Design this animation using SliverAppBar flutter

Question

Here's what I want to build but I am able to achieve this

by the following code,

@override
  Widget build(BuildContext context) {
    return Scaffold(
      body: NestedScrollView(
        headerSliverBuilder: (BuildContext context, bool innerBoxIsScrolled){
          return [
            SliverAppBar(
              expandedHeight: 120,
              floating: false,
              pinned: false,
              flexibleSpace: Container(
                padding: EdgeInsets.all(10),
                height: 160,
                width: double.infinity,
                child: Column(
                  mainAxisAlignment: MainAxisAlignment.end,
                  crossAxisAlignment: CrossAxisAlignment.start,
                  children: <Widget>[
                    Align(
                        alignment: Alignment.topCenter,
                        child: Text(
                          'Hello World',
                        )),
                    Padding(
                      padding: EdgeInsets.only(top: 16),
                      child: Image.asset(
                        'assets/images/banner.png',
                      ),
                    ),
                  ],
                ),
              ),
            ),
          ];
        },
        body: ListView.builder(),
      ),
    );
  }

I have tried SliverAppBar using flexible and expanded, but was not able to achieve.

---

Update - First element of list is going behind text field. I want to scroll only when the animation is completed

Answer 1

updated answer,

   @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: CustomScrollView(
        slivers: <Widget>[
          TransitionAppBar(
            extent: 250,
            avatar: Text("Rancho"),
            title: Container(
              margin: EdgeInsets.symmetric(horizontal: 20.0, vertical: 0),
              decoration: BoxDecoration(
                  color: Colors.grey[200],
                  borderRadius: BorderRadius.all(Radius.circular(5.0))),
              child: Row(children: <Widget>[
                Padding(
                  padding: EdgeInsets.only(left: 20.0, right: 10.0),
                  child: Icon(Icons.search),
                ),
                Expanded(
                  child: TextFormField(
                    keyboardType: TextInputType.text,
                    textInputAction: TextInputAction.done,
                    cursorColor: Colors.black,
                    autofocus: false,
                    style: TextField_Style,
                    decoration: InputDecoration(
                        filled: true,
                        fillColor: Colors.transparent,
                        contentPadding:
                            EdgeInsets.symmetric(vertical: 10, horizontal: 15),
                        hintText: "Search",
                        border: InputBorder.none,
                        disabledBorder: OutlineInputBorder(
                          borderSide: new BorderSide(color: Colors.transparent),
                          borderRadius: new BorderRadius.circular(2),
                        ),
                        focusedBorder: OutlineInputBorder(
                          borderSide: new BorderSide(color: Colors.transparent),
                          borderRadius: new BorderRadius.circular(2),
                        )),
                  ),
                )
              ]),
            ),
          ),
          SliverList(
              delegate: SliverChildBuilderDelegate((context, index) {
            return Container(
                color: Colors.blue,
                child: ListTile(
                  title: Text("${index}a"),
                ));
          }, childCount: 25))
        ],
      ),
    );
  } 

.

class TransitionAppBar extends StatelessWidget {
  final Widget avatar;
  final Widget title;
  final double extent;

  TransitionAppBar({this.avatar, this.title, this.extent = 250, Key key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return SliverPersistentHeader(
      pinned: true,
      delegate: _TransitionAppBarDelegate(
        avatar: avatar,
        title: title,
        extent: extent > 200 ? extent : 200
      ),
    );
  }
}

class _TransitionAppBarDelegate extends SliverPersistentHeaderDelegate {
  final _avatarMarginTween = EdgeInsetsTween(
      begin: EdgeInsets.only(bottom: 70, left: 30),
      end: EdgeInsets.only(left: 0.0, top: 30.0));
  final _avatarAlignTween =
      AlignmentTween(begin: Alignment.bottomLeft, end: Alignment.topCenter);

  final Widget avatar;
  final Widget title;
  final double extent;

  _TransitionAppBarDelegate({this.avatar, this.title, this.extent = 250})
      : assert(avatar != null),
        assert(extent == null || extent >= 200),
        assert(title != null);

  @override
  Widget build(
      BuildContext context, double shrinkOffset, bool overlapsContent) {
    double tempVal = 34 * maxExtent / 100;
    final progress =  shrinkOffset > tempVal ? 1.0 : shrinkOffset / tempVal;
    print("Objechjkf === ${progress} ${shrinkOffset}");
    final avatarMargin = _avatarMarginTween.lerp(progress);
    final avatarAlign = _avatarAlignTween.lerp(progress);

    return Stack(
      children: <Widget>[
        AnimatedContainer(
          duration: Duration(milliseconds: 100),
          height: shrinkOffset * 2,
          constraints: BoxConstraints(maxHeight: minExtent),
          color: Colors.redAccent,
        ),
        Padding(
          padding: avatarMargin,
          child: Align(
            alignment: avatarAlign,
              child: avatar
          ),
        ),
        Padding(
          padding: EdgeInsets.only(bottom: 10),
          child: Align(
            alignment: Alignment.bottomCenter,
            child: title,
          ),
        )
      ],
    );
  }

  @override
  double get maxExtent => extent;

  @override
  double get minExtent => (maxExtent * 68) / 100;

  @override
  bool shouldRebuild(_TransitionAppBarDelegate oldDelegate) {
    return avatar != oldDelegate.avatar || title != oldDelegate.title;
  }
}