Flutter：实现一个滑动头部折叠的动画效果

  Android和iOS中都有类似的滑动折叠效果，Flutter官方也提供了NestedScrollView控件来实现类似的效果，但是因为Flutter的一些特性，布局容易出现溢出，这些坑需要自己处理。

  先上效果图： 头部折叠.gif
  效果实现是基于Google的gallery demo中的tabs_demo来实现的，主要是通过NestedScrollView控件来实现。

  头部为一个SliverAppBar，折叠部分的内容都放在了flexibleSpace中。

  全部代码如下：

import 'package:flutter/material.dart';

// Each TabBarView contains a _Page and for each _Page there is a list
// of _CardData objects. Each _CardData object is displayed by a _CardItem.

const String _kGalleryAssetsPackage = 'flutter_gallery_assets';

class _Page {
  _Page({this.label});

  final String label;

  String get id => label[0];

  @override
  String toString() => '$runtimeType("$label")';
}

class _CardData {
  const _CardData({this.title, this.imageAsset, this.imageAssetPackage});

  final String title;
  final String imageAsset;
  final String imageAssetPackage;
}

final Map<_Page, List<_CardData>> _allPages = <_Page, List<_CardData>>{
  new _Page(label: 'LEFT'): <_CardData>[
    const _CardData(
      title: 'Vintage Bluetooth Radio',
      imageAsset: 'shrine/products/radio.png',
      imageAssetPackage: _kGalleryAssetsPackage,
    ),
    const _CardData(
      title: 'Sunglasses',
      imageAsset: 'shrine/products/sunnies.png',
      imageAssetPackage: _kGalleryAssetsPackage,
    ),
    const _CardData(
      title: 'Clock',
      imageAsset: 'shrine/products/clock.png',
      imageAssetPackage: _kGalleryAssetsPackage,
    ),
    const _CardData(
      title: 'Red popsicle',
      imageAsset: 'shrine/products/popsicle.png',
      imageAssetPackage: _kGalleryAssetsPackage,
    ),
    const _CardData(
      title: 'Folding Chair',
      imageAsset: 'shrine/products/lawn_chair.png',
      imageAssetPackage: _kGalleryAssetsPackage,
    ),
    const _CardData(
      title: 'Green comfort chair',
      imageAsset: 'shrine/products/chair.png',
      imageAssetPackage: _kGalleryAssetsPackage,
    ),
    const _CardData(
      title: 'Old Binoculars',
      imageAsset: 'shrine/products/binoculars.png',
      imageAssetPackage: _kGalleryAssetsPackage,
    ),
    const _CardData(
      title: 'Teapot',
      imageAsset: 'shrine/products/teapot.png',
      imageAssetPackage: _kGalleryAssetsPackage,
    ),
    const _CardData(
      title: 'Blue suede shoes',
      imageAsset: 'shrine/products/chucks.png',
      imageAssetPackage: _kGalleryAssetsPackage,
    ),
  ],
  new _Page(label: 'RIGHT'): <_CardData>[
    const _CardData(
      title: 'Beachball',
      imageAsset: 'shrine/products/beachball.png',
      imageAssetPackage: _kGalleryAssetsPackage,
    ),
    const _CardData(
      title: 'Dipped Brush',
      imageAsset: 'shrine/products/brush.png',
      imageAssetPackage: _kGalleryAssetsPackage,
    ),
    const _CardData(
      title: 'Perfect Goldfish Bowl',
      imageAsset: 'shrine/products/fish_bowl.png',
      imageAssetPackage: _kGalleryAssetsPackage,
    ),
  ],
};

class _CardDataItem extends StatelessWidget {
  const _CardDataItem({this.page, this.data});

  static const double height = 272.0;
  final _Page page;
  final _CardData data;

  @override
  Widget build(BuildContext context) {
    return new Card(
      child: new Padding(
        padding: const EdgeInsets.all(16.0),
        child: new Column(
          crossAxisAlignment: CrossAxisAlignment.stretch,
          mainAxisAlignment: MainAxisAlignment.start,
          children: <Widget>[
            new Align(
              alignment:
                  page.id == 'L' ? Alignment.centerLeft : Alignment.centerRight,
              child: new CircleAvatar(child: new Text('${page.id}')),
            ),
            new SizedBox(
              width: 144.0,
              height: 144.0,
              child: new Image.asset(
                data.imageAsset,
                package: data.imageAssetPackage,
                fit: BoxFit.contain,
              ),
            ),
            new Center(
              child: new Text(
                data.title,
                style: Theme.of(context).textTheme.title,
              ),
            ),
          ],
        ),
      ),
    );
  }
}

class TabsDemo extends StatelessWidget {
  static const String routeName = '/material/tabs';

  @override
  Widget build(BuildContext context) {
    return new DefaultTabController(
      length: _allPages.length,
      child: new Scaffold(
        body: new NestedScrollView(
          headerSliverBuilder: (BuildContext context, bool innerBoxIsScrolled) {
            return <Widget>[
              new SliverOverlapAbsorber(
                handle:
                    NestedScrollView.sliverOverlapAbsorberHandleFor(context),
                child: new SliverAppBar(
                  pinned: true,
                  expandedHeight: 300.0,
                  // 这个高度必须比flexibleSpace高度大
                  forceElevated: innerBoxIsScrolled,
                  bottom: PreferredSize(
                      child: new Container(
                        child: new TabBar(
                          tabs: _allPages.keys
                              .map(
                                (_Page page) => new Tab(
                                      child: new Tab(text: page.label),
                                    ),
                              )
                              .toList(),
                        ),
                        color: Colors.redAccent[200],
                      ),
                      preferredSize: new Size(double.infinity, 46.0)),
                  // 46.0为TabBar的高度，也就是tabs.dart中的_kTabHeight值，因为flutter不支持反射所以暂时没法通过代码获取
                  flexibleSpace: new Container(
                    child: new Column(
                      children: <Widget>[
                        new AppBar(
                          title: Text("this is title"),
                        ),
                        new Expanded(
                          child: new Container(
                            child: Image.asset(
                              "images/test.jpg",
                              repeat: ImageRepeat.repeat,
                            ),
                            width: double.infinity,
                          ),
                        )
                      ],
                    ),
                  ),
                ),
              ),
            ];
          },
          body: new TabBarView(
            children: _allPages.keys.map((_Page page) {
              return new SafeArea(
                top: false,
                bottom: false,
                child: new Builder(
                  builder: (BuildContext context) {
                    return new CustomScrollView(
                      key: new PageStorageKey<_Page>(page),
                      slivers: <Widget>[
                        new SliverOverlapInjector(
                          handle: NestedScrollView
                              .sliverOverlapAbsorberHandleFor(context),
                        ),
                        new SliverPadding(
                          padding: const EdgeInsets.symmetric(
                            vertical: 8.0,
                            horizontal: 16.0,
                          ),
                          sliver: new SliverFixedExtentList(
                            itemExtent: _CardDataItem.height,
                            delegate: new SliverChildBuilderDelegate(
                              (BuildContext context, int index) {
                                final _CardData data = _allPages[page][index];
                                return new Padding(
                                  padding: const EdgeInsets.symmetric(
                                    vertical: 8.0,
                                  ),
                                  child: new _CardDataItem(
                                    page: page,
                                    data: data,
                                  ),
                                );
                              },
                              childCount: _allPages[page].length,
                            ),
                          ),
                        ),
                      ],
                    );
                  },
                ),
              );
            }).toList(),
          ),
        ),
      ),
    );
  }
}