WP #23906 example

class Drafts_For_Friends {

    function __construct() {
        add_action( 'init', array( $this, 'init' ) );
    }

    function init() {
        // ...
        add_action( 'save_post',        array( $this, 'save_post' ), 10, 2 );
    }

    /**
     * Performs any extra processing when a post is saved
     * @param int $post_id The ID of the current revision of the edited post
     * @param object $revision The full object of the current revision of the edited post
     */
    public function save_post( $post_id, $post )
    {
        if( 1 !== did_action( 'save_post' ) )
            return;

        $ignored_actions = array( 'trash', 'untrash', 'restore' );

        // 3.6-alpha-23646 passes the canonical $post, while previous versions passed the revision, so we need to check which one we have
        //if ( 'revision' == $post->post_type )
            //$post = get_post( $post->post_parent );

        // Avoid processing the post if it's not in the desired state
        if( isset( $_GET[ 'action' ] ) && in_array( $_GET[ 'action' ], $ignored_actions ) )
            return;

        if( !$post || 'post' != $post->post_type || ! current_user_can( self::REQUIRED_CAPABILITY, $post_id ) )
            return;

        if( ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE ) || 'auto-draft' == $post->post_status )
            return;

        // Process the post
        $status_messages = $this->process_metabox_actions( $post->ID, $this->validate_metabox_actions( $_POST ) );
    }

    /**
     * Processes actions triggered in metaboxes
     *
     * @since 2.3
     *
     * @param int $post_id The ID of the canonical post that the current revision being saved is attached to
     * @param array $actions The metabox actions that were activated
     * @return string The status method from the action taken
     */
    protected function process_metabox_actions( $post_id, $actions )
    {
        $status_messages = array();

        if ( isset( $actions['share'] ) )
            $status_messages[] = $this->process_post_options( $post_id, $actions['share']['expires'], $actions['share']['measure'] );

        if ( isset( $actions['extend'] ) )
            $status_messages[] = $this->process_extend( $actions['extend']['key'], $actions['extend']['expires'], $actions['extend']['measure'] );

        if ( isset( $actions['email'] ) )
            $status_messages[] = $this->process_email_share_url( $actions['email']['key'], $actions['email']['guest_email_address'] );

        if ( isset( $actions['unshare'] ) )
            $status_messages[] = $this->process_delete( $actions['unshare']['key'] );

        return $status_messages;
    }
}