创建滚动视差效果

当你在应用程序中滚动包含图片（例如）的卡片列表时，你可能会注意到这些图片似乎比屏幕的其余部分滚动得更慢。这看起来几乎就像列表中的卡片在前景中，而图片本身则在远处的背景中。这种效果被称为视差效果。

在本教程中，你将通过构建一个卡片列表（带有圆角，包含一些文本）来创建视差效果。每张卡片也包含一张图片。当卡片向上滑动时，每张卡片内的图片会向下滑动。

以下动画展示了应用程序的行为

要显示视差滚动图片列表，你必须首先显示一个列表。

创建一个名为 ParallaxRecipe 的新无状态 widget。在 ParallaxRecipe 中，使用 SingleChildScrollView 和 Column 构建一个 widget 树，它们共同形成一个列表。

class ParallaxRecipe extends StatelessWidget {
  const ParallaxRecipe({super.key});

  @override
  Widget build(BuildContext context) {
    return const SingleChildScrollView(child: Column(children: []));
  }
}

每个列表项都显示一个圆角矩形背景图片，代表世界上的七个地点之一。背景图片之上堆叠着地点名称及其国家，位于左下角。背景图片和文本之间是一个深色渐变，这改善了文本在背景上的可读性。

实现一个名为 LocationListItem 的无状态 widget，它由前面提到的视觉元素组成。目前，背景使用静态的 Image widget。稍后，你将用视差版本替换该 widget。

@immutable
class LocationListItem extends StatelessWidget {
  const LocationListItem({
    super.key,
    required this.imageUrl,
    required this.name,
    required this.country,
  });

  final String imageUrl;
  final String name;
  final String country;

  @override
  Widget build(BuildContext context) {
    return Padding(
      padding: const EdgeInsets.symmetric(horizontal: 24, vertical: 16),
      child: AspectRatio(
        aspectRatio: 16 / 9,
        child: ClipRRect(
          borderRadius: BorderRadius.circular(16),
          child: Stack(
            children: [
              _buildParallaxBackground(context),
              _buildGradient(),
              _buildTitleAndSubtitle(),
            ],
          ),
        ),
      ),
    );
  }

  Widget _buildParallaxBackground(BuildContext context) {
    return Positioned.fill(child: Image.network(imageUrl, fit: BoxFit.cover));
  }

  Widget _buildGradient() {
    return Positioned.fill(
      child: DecoratedBox(
        decoration: BoxDecoration(
          gradient: LinearGradient(
            colors: [Colors.transparent, Colors.black.withValues(alpha: 0.7)],
            begin: Alignment.topCenter,
            end: Alignment.bottomCenter,
            stops: const [0.6, 0.95],
          ),
        ),
      ),
    );
  }

  Widget _buildTitleAndSubtitle() {
    return Positioned(
      left: 20,
      bottom: 20,
      child: Column(
        mainAxisSize: MainAxisSize.min,
        crossAxisAlignment: CrossAxisAlignment.start,
        children: [
          Text(
            name,
            style: const TextStyle(
              color: Colors.white,
              fontSize: 20,
              fontWeight: FontWeight.bold,
            ),
          ),
          Text(
            country,
            style: const TextStyle(color: Colors.white, fontSize: 14),
          ),
        ],
      ),
    );
  }
}

接下来，将列表项添加到你的 ParallaxRecipe widget。

class ParallaxRecipe extends StatelessWidget {
  const ParallaxRecipe({super.key});

  @override
  Widget build(BuildContext context) {
    return SingleChildScrollView(
      child: Column(
        children: [
          for (final location in locations)
            LocationListItem(
              imageUrl: location.imageUrl,
              name: location.name,
              country: location.place,
            ),
        ],
      ),
    );
  }
}

你现在有了一个典型的、可滚动的卡片列表，显示了世界上七个独特的地点。在下一步中，你将为背景图片添加视差效果。

视差滚动效果是通过使背景图片与列表其余部分向相反方向轻微移动来实现的。当列表项向上滑动时，每个背景图片会稍微向下滑动。相反，当列表项向下滑动时，每个背景图片会稍微向上滑动。从视觉上看，这产生了视差效果。

视差效果取决于列表项在其祖先 Scrollable 中的当前位置。随着列表项的滚动位置变化，列表项背景图片的位置也必须随之改变。这是一个有趣的问题。列表项在 Scrollable 中的位置直到 Flutter 的布局阶段完成后才能获得。这意味着背景图片的位置必须在绘制阶段确定，而绘制阶段在布局阶段之后。幸运的是，Flutter 提供了一个名为 Flow 的 widget，它专门设计用于在子 widget 绘制之前立即控制其变换。换句话说，你可以拦截绘制阶段并控制子 widget 的重新定位，随心所欲。

用 Flow widget 包装你的背景 Image widget。

Widget _buildParallaxBackground(BuildContext context) {
  return Flow(
    children: [Image.network(imageUrl, fit: BoxFit.cover)],
  );
}

引入一个新的 FlowDelegate，名为 ParallaxFlowDelegate。

Widget _buildParallaxBackground(BuildContext context) {
  return Flow(
    delegate: ParallaxFlowDelegate(),
    children: [Image.network(imageUrl, fit: BoxFit.cover)],
  );
}

class ParallaxFlowDelegate extends FlowDelegate {
  ParallaxFlