Flutter: How manage transition with MediaQuery's values on animatedTransition Widget - flutter

Question

first at all, i apologize for my english.
And second i have a trouble manage values for animateTransition. As you can see i have a card widget with a position at the right of the screen and MaterialButton in the center of the screen.
The positioned widget's values are give by MediaQuery Class and after save in the position variable, but when i try change the position's value and call setSate function, the build method redo and initialize the variables and again the new value is the same always because of that (i think that), and for that never will see a transition.
i show the code.
class _DashboardState extends State<Dashboard> {
final double blur = 30;
final double offset = 20;
final double top = 25;
#override
Widget build(BuildContext context) {
final size = MediaQuery.of(context).size;
double position = -(size.width) * 0.8;
return WillPopScope(
onWillPop: () async =>
SystemChannels.platform.invokeMethod('SystemNavigator.pop'),
child: MaterialApp(
home: Scaffold(
body: SafeArea(
child: Stack(
alignment: Alignment.center,
children: <Widget>[
Positioned(
right: position,
child: AnimatedContainer(
duration: Duration(milliseconds: 500),
curve: Curves.decelerate,
child: Card(
elevation: 8,
color: Colors.red,
shape: RoundedRectangleBorder(
borderRadius: BorderRadius.circular(10),
),
child: Container(
width: size.width * 0.9,
height: size.height * 0.8,
),
),
),
),
Center(
child: MaterialButton(
color: Colors.black,
child: Text("mover"),
onPressed: () {
setState(() {
position = 0;
});
},
),
)
],
),
),
),
),
);
}
}
when i try to take out the variables i can't because de context dependency in mediaQuery and i can't initialize the request variables to get size of device, and put in the initial state. any advice for manage this type of problem.
Thanks for your attention