library2.dart

import 'package:flutter/material.dart';

boxDeco(double aA,double bB){

dynamic boxDeco = new BoxDecoration(
              border: new Border.all(
                color:Colors.black,
                width: 2.0,
                style: BorderStyle.solid,
              ),
              color: Colors.blue,
              borderRadius: BorderRadius.all(
                new Radius.circular(
                  10.0
                )
              ),
              boxShadow: [new BoxShadow(
                color: Colors.grey,
                offset: new Offset(aA, bB),
                blurRadius: 5.0,
              )],
            );
            return boxDeco;
}
Shadow b = new Shadow(
       color: Colors.grey[200],
       offset: Offset(3.0, 3.0),
       blurRadius: 3.0,
  );
Shadow d = new Shadow();

main3.dart

import 'package:flutter/material.dart';
import 'library.dart';


void main() => runApp(MeineApp());

class MeineApp extends StatefulWidget {
  @override
  _MeineAppState createState() => _MeineAppState();
}

class _MeineAppState extends State<MeineApp> {
  double aA=0;
  double bB=0;

  List<Shadow> schatten = [d];
  List<Shadow> schatten2 = [b];
  dynamic mneueBox=boxDeco(0.0, 0.0);
  void neueBox(){
    setState(() {
     aA=aA + 1.0;
     bB=bB + 1.0;
     mneueBox=boxDeco(aA, bB); 
    });
  }
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: "StateDemo",
      home: new Scaffold(
        body: new Row(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
           new Center(
             child: new Container(
               width: 50.0,
               height: 50.0,
               child: Center(child: new Text("Hallo"),),
               decoration: mneueBox,
             ),
           ),
           new IconButton(
             color: Colors.deepOrange,
             icon:Icon(Icons.star) ,
             onPressed:neueBox,
           ),
           new Text(
             "Guten Morgen liebe Sorgen :-)",
             style: TextStyle(
               color: Colors.black,
               fontSize: 20.0,
               fontWeight: FontWeight.normal,
               decoration: TextDecoration.underline,
               decorationStyle: TextDecorationStyle.solid,
               shadows: Shadow.lerpList(schatten, schatten2, 0.8),
             ),
           ),
          ],
        ),
      ),
      
    );
  }
}