T5 Silent Flush - Flushes rewrite rules whenever a custom post type or taxonomy is registered and not already part of these rules. This is a soft flush, the .htaccess will not be rewritten.

t5-silent-flush.php

<?php # -*- coding: utf-8 -*-
/**
 * Plugin Name: T5 Silent Flush
 * Description: Flushes rewrite rules whenever a custom post type or taxonomy is registered and not already part of these rules. This is a soft flush, the .htaccess will not be rewritten.
 * Version:     2013.05.04
 * Author:      Thomas Scholz <info@toscho.de>
 * Author URI:  http://toscho.de
 * License:     MIT
 * License URI: http://www.opensource.org/licenses/mit-license.php
 */

add_action( 'registered_post_type', 't5_silent_flush_cpt', 10, 2 );
add_action( 'registered_taxonomy',  't5_silent_flush_tax', 10, 3 );

/**
 * Flush rules for custom post types.
 *
 * @wp-hook registered_post_type
 * @param   string $post_type
 * @param   stdClass $args
 * @return  void
 */
function t5_silent_flush_cpt( $post_type, $args )
{
	if ( ! t5_silent_flush_args_check( $args ) )
		return;

	$slug  = t5_silent_flush_get_slug( $args->rewrite, $post_type );
	$rules = get_option( 'rewrite_rules' );

	if ( ! isset ( $rules[ $slug . '/?$' ] ) )
		flush_rewrite_rules( FALSE );
}

/**
 * Flush rules for custom post taxonomies.
 *
 * @wp-hook registered_taxonomy
 * @param   string $taxonomy
 * @param   string $object_type Post, user, comment
 * @param   array  $args Will be converted to object
 * @return  void
 */
function t5_silent_flush_tax( $taxonomy, $object_type, $args )
{
	// No idea why we get an array here, but an object for post types.
	// Objects are easier to write, so ...
	$args = (object) $args;
	if ( ! t5_silent_flush_args_check( $args ) )
		return;

	$slug  = t5_silent_flush_get_slug( $args->rewrite, $taxonomy );
	$rules = get_option( 'rewrite_rules' );

	if ( ! isset ( $rules[ $slug . '/(.+?)/?$' ] ) )
		flush_rewrite_rules( FALSE );
}

/**
 * Basic check on the arguments of the registered object.
 *
 * @param  stdClass $args
 * @return boolean
 */
function t5_silent_flush_args_check( $args )
{
    if ( $args->_builtin )
        return FALSE;

	if ( ! $args->public )
		return FALSE;

	if ( ! $args->rewrite )
		return FALSE;

	return TRUE;
}

/**
 * Get the slug to search for in current rewrite rules.
 *
 * @param  array $rewrite
 * @param  string $name
 * @return string
 */
function t5_silent_flush_get_slug( $rewrite, $name )
{
	if ( ! isset ( $rewrite[ 'slug' ] ) )
		return $name;

	if ( ! is_string( $rewrite[ 'slug' ] ) )
		return $name;

	return $rewrite[ 'slug' ];
}