How can I make a gradient shadow around any widget in flutter?

Example:

Please notice that the color of the widget doesn't matter, only the color of the shadow around it.

I've been trying to edit the source code of BoxShadow, which is only like 100 lines long, but I'm not having much success.

Any help would be appreciated! Thanks!

By using this, I implemented what you want.

import 'package:flutter/material.dart';

import 'gradient_decoration.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
        visualDensity: VisualDensity.adaptivePlatformDensity,
      ),
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  final String title;

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  @override
  void initState() {
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: _buildBody(),
      floatingActionButton: FloatingActionButton(
        onPressed: () {},
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ),
    );
  }

  Widget _buildBody() {
    return Center(
      child: Container(
        width: 300,
        height: 80,
        decoration: GradientShadowBoxDecoration(
            borderRadius: BorderRadius.circular(20),
            boxShadow: [
              BoxShadow(
                offset: Offset(0.0, 0.0),
                blurRadius: 25,
              ),
            ],
            gradient: LinearGradient(
              begin: Alignment.centerLeft,
              end: Alignment.centerRight,
              colors: [
                Colors.blue,
                Colors.purple,
              ],
            )),
        child: Center(
          child: Text(
            'Login',
            style: TextStyle(
              fontSize: 25.0,
              fontWeight: FontWeight.bold,
              color: Colors.white,
            ),
          ),
        ),
      ),
    );
  }
}