Using previous_post_link and next_post_link to wrap around post sequence

Question

For a single custom post type, I'm using previous_post_link and next_post_link with simple arrows. (Although text arrows are shown here, in the actual I'm using graphics with the text hidden via CSS.) Code:

<nav class="arrowNav">
    <div class="nav-previous">
       <?php previous_post_link('%link', '«'); ?>
    </div>

    <div class="nav-next">
       <?php next_post_link('%link', '»'); ?>
    </div>
</nav>

This works fine except for the edge cases. I'd like the posts to wrap around: that is, for the first post, I'd like previous_post_link to link to the last post, and for the last post next_post_link would link to the first post. It appears the default for both those cases is null. How do I test for first and last post status and produce links for each?

Answer 1

Add following custom functions in functions.php file and instead of calling previous_post_link and next_post_link functions call custom_next_post_link and custom_previous_post_link custom functions respectively.

 function custom_adjacent_post_link( $format, $link, $in_same_cat = false, $excluded_categories = '', $previous = true ) {
if ( $previous && is_attachment() )
        $post = get_post( get_post()->post_parent );
else
        $post = get_adjacent_post( $in_same_cat, $excluded_categories, $previous );

if ( ! $post ) {            
    $post =  '';
    $args = 'posts_per_page=1&orderby=date&ignore_sticky_posts=1';

    if($previous)
        $args .= '&order=DESC';
    else
        $args .= '&order=ASC';

   $the_query = new WP_Query( $args );
    while ( $the_query->have_posts() ) :
         $the_query->the_post();
         $post = get_post(get_the_ID());   
    endwhile;
    wp_reset_postdata();
} 

$title = $post->post_title;

if ( empty( $post->post_title ) )
        $title = $previous ? __( 'Previous Post' ) : __( 'Next Post' );

$title = apply_filters( 'the_title', $title, $post->ID );
$date = mysql2date( get_option( 'date_format' ), $post->post_date );
$rel = $previous ? 'prev' : 'next';

$string = '<a href="' . get_permalink( $post ) . '" rel="'.$rel.'">';
$inlink = str_replace( '%title', $title, $link );
$inlink = str_replace( '%date', $date, $inlink );
$inlink = $string . $inlink . '</a>';

$output = str_replace( '%link', $inlink, $format );
$adjacent = $previous ? 'previous' : 'next';
echo apply_filters( "{$adjacent}_post_link", $output, $format, $link, $post );
}
function custom_previous_post_link($format='&laquo; %link', $link='%title', $in_same_cat = false, $excluded_categories = '') {
custom_adjacent_post_link($format, $link, $in_same_cat, $excluded_categories, true);
}
function custom_next_post_link($format='%link &raquo;', $link='%title', $in_same_cat = false, $excluded_categories = '') {
custom_adjacent_post_link($format, $link, $in_same_cat, $excluded_categories, false);
}

Answer 2

The accepted answer works, but probly should have used get_posts() instead of a whole wpquery and loop:

    if ( ! $post ) { 
    $args = array('posts_per_page'=> 1,
            'orderby'=> 'date',
            'ignore_sticky_posts' => 1
            );
    if($previous)
        $args['order']='DESC';
    else
       $args['order']='ASC';
    $adjposts = get_posts($args);
    $post = $adjposts[0];   
} 

It's generally frowned upon to use wpquery if you dont need to. With full credit, otherwise, to Vinod Dalvi.