' ); ?></option>
-				<option value="month"<?php selected( 'month', $minor ); ?>><?php _e( 'Monthly archives', 'jetpack' ); ?></option>
-				<option value="year"<?php selected( 'year', $minor ); ?>><?php _e( 'Yearly archives', 'jetpack' ); ?></option>
-				<?php
+				$minor_conditions[''] = __( 'All date archives', 'jetpack' );
+				$minor_conditions['day'] = __( 'Daily archives', 'jetpack' );
+				$minor_conditions['month'] = __( 'Monthly archives', 'jetpack' );
+				$minor_conditions['year'] = __( 'Yearly archives', 'jetpack' );
 			break;
 			case 'page':
-				// Previously hardcoded post type options.
-				if ( ! $minor )
-					$minor = 'post_type-page';
-				else if ( 'post' == $minor )
-					$minor = 'post_type-post';
+				$minor_conditions['front'] = __( 'Front page', 'jetpack' );
+				$minor_conditions['posts'] = __( 'Posts page', 'jetpack' );
+				$minor_conditions['archive'] = __( 'Archive page', 'jetpack' );
+				$minor_conditions['404'] = __( '404 error page', 'jetpack' );
+				$minor_conditions['search'] = __( 'Search results', 'jetpack' );
 
-				?>
-				<option value="front" <?php selected( 'front', $minor ); ?>><?php _e( 'Front page', 'jetpack' ); ?></option>
-				<option value="posts" <?php selected( 'posts', $minor ); ?>><?php _e( 'Posts page', 'jetpack' ); ?></option>
-				<option value="archive" <?php selected( 'archive', $minor ); ?>><?php _e( 'Archive page', 'jetpack' ); ?></option>
-				<option value="404" <?php selected( '404', $minor ); ?>><?php _e( '404 error page', 'jetpack' ); ?></option>
-				<option value="search" <?php selected( 'search', $minor ); ?>><?php _e( 'Search results', 'jetpack' ); ?></option>
-				<optgroup label="<?php esc_attr_e( 'Post type:', 'jetpack' ); ?>">
-					<?php
+				$post_type_values = array();
+				$post_types = get_post_types( array( 'public' => true ), 'objects' );
 
-					$post_types = get_post_types( array( 'public' => true ), 'objects' );
+				foreach ( $post_types as $post_type ) {
+					$post_type_values[ 'post_type-' . $post_type->name ] = $post_type->labels->singular_name;
+				}
 
-					foreach ( $post_types as $post_type ) {
-						?>
-						<option value="<?php echo esc_attr( 'post_type-' . $post_type->name ); ?>" <?php selected( 'post_type-' . $post_type->name, $minor ); ?>><?php echo esc_html( $post_type->labels->singular_name ); ?></option>
-						<?php
-					}
+				$minor_conditions[ __( 'Post type:', 'jetpack' ) ] = $post_type_values;
 
-					?>
-				</optgroup>
-				<optgroup label="<?php esc_attr_e( 'Static page:', 'jetpack' ); ?>">
-					<?php
+				$static_pages = array();
 
-					echo str_replace( ' value="' . esc_attr( $minor ) . '"', ' value="' . esc_attr( $minor ) . '" selected="selected"', preg_replace( '/<\/?select[^>]*?>/i', '', wp_dropdown_pages( array( 'echo' => false ) ) ) );
+				$pages = get_pages( array( 'sort_order' => 'menu_order' ) );
+				$parent_depths = array();
+				
+				foreach ( $pages as $page ) {
+					if ( isset( $parent_depths[ $page->post_parent ] ) ) {
+						$depth = $parent_depths[ $page->post_parent ] + 1;
+					}
+					else {
+						$depth = 0;
+					}
+					
+					$parent_depths[ $page->ID ] = $depth;
 
-					?>
-				</optgroup>
-				<?php
+					$static_pages[ $page->ID ] = str_repeat( "&nbsp;", $depth * 2 ) . $page->post_title;
+				}
+
+				$minor_conditions[ __( 'Static page:', 'jetpack' ) ] = $static_pages;
 			break;
 			case 'taxonomy':
-				?>
-				<option value=""><?php _e( 'All taxonomy pages', 'jetpack' ); ?></option>
-				<?php
+				$minor_conditions[''] = __( 'All taxonomy pages', 'jetpack' );
 
