Adds a view all posts page to any archive.

class.View_All_Posts.php

<?php

/**
 * Adds a view all posts page to any archive.
 *
 * @author   Thomas Scholz http://toscho.de
 * @version  1.1
 * @license: GPL2
 * License URI: http://www.gnu.org/licenses/gpl-2.0.html

    Copyright 2010  Thomas Scholz  (email: info@toscho.de)

    This program is free software; you can redistribute it and/or
    modify it under the terms of the GNU General Public License
    version 2, as published by the Free Software Foundation.

    You may NOT assume that you can use any other version of the GPL.

    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.

    The license for this software can be found here:
    http://www.gnu.org/licenses/gpl-2.0.html
 */
class View_All_Posts
{
	/**
	 * GET parameter to trigger a complete, not paged archive.
	 * @var string
	 */
	protected $all_param = 'all';

	public function __construct()
	{
		/* Register the query argument. */
		add_filter('query_vars', array ( $this, 'add_query_arg') );

		/* Hook into the query. */
		add_action('pre_get_posts', array ( $this, 'view_all_posts') );
	}

	/**
	 * Registers the query arg in WordPress.
	 * Otherwise it will be unset.
	 *
	 * @param  array $vars Already registered query args.
	 * @return array
	 */
	public function add_query_arg( array $vars )
	{
		return array_merge( $vars, array ( $this->query_arg ) );
	}

	/**
	 * Alters the query to remove the paging.
	 * @return void
	 */
	public function view_all_posts()
	{
		if ( ! $this->is_all_posts() )
		{
			return;
		}

		$GLOBALS['wp_query']->query_vars['nopaging'] = TRUE;

		return;
	}

	/**
	 * Are we there already?
	 *
	 * @return bool
	 */
	public function is_all_posts()
	{
		return isset ( $_GET[$this->all_param] );
	}

	/**
	 * Creates the markup for the link.
	 *
	 * Usage in archive.php, category.php or search.php:
	 * $GLOBALS['view_all_posts']->get_allposts_link();
	 *
	 * @param  string $text Linktext
	 * @param  bool   $print echo or return
	 * @return string|void
	 */
	public function get_allposts_link(
		$text   = 'View all posts'
	,	$before = '<p class="allpostslink">'
	,	$after  = '</p>'
	,	$print  = TRUE
	)
	{
		if ( $this->is_all_posts()
		or $GLOBALS['wp_query']->found_posts <= get_option('posts_per_page')
		)
		{	// No link needed.
			return;
		}

		if ( isset ( $_SERVER['QUERY_STRING'] )
			&& ! empty ( $_SERVER['QUERY_STRING'] )
		)
		{
			/* We have already visible GET parameters: /?hello=world. */
			$new_url = $_SERVER['REQUEST_URI'] . '&amp;';
		}
		else
		{
			/* Note the difference: REQUEST_URL doesn't include
			 * the query string while REQUEST_URI does. */
			$new_url = $_SERVER['REQUEST_URL'] . '?';
		}

		$link = "$before<a href='$new_url$this->all_param'>$text</a>$after";

		if ( $print )
		{
			print $link;
			return;
		}
		return $link;
	}

	/**
	 * Prevents indexing from search engines.
	 *
	 * Add this as an action to 'wp_head'.
	 *
	 * @return void
	 */
	public function meta_noindex()
	{
		if ( $this->is_all_posts() )
		{
			print '<meta name="robots" content="noindex">';
		}
	}
}

$GLOBALS['view_all_posts'] = new View_All_Posts;

// Comment this out, if you don’t need it.
add_action(
	'wp_head'
,	array ( $GLOBALS['view_all_posts'], 'meta_noindex' )
);