bp_core_register_subnav_screen_function( array|string $args = '', string $component = 'members' )

function bp_core_register_subnav_screen_function( $args = '', $component = 'members' ) {
	$bp = buddypress();

	$r = wp_parse_args( $args, array(
		'slug'              => false, // URL slug for the screen.
		'parent_slug'       => false, // URL slug of the parent screen.
		'user_has_access'   => true,  // Can the user visit this screen?
		'no_access_url'     => '',
		'site_admin_only'   => false, // Can only site admins visit this screen?
		'screen_function'   => false, // The name of the function to run when clicked.
	) );

	/*
	 * Hook the screen function for the added subnav item. But this only needs to
	 * be done if this subnav item is the current view, and the user has access to the
	 * subnav item. We figure out whether we're currently viewing this subnav by
	 * checking the following two conditions:
	 *   (1) Either:
	 *       (a) the parent slug matches the current_component, or
	 *       (b) the parent slug matches the current_item
	 *   (2) And either:
	 *       (a) the current_action matches $slug, or
	 *       (b) there is no current_action (ie, this is the default subnav for the parent nav)
	 *       and this subnav item is the default for the parent item (which we check by
	 *       comparing this subnav item's screen function with the screen function of the
	 *       parent nav item in the component's primary nav). This condition only arises
	 *       when viewing a user, since groups should always have an action set.
	 */

	// If we *don't* meet condition (1), return.
	if ( ! bp_is_current_component( $r['parent_slug'] ) && ! bp_is_current_item( $r['parent_slug'] ) ) {
		return;
	}

	$parent_nav = $bp->{$component}->nav->get_primary( array( 'slug' => $r['parent_slug'] ), false );
	if ( is_array( $parent_nav ) ) {
 		$parent_nav = reset( $parent_nav );
 	}

	// If we *do* meet condition (2), then the added subnav item is currently being requested.
	if ( ( bp_current_action() && bp_is_current_action( $r['slug'] ) ) || ( bp_is_user() && ! bp_current_action() && ! empty( $parent_nav->screen_function ) && $r['screen_function'] == $parent_nav->screen_function ) ) {

		// If this is for site admins only and the user is not one, don't create the subnav item.
		if ( ! empty( $r['site_admin_only'] ) && ! bp_current_user_can( 'bp_moderate' ) ) {
			return false;
		}

		$hooked = bp_core_maybe_hook_new_subnav_screen_function( $r, $component );

		// If redirect args have been returned, perform the redirect now.
		if ( ! empty( $hooked['status'] ) && 'failure' === $hooked['status'] && isset( $hooked['redirect_args'] ) ) {
			bp_core_no_access( $hooked['redirect_args'] );
		}
	}
}