 				$taxonomies = get_taxonomies( array( '_builtin' => false ), 'objects' );
 				usort( $taxonomies, array( __CLASS__, 'strcasecmp_name' ) );
 
 				foreach ( $taxonomies as $taxonomy ) {
-					?>
-					<optgroup label="<?php esc_attr_e( $taxonomy->labels->name . ':', 'jetpack' ); ?>">
-						<option value="<?php echo esc_attr( $taxonomy->name ); ?>" <?php selected( $taxonomy->name, $minor ); ?>><?php echo 'All ' . esc_html( $taxonomy->name ) . ' pages'; ?></option>
-						<?php
-						
-						$terms = get_terms( array( $taxonomy->name ), array( 'number' => 1000, 'hide_empty' => false ) );
-						foreach ( $terms as $term ) {
-							?>
-							<option value="<?php echo esc_attr( $taxonomy->name . '_tax_' . $term->term_id ); ?>" <?php selected( $taxonomy->name . '_tax_' . $term->term_id, $minor ); ?>><?php echo esc_html( $term->name ); ?></option>
-							<?php
+					$sub_taxonomies = array(
+						$taxonomy->name => sprintf( __( 'All %s pages', 'jetpack' ), $taxonomy->name ),
+					);
+
+					$terms = get_terms( array( $taxonomy->name ), array( 'number' => 1000, 'hide_empty' => false ) );
+					foreach ( $terms as $term ) {
+						$sub_taxonomies[ $taxonomy->name . '_tax_' . $term->term_id ] = $term->name;
+					}
+
+					$minor_conditions[ $taxonomy->labels->name . ':' ] = $sub_taxonomies;
+				}
+			break;
+		}
+
+		return $minor_conditions;
+	}
+
+	/**
+	 * The callback that does the actual filtering.
+	 *
+	 * @param string $major The "major" filter category.
+	 * @param string $minor The "minor" filter value.
+	 * @return boolean Whether this condition is met in the current context.
+	 */
+	public static function condition_callback( $major, $minor ) {
+		switch ( $major ) {
+			case 'date':
+				switch ( $minor ) {
+					case '':
+						return is_date();
+					break;
+					case 'month':
+						return is_month();
+					break;
+					case 'day':
+						return is_day();
+					break;
+					case 'year':
+						return is_year();
+					break;
+				}
+			break;
+			case 'page':
+				// Previously hardcoded post type options.
+				if ( 'post' == $minor )
+					$minor = 'post_type-post';
+				else if ( ! $minor )
+					$minor = 'post_type-page';
+
+				switch ( $minor ) {
+					case '404':
+						return is_404();
+					break;
+					case 'search':
+						return is_search();
+					break;
+					case 'archive':
+						return is_archive();
+					break;
+					case 'posts':
+						return $wp_query->is_posts_page;
+					break;
+					case 'home':
+						return is_home();
+					break;
+					case 'front':
+						if ( current_theme_supports( 'infinite-scroll' ) )
+							return is_front_page();
+						else {
+							return is_front_page() && !is_paged();
 						}
+					break;
+					default:
+						if ( substr( $minor, 0, 10 ) == 'post_type-' )
+							return is_singular( substr( $minor, 10 ) );
+						else {
+							// $minor is a page ID -- check if we're either looking at that particular page itself OR looking at the posts page, with the correct conditions
+							return ( is_page( $minor ) || ( get_option( 'show_on_front' ) == 'page' && $wp_query->is_posts_page && get_option( 'page_for_posts' ) == $minor ) );
+						}
+					break;
+				}
+			break;
+			case 'tag':
+				if ( ! $minor && is_tag() )
+					return true;
+				else if ( is_singular() && $minor && has_tag( $minor ) )
+					return true;
+				else {
+					$tag = get_tag( $minor );
 
