How To Use Custom Single Child Layout Widget Using Flutter

import 'package:flutter/material.dart';
void main() => runApp(MyApp());
class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      debugShowCheckedModeBanner: false,
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(),
    );
  }
}

class MyHomePage extends StatefulWidget {
  @override
  MyHomePageState createState() {
    return new MyHomePageState();
  }
}

class MyHomePageState extends State<MyHomePage> with SingleTickerProviderStateMixin {
  @override
  Widget build(BuildContext context) {
    Size size = MediaQuery.of(context).size;
    return Scaffold(
      backgroundColor: Colors.black,
      appBar: AppBar(
        backgroundColor: Colors.deepPurple[600],
          title: Text("Custom Single Child Layout Widget"
          )),
      body: Container(
        height: 100.0,
        width: 100.0,
        color: Colors.deepPurple[300],
        child: CustomSingleChildLayout(
          delegate: _MySingleChildLayoutDelegate(
            widgetSize: size,
          ),
        ),
      ),
    );
  }
}

class _MySingleChildLayoutDelegate extends SingleChildLayoutDelegate {
  _MySingleChildLayoutDelegate({@required this.widgetSize});
  final Size widgetSize;
  @override
  BoxConstraints getConstraintsForChild(BoxConstraints constraints) {
    return BoxConstraints.expand(width: 120.0, height: 120.0);
  }
  @override
  Offset getPositionForChild(Size size, Size childSize) {
    return Offset(widgetSize.width / 2, widgetSize.height / 2);
  }
  @override
  bool shouldRelayout(_MySingleChildLayoutDelegate oldDelegate) {
    return widgetSize != oldDelegate.widgetSize;
  }
}