How to use Dismissible on a Card object

If I have a Card object, how can I use the Dismissable widget on top of that so when the user swipe the card it executes an onDismissed function, I tried the following example but it asks for an identifier for the key:

  Widget build(BuildContext context) {
    return new Dismissible(
    child:  new Card(
//some code

    ),
     key:/*how this works*/
); 
    }

is this doable by any mean ?