Wordpress: displaying an error message - hook admin_notices fails on wp_insert_post_data or publish_post

Question

I\'m adding validation so if a post is in a particular category, it needs certain Custom Fields to be set.

This should be easy hooking wp_insert_post_data

Answer 1

You can simply do like WordPress do : using transients like this :

function set_post_pending($data, $postarr) {
    // If it's not valid...
        $error = "You are missing some Custom Fields.";
        set_transient( get_current_user_id().'missingfield', $error );
        $data['post_status'] = 'pending';
    return $data;
}
add_filter('wp_insert_post_data', 'set_post_pending',1,2);

function show_admin_notice() {
    if($out = get_transient( get_current_user_id().'missingfield' ) ) {
        delete_transient( get_current_user_id().'missingfield' );
        echo "
$out
";
    }
    // return false; // nothing to return here
}
add_action('admin_notices', "session_admin_notice");

ps : avoid $_SESSION in WordPress, thx