在这个例子中，我如何在SliverAppBar中创建一个部件的动画？

import 'package:flutter/material.dart';

class MyApp extends StatefulWidget {
  @override
  _MyAppState createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> with SingleTickerProviderStateMixin {
  AnimationController _animationController;
  Animation<double> _animation;

  @override
  void initState() {
    super.initState();
    _animationController = AnimationController(
      vsync: this,
      duration: Duration(seconds: 1),
    );
    _animation = Tween<double>(begin: 0, end: 1).animate(_animationController);
  }

  @override
  void dispose() {
    _animationController.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: CustomScrollView(
        slivers: <Widget>[
          SliverAppBar(
            title: Text('Animated Widget'),
            expandedHeight: 200,
            flexibleSpace: FlexibleSpaceBar(
              background: AnimatedBuilder(
                animation: _animation,
                builder: (BuildContext context, Widget child) {
                  return Opacity(
                    opacity: _animation.value,
                    child: Container(
                      decoration: BoxDecoration(
                        image: DecorationImage(
                          image: AssetImage('assets/image.jpg'),
                          fit: BoxFit.cover,
                        ),
                      ),
                    ),
                  );
                },
              ),
            ),
          ),
          SliverList(
            delegate: SliverChildBuilderDelegate(
              (BuildContext context, int index) {
                return ListTile(
                  title: Text('Item $index'),
                );
              },
              childCount: 20,
            ),
          ),
        ],
      ),
    );
  }
}

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