-						?>
-					</optgroup>
-					<?php
-					
-				
+					if ( $tag && is_tag( $tag->slug ) )
+						return true;
 				}
 			break;
+			case 'category':
+				if ( ! $minor && is_category() )
+					return true;
+				else if ( is_category( $minor ) )
+					return true;
+				else if ( is_singular() && $minor && in_array( 'category', get_post_taxonomies() ) &&  has_category( $minor ) )
+					return true;
+			break;
+			case 'author':
+				if ( ! $minor && is_author() )
+					return true;
+				else if ( $minor && is_author( $minor ) )
+					return true;
+				else if ( is_singular() && $minor && $minor == $post->post_author )
+					return true;
+			break;
+			case 'taxonomy':
+				$term = explode( '_tax_', $minor ); // $term[0] = taxonomy name; $term[1] = term id
+				$terms = get_the_terms( $post->ID, $minor ); // Does post have terms in taxonomy?
+				if ( is_tax( $term[0], $term[1] ) )
+					return true;
+				else if ( is_singular() && $term[1] && has_term( $term[1], $term[0] ) )
+					return true;
+				else if ( is_singular() && $terms & !is_wp_error( $terms ) )
+					return true;
+			break;
+		}
+
+		return false;
+	}
+
+	/**
+	 * Provided a second level of granularity for widget conditions.
+	 */
+	public static function widget_conditions_options_echo( $major = '', $selected_value = '' ) {
+		if ( $major ) {
+			$minor_conditions = apply_filters( 'widget_visibility_minor_conditions', array(), $major );
+
+			foreach ( $minor_conditions as $key => $val ) {
+				self::do_widget_conditions_options_echo( $selected_value, $key, $val );
+			}
+		}
+	}
+
+	private static function do_widget_conditions_options_echo( $selected_value, $minor_key, $minor_value ) {
+		if ( is_array( $minor_value ) ) {
+			?>
+			<optgroup label="<?php echo esc_attr( $minor_key ); ?>">
+				<?php
+
+				foreach ( $minor_value as $grouped_minor_key => $grouped_minor_value ) {
+					self::do_widget_conditions_options_echo( $selected_value, $grouped_minor_key, $grouped_minor_value );
+				}
+
+				?>
+			</optgroup>
+			<?php
+		}
+		else {
+			?>
+			<option value="<?php echo esc_attr( $minor_key ); ?>" <?php selected( $minor_key, $selected_value ); ?>><?php echo esc_html( $minor_value ); ?></option>
+			<?php
 		}
 	}
 
@@ -169,6 +347,8 @@ class Jetpack_Widget_Conditions {
 		if ( empty( $conditions['rules'] ) )
 			$conditions['rules'][] = array( 'major' => '', 'minor' => '' );
 
+		$major_conditions = apply_filters( 'widget_visibility_major_conditions', array() );
+
 		?>
 		<div class="widget-conditional <?php if ( empty( $_POST['widget-conditions-visible'] ) || $_POST['widget-conditions-visible'] == '0' ) { ?>widget-conditional-hide<?php } ?>">
 			<input type="hidden" name="widget-conditions-visible" value="<?php if ( isset( $_POST['widget-conditions-visible'] ) ) { echo esc_attr( $_POST['widget-conditions-visible'] ); } else { ?>0<?php } ?>" />
@@ -187,12 +367,9 @@ class Jetpack_Widget_Conditions {
 							<div class="alignleft">
 								<select class="conditions-rule-major" name="conditions[rules_major][]">
 									<option value="" <?php selected( "", $rule['major'] ); ?>><?php echo esc_html_x( '-- Select --', 'Used as the default option in a dropdown list', 'jetpack' ); ?></option>
-									<option value="category" <?php selected( "category", $rule['major'] ); ?>><?php esc_html_e( 'Category', 'jetpack' ); ?></option>
-									<option value="author" <?php selected( "author", $rule['major'] ); ?>><?php echo esc_html_x( 'Author', 'Noun, as in: "The author of this post is..."', 'jetpack' ); ?></option>
-									<option value="tag" <?php selected( "tag", $rule['major'] ); ?>><?php echo esc_html_x( 'Tag', 'Noun, as in: "This post has one tag."', 'jetpack' ); ?></option>
-									<option value="date" <?php selected( "date", $rule['major'] ); ?>><?php echo esc_html_x( 'Date', 'Noun, as in: "This page is a date archive."', 'jetpack' ); ?></option>
-									<option value="page" <?php selected( "page", $rule['major'] ); ?>><?php echo esc_html_x( 'Page', 'Example: The user is looking at a page, not a post.', 'jetpack' ); ?></option>
-									<option value="taxonomy" <?php selected( "taxonomy", $rule['major'] ); ?>><?php echo esc_html_x( 'Taxonomy', 'Noun, as in: "This post has one taxonomy."', 'jetpack' ); ?></option>
+									<?php foreach ( $major_conditions as $major_condition_key => $major_condition_meta ) { ?>
+										<option value="<?php echo esc_attr( $major_condition_key ); ?>" <?php selected( $major_condition_key, $rule['major'] ); ?>><?php echo esc_html( $major_condition_meta['title'] ); ?></option>
+									<?php } ?>
 								</select>
 								<?php _ex( 'is', 'Widget Visibility: {Rule Major [Page]} is {Rule Minor [Search results]}', 'jetpack' ); ?>
 								<select class="conditions-rule-minor" name="conditions[rules_minor][]" <?php if ( ! $rule['major'] ) { ?> disabled="disabled"<?php } ?> data-loading-text="<?php esc_attr_e( 'Loading...', 'jetpack' ); ?>">
@@ -322,107 +499,16 @@ class Jetpack_Widget_Conditions {
 
