使用挂钩save_post 并在“正常”保存元字段例程之后执行保存的函数中放置一个if。

add_action( \'save_post\', \'this_is_the_function_name\' ); 

function this_is_the_function_name( $post_id ) { 

    // No auto saves 
    if( defined( \'DOING_AUTOSAVE\' ) && DOING_AUTOSAVE ) return; 

    // make sure the current user can edit the post 
    if( ! current_user_can( \'edit_post\' ) ) return;

    // assure the post type
    if ( ! get_post_type($post_id) == \'cards\' ) return;

    // assure the data are sended by the form
    if (
       ! isset($_POST[\'my_cards_activity\']) ||
       ! isset($_POST[\'my_cards_datetime\']) || ! isset(\'my_cards_user\')
    ) return;

    $activity = $_POST[\'my_cards_activity\'];

    // and now the simply logic
    if ( $activity == 1 ) {
        $datetime = $_POST[\'my_cards_datetime\'];
        $user = $_POST[\'my_cards_user\'];
    } else {
        $datetime = \'\';
        $user = \'\';    
    }

    // save data as array
    $meta_data = compact("activity", "datetime", "user");
    update_post_meta($post_id, \'card_data\', $meta_data);

    // if you want can save data as 3 different meta fields in this case
    // delete the previous 2 lines and uncommente the following
    // update_post_meta($post_id, \'card_activity\', $activity);
    // update_post_meta($post_id, \'datetime\', $datetime);
    // update_post_meta($post_id, \'user\', $user);        

} 

看见Codex 适用于：

• save_post action hook
• get_post_type
• update_post_meta