Comments: Allow comment types to register a display callback

src/wp-includes/class-walker-comment.php

@@ -158,6 +158,8 @@ public function display_element( $element, &$children_elements, $max_depth, $dep
 	 * @since 2.7.0
 	 * @since 5.9.0 Renamed `$comment` to `$data_object` and `$id` to `$current_object_id`
 	 *              to match parent class for PHP 8 named parameter support.
+	 * @since 7.1.0 Comments of a registered comment type with a `render_callback`
+	 *              are rendered via that callback.
 	 *
 	 * @see Walker::start_el()
 	 * @see wp_list_comments()
@@ -185,6 +187,19 @@ public function start_el( &$output, $data_object, $depth = 0, $args = array(), $
 			return;
 		}
 
+		/*
+		 * Allow a registered comment type to render itself. An explicit `callback`
+		 * argument passed to wp_list_comments() takes precedence and is handled above.
+		 */
+		$comment_type_object = get_comment_type_object( $comment->comment_type );
+
+		if ( $comment_type_object && is_callable( $comment_type_object->render_callback ) ) {
+			ob_start();
+			call_user_func( $comment_type_object->render_callback, $comment, $args, $depth );
+			$output .= ob_get_clean();
+			return;
+		}
+
 		if ( 'comment' === $comment->comment_type ) {
 			add_filter( 'comment_text', array( $this, 'filter_comment_text' ), 40, 2 );
 		}

src/wp-includes/class-wp-comment-type.php

@@ -99,6 +99,48 @@ final class WP_Comment_Type {
 	 */
 	public $_builtin = false;
 
+	/**
+	 * Callback used to render a comment of this type in comment lists.
+	 *
+	 * When set to a callable, {@see Walker_Comment} invokes it to render a
+	 * comment of this type, receiving the same arguments as the `callback`
+	 * argument of wp_list_comments(): the comment object, the arguments array,
+	 * and the depth. The precedence chain is: an explicit `callback` passed to
+	 * wp_list_comments(), then this callback, then the default markup.
+	 *
+	 * Like the `callback` argument of wp_list_comments(), the callback must only
+	 * output the opening of the list element; {@see Walker_Comment::end_el()}
+	 * (or the `end-callback` argument) closes the element after any child
+	 * comments have been rendered. Which element that is depends on the `style`
+	 * argument, so the callback has to open a `<div>` when `$args['style']` is
+	 * 'div' and an `<li>` otherwise, the way {@see Walker_Comment::comment()}
+	 * does. Opening the wrong one leaves the markup mismatched.
+	 *
+	 * The callback must echo its output. Unlike the `render_callback` argument
+	 * of register_block_type(), a returned string is discarded.
+	 *
+	 * Output from the callback is printed unescaped; the callback is
+	 * responsible for escaping all output.
+	 *
+	 * The built-in comment types register without a callback and cannot be
+	 * re-registered, but setting one on them through the
+	 * {@see 'register_comment_type_args'} filter is supported. It grants no more
+	 * than the `callback` argument of wp_list_comments() already does. Note that
+	 * a callback on the 'comment' type also takes over the walker's handling of
+	 * unapproved comments, which strips links from a pending comment's text for
+	 * everyone but its author.
+	 *
+	 * Only applies when comments are rendered via wp_list_comments() (classic
+	 * themes). Block themes render comments through the `core/comment-template`
+	 * block and do not invoke this callback.
+	 *
+	 * Default null.
+	 *
+	 * @since 7.1.0
+	 * @var callable|null
+	 */
+	public $render_callback = null;
+
 	/**
 	 * Whether the comment type is hierarchical.
 	 *
@@ -183,11 +225,12 @@ public function set_props( $args ) {
 		 * treated as a provided value and overwrite the default name with false.
 		 */
 		$defaults = array(
-			'labels'      => array(),
-			'description' => '',
-			'public'      => true,
-			'internal'    => false,
-			'_builtin'    => false,
+			'labels'          => array(),
+			'description'     => '',
+			'public'          => true,
+			'internal'        => false,
+			'render_callback' => null,
+			'_builtin'        => false,
 		);
 
 		$args = array_merge( $defaults, $args );

src/wp-includes/comment.php

@@ -434,22 +434,35 @@ function create_initial_comment_types() {
  * @param array|string $args {
  *     Optional. Array or string of arguments for registering a comment type. Default empty array.
  *
- *     @type string     $label       Name of the comment type. Usually plural.
- *                                   Default is the value of $labels['name'].
- *     @type string[]   $labels      An array of labels for this comment type. If not set, the
- *                                   default comment labels are used. See get_comment_type_labels()
- *                                   for a full list of supported labels.
- *     @type string     $description A short descriptive summary of what the comment type is.
- *                                   Default empty.
- *     @type bool       $public      Whether the comment type is intended for use publicly either via
- *                                   the admin interface or by front-end users. Core does not
- *                                   currently act on this argument. Default true.
- *     @type bool       $internal    Whether the comment type is for internal use only. Internal types
- *                                   are excluded from comment queries and counts by default, through
- *                                   the {@see 'default_excluded_comment_types'} filter. Default false.
- *     @type bool       $_builtin    For internal core use only. Marks the type as native to
- *                                   WordPress, which blocks it from being re-registered or
- *                                   unregistered. Default false.
+ *     @type string     $label           Name of the comment type. Usually plural.
+ *                                       Default is the value of $labels['name'].
+ *     @type string[]   $labels          An array of labels for this comment type. If not set, the
+ *                                       default comment labels are used. See get_comment_type_labels()
+ *                                       for a full list of supported labels.
+ *     @type string     $description     A short descriptive summary of what the comment type is.
+ *                                       Default empty.
+ *     @type bool       $public          Whether the comment type is intended for use publicly either
+ *                                       via the admin interface or by front-end users. Core does not
+ *                                       currently act on this argument. Default true.
+ *     @type bool       $internal        Whether the comment type is for internal use only. Internal
+ *                                       types are excluded from comment queries and counts by
+ *                                       default, through the
+ *                                       {@see 'default_excluded_comment_types'} filter.
+ *                                       Default false.
+ *     @type callable   $render_callback Callback used to render a comment of this type in comment
+ *                                       lists. Receives the same arguments as the `callback` argument
+ *                                       of wp_list_comments() (the comment, the arguments, and the
+ *                                       depth) and, like it, must echo only the opening of the list
+ *                                       element - a `<div>` when the `style` argument is 'div' and an
+ *                                       `<li>` otherwise; Walker_Comment::end_el() closes the element.
+ *                                       A returned string is discarded. An explicit
+ *                                       wp_list_comments() `callback` takes precedence. Output is
+ *                                       printed unescaped; the callback must escape all output.
+ *                                       Applies only when comments are rendered via
+ *                                       wp_list_comments() (classic themes). Default null.
+ *     @type bool       $_builtin        For internal core use only. Marks the type as native to
+ *                                       WordPress, which blocks it from being re-registered or
+ *                                       unregistered. Default false.
  * }
  * @return WP_Comment_Type|WP_Error The registered comment type object on success,
  *                                  WP_Error object on failure.