bp_core_register_nav_screen_function( array|string $args = '' )

function bp_core_register_nav_screen_function( $args = '' ) {
	$bp = buddypress();

	$defaults = array(
		'name'                    => false, // Display name for the nav item.
		'slug'                    => false, // URL slug for the nav item.
		'item_css_id'             => false, // The CSS ID to apply to the HTML of the nav item.
		'show_for_displayed_user' => true,  // When viewing another user does this nav item show up?
		'site_admin_only'         => false, // Can only site admins see this nav item?
		'position'                => 99,    // Index of where this nav item should be positioned.
		'screen_function'         => false, // The name of the function to run when clicked.
		'default_subnav_slug'     => false  // The slug of the default subnav item to select when clicked.
	);

	$r = wp_parse_args( $args, $defaults );

	// If we don't have the required info we need, don't register this screen function.
	if ( empty( $r['slug'] ) ) {
		return false;
	}

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

	/**
	 * If this nav item is hidden for the displayed user, and
	 * the logged in user is not the displayed user
	 * looking at their own profile, don't don't register this screen function.
	 */
	if ( empty( $r['show_for_displayed_user'] ) && ! bp_user_has_access() ) {
		return false;
	}

	/**
	 * If the nav item is visible, we are not viewing a user, and this is a root
	 * component, don't attach the default subnav function so we can display a
	 * directory or something else.
	 */
	if ( ( -1 != $r['position'] ) && bp_is_root_component( $r['slug'] ) && ! bp_displayed_user_id() ) {
		return;
	}

	// Look for current component.
	if ( bp_is_current_component( $r['slug'] ) || bp_is_current_item( $r['slug'] ) ) {

		// The requested URL has explicitly included the default subnav
		// (eg: http://example.com/members/membername/activity/just-me/)
		// The canonical version will not contain this subnav slug.
		if ( ! empty( $r['default_subnav_slug'] ) && bp_is_current_action( $r['default_subnav_slug'] ) && ! bp_action_variable( 0 ) ) {
			unset( $bp->canonical_stack['action'] );
		} elseif ( ! bp_current_action() ) {

			// Add our screen hook if screen function is callable.
			if ( is_callable( $r['screen_function'] ) ) {
				add_action( 'bp_screens', $r['screen_function'], 3 );
			}

			if ( ! empty( $r['default_subnav_slug'] ) ) {

				/**
				 * Filters the default component subnav item.
				 *
				 * @since BuddyPress 1.5.0
				 *
				 * @param string $value The slug of the default subnav item
				 *                      to select when clicked.
				 * @param array  $r     Parsed arguments for the nav item.
				 */
				$bp->current_action = apply_filters( 'bp_default_component_subnav', $r['default_subnav_slug'], $r );
			}
		}
	}

	/**
	 * Fires after the screen function for an item in the BuddyPress main
	 * navigation is registered.
	 *
	 * @since BuddyPress 2.4.0
	 *
	 * @param array $r        Parsed arguments for the nav item.
	 * @param array $args     Originally passed in arguments for the nav item.
	 * @param array $defaults Default arguments for a nav item.
	 */
	do_action( 'bp_core_register_nav_screen_function', $r, $args, $defaults );
}