Import featured image on demand

gistfile1.php

add_filter(
	'get_post_metadata',
	array($this, 'maybe_grab_simpleview_photo_for_featured_image'),
	10,
	4
);

/**
 * On the request for a post's featured image, it goes and retrieves it from the 3rd party site,
 * attaches it to the current post, then sets it as a featured image. This only happens once per post.
 */
public function maybe_grab_simpleview_photo_for_featured_image($value, $object_id, $meta_key, $single) {
	// someone's checking for featured image presence
	if ($meta_key == '_thumbnail_id') {
		$post = get_post($object_id);
		if ($post
			&& $post->post_type == 'listing'
			&& !get_post_meta($object_id, '_cf_imported_thumbnail', true) // haven't already imported
			&& !get_post_meta($object_id, '_cf_attempted_thumb_import', true) // AND haven't already attempted to import
			) {
			$post_id = $object_id;
			$listing = new CF_SimpleView_Listing($post_id);
			$url = $listing->get_photo_url();
			if ($url) {
				if (!function_exists('media_sideload_image')) {
					require_once ABSPATH.'wp-admin/includes/media.php';
					require_once ABSPATH.'wp-admin/includes/file.php';
					require_once ABSPATH.'wp-admin/includes/image.php';
				}
				// retrieve the remote image, and attach to our post
				$img_html = media_sideload_image($url, $post_id);
				if ($img_html && !is_wp_error($img_html)) {
					// Now make it the featured image for the post
					$images = get_children(array(
							'post_parent' => $post_id,
							'post_type' => 'attachment',
							'post_mime_type' => 'image'
					));
					if ($images) {
						$first_image = array_shift($images);
						update_post_meta($post_id, '_thumbnail_id', $first_image->ID);
						update_post_meta($post_id, '_cf_imported_thumbnail', true);
					}
				}
				else {
					// couldn't grab the image from SimpleView...don't keep trying
					update_post_meta($post_id, '_cf_attempted_thumb_import', true);
				}
			}
		}
	}
	return $value;
}