 		$condition_result = false;
 
-		foreach ( $instance['conditions']['rules'] as $rule ) {
-			switch ( $rule['major'] ) {
-				case 'date':
-					switch ( $rule['minor'] ) {
-						case '':
-							$condition_result = is_date();
-						break;
-						case 'month':
-							$condition_result = is_month();
-						break;
-						case 'day':
-							$condition_result = is_day();
-						break;
-						case 'year':
-							$condition_result = is_year();
-						break;
-					}
-				break;
-				case 'page':
-					// Previously hardcoded post type options.
-					if ( 'post' == $rule['minor'] )
-						$rule['minor'] = 'post_type-post';
-					else if ( ! $rule['minor'] )
-						$rule['minor'] = 'post_type-page';
-
-					switch ( $rule['minor'] ) {
-						case '404':
-							$condition_result = is_404();
-						break;
-						case 'search':
-							$condition_result = is_search();
-						break;
-						case 'archive':
-							$condition_result = is_archive();
-						break;
-						case 'posts':
-							$condition_result = $wp_query->is_posts_page;
-						break;
-						case 'home':
-							$condition_result = is_home();
-						break;
-						case 'front':
-							if ( current_theme_supports( 'infinite-scroll' ) )
-								$condition_result = is_front_page();
-							else {
-								$condition_result = is_front_page() && !is_paged();
-							}
-						break;
-						default:
-							if ( substr( $rule['minor'], 0, 10 ) == 'post_type-' )
-								$condition_result = is_singular( substr( $rule['minor'], 10 ) );
-							else {
-								// $rule['minor'] is a page ID -- check if we're either looking at that particular page itself OR looking at the posts page, with the correct conditions
-								
-								$condition_result = ( is_page( $rule['minor'] ) || ( get_option( 'show_on_front' ) == 'page' && $wp_query->is_posts_page && get_option( 'page_for_posts' ) == $rule['minor'] ) );
-							}
-						break;
-					}
-				break;
-				case 'tag':
-					if ( ! $rule['minor'] && is_tag() )
-						$condition_result = true;
-					else if ( is_singular() && $rule['minor'] && has_tag( $rule['minor'] ) )
-						$condition_result = true;
-					else {
-						$tag = get_tag( $rule['minor'] );
+		$major_conditions = apply_filters( 'widget_visibility_major_conditions', array() );
 
-						if ( $tag && is_tag( $tag->slug ) )
-							$condition_result = true;
-					}
-				break;
-				case 'category':
-					if ( ! $rule['minor'] && is_category() )
-						$condition_result = true;
-					else if ( is_category( $rule['minor'] ) )
-						$condition_result = true;
-					else if ( is_singular() && $rule['minor'] && in_array( 'category', get_post_taxonomies() ) &&  has_category( $rule['minor'] ) )
-						$condition_result = true;
-				break;
-				case 'author':
-					if ( ! $rule['minor'] && is_author() )
-						$condition_result = true;
-					else if ( $rule['minor'] && is_author( $rule['minor'] ) )
-						$condition_result = true;
-					else if ( is_singular() && $rule['minor'] && $rule['minor'] == $post->post_author )
-						$condition_result = true;
-				break;
-				case 'taxonomy':
-					$term = explode( '_tax_', $rule['minor'] ); // $term[0] = taxonomy name; $term[1] = term id
-					$terms = get_the_terms( $post->ID, $rule['minor'] ); // Does post have terms in taxonomy?
-					if ( is_tax( $term[0], $term[1] ) )
-						$condition_result = true;
-					else if ( is_singular() && $term[1] && has_term( $term[1], $term[0] ) )
-						$condition_result = true;
-					else if ( is_singular() && $terms & !is_wp_error( $terms ) )
-						$condition_result = true;
-				break;
+		foreach ( $instance['conditions']['rules'] as $rule ) {
+			if ( isset( $major_conditions[ $rule['major'] ] ) ) {
+				$condition_result = call_user_func( $major_conditions[ $rule['major'] ]['callback'], $rule['major'], $rule['minor'] );
 			}
 
-			if ( $condition_result )
+			if ( $condition_result ) {
 				break;
+			}
 		}
 
 		if ( ( 'show' == $instance['conditions']['action'] && ! $condition_result ) || ( 'hide' == $instance['conditions']['action'] && $condition_result ) )