AnimatedBackground class for the Flutter scrolling animation demo

animated-bg.dart

import 'package:flutter/material.dart';
import 'dart:math' as math;

class AnimatedBackground extends StatefulWidget {

    AnimatedBackground({Key key, this.controller}) : super(key: key);

    final ScrollController controller;

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

class _AnimatedBackgroundState extends State<AnimatedBackground> {

    get offset => widget.controller.hasClients ? widget.controller.offset : 0;

    @override
    Widget build(BuildContext context) {
    
        return AnimatedBuilder(

            animation: widget.controller,
            builder: (BuildContext context, Widget child) {

                return  OverflowBox(
                    
                    maxWidth: double.infinity,
                    alignment: Alignment(4, 3),
                    child: Transform.rotate(
                        angle: ((math.pi * offset) / -1024),
                        child: Icon(Icons.settings, size: 512, color: Colors.white)
                    )
                );
            }
        );
    }
}