下拉放大背景在个人主页的设计中比较常见,但是我没在Flutter的文章中找到实现方式。
结合了几个文章的方案后，我尝试修改实现方式，最终结合CustomScrollView 和Listener的方法实现了以下的方案

CustomScrollView

可以说列表视图想要自定义滚动效果，绕不过CustomScrollView
详细的定义可以参见其他博客https://juejin.im/post/5bceb534e51d457aa4596f9a
这里不详细赘述

SliverPersistentHeader 可高度自定制的头部伸缩视图

很多文章里推荐大家用SliverAppBar来实现头部，这种方案并不是很灵活
SliverPersistentHeader 可以说对下拉放大背景图的支持更好一些

SliverPersistentHeader(
              delegate: _SVPersonalAppBarDelegate(
                  minHeight: defaultHeight,
                  maxHeight: maxHeight,
                  child: _createImageWidget()),
            )

  _createImageWidget() {
    return Container(
      child: Image.network(
        'https://pic4.zhimg.com/80/v2-b02e601349241df0e3f25fd1ec622155_1440w.jpg',
        fit: BoxFit.cover,
      ),
    );
  }

SliverPersistentHeader 需要我们传入一个SliverPersistentHeaderDelegate对象，SliverPersistentHeaderDelegate是一个抽象类，所以需要我们自己去继承实现一个子类

class _SVPersonalAppBarDelegate extends SliverPersistentHeaderDelegate {
  _SVPersonalAppBarDelegate({
    @required this.minHeight,
    @required this.maxHeight,
    @required this.child,
  });

  final double minHeight;
  final double maxHeight;
  final Widget child;

  @override
  double get minExtent => minHeight;

  @override
  double get maxExtent => max(maxHeight, minHeight);

  @override
  Widget build(
      BuildContext context, double shrinkOffset, bool overlapsContent) {
    return new SizedBox.expand(child: child);
  }

  @override
  bool shouldRebuild(_SVPersonalAppBarDelegate oldDelegate) {
    return maxHeight != oldDelegate.maxHeight ||
        minHeight != oldDelegate.minHeight ||
        child != oldDelegate.child;
  }
}

完成头部视图的定制后，我们需要关心的几个问题

1. 放大后，解除触摸，如何让他自动重置到正常状态?
2. 从顶部快速滑下后，如何制造弹性放大回置的动画？

我通过引入Listener 和状态监听来完成了这两部份的实现
完整代码如下

import 'dart:math';

import 'package:flutter/material.dart';

enum SVDragState {
    SVDragStateIdle,
    SVDragStateBegin,
    SVDrageStateEnd,
}

class SVPersonalInfoPage extends StatefulWidget {
  @override
  _SVPersonalInfoPageState createState() => _SVPersonalInfoPageState();
}

class _SVPersonalInfoPageState extends State<SVPersonalInfoPage> {
  double startOffsetY;
  static double defaultHeight = 250;
  static double maxHeight = 350.0;
  static double offsetY = 0;
  static double distance = maxHeight - defaultHeight;
  SVDragState dragState = SVDragState.SVDragStateIdle;
  final ScrollController controller =
      ScrollController(initialScrollOffset: distance);

  @override
  void initState() {
    controller.addListener(() {
      offsetY = controller.offset;
      if (offsetY <= 0) {
        controller.jumpTo(0);
        _resetWithAnimation(true);
      }
    });

    super.initState();
  }

  _resetWithAnimation(bool delay) {
    if (!delay) {
       if (controller.offset < distance && dragState == SVDragState.SVDrageStateEnd) {
        dragState = SVDragState.SVDragStateIdle;
        controller.animateTo(distance,
            duration: Duration(milliseconds: 300), curve: Curves.easeInOut);
            return;
      }
    }
    Future.delayed(Duration(milliseconds: 200)).then((value) {
      if (controller.offset < distance && dragState == SVDragState.SVDrageStateEnd) {
        dragState = SVDragState.SVDragStateIdle;
        controller.animateTo(distance,
            duration: Duration(milliseconds: 300), curve: Curves.easeInOut);
            return;
      }
    });
  }

  @override
  Widget build(BuildContext context) {
    return Listener(
        onPointerDown: (e) { dragState = SVDragState.SVDragStateBegin; },
        onPointerUp: (e) { 
          dragState = SVDragState.SVDrageStateEnd;
          _resetWithAnimation(false);  
        },
        child: CustomScrollView(
          controller: controller,
          slivers: <Widget>[
            SliverPersistentHeader(
              delegate: _SVPersonalAppBarDelegate(
                  minHeight: defaultHeight,
                  maxHeight: maxHeight,
                  child: _createImageWidget()),
            ),
            SliverList(delegate: SliverChildBuilderDelegate(
              (context, index) {
                return Container(
                  child: Text(
                    "This is item $index",
                    style: TextStyle(fontSize: 20),
                  ),
                  color: Colors.redAccent,
                );
              },
            ))
          ],
        ));
  }

  _createImageWidget() {
    return Container(
      child: Image.network(
        'https://pic4.zhimg.com/80/v2-b02e601349241df0e3f25fd1ec622155_1440w.jpg',
        fit: BoxFit.cover,
      ),
    );
  }
}

class _SVPersonalAppBarDelegate extends SliverPersistentHeaderDelegate {
  _SVPersonalAppBarDelegate({
    @required this.minHeight,
    @required this.maxHeight,
    @required this.child,
  });

  final double minHeight;
  final double maxHeight;
  final Widget child;

  @override
  double get minExtent => minHeight;

  @override
  double get maxExtent => max(maxHeight, minHeight);

  @override
  Widget build(
      BuildContext context, double shrinkOffset, bool overlapsContent) {
    return new SizedBox.expand(child: child);
  }

  @override
  bool shouldRebuild(_SVPersonalAppBarDelegate oldDelegate) {
    return maxHeight != oldDelegate.maxHeight ||
        minHeight != oldDelegate.minHeight ||
        child != oldDelegate.child;
  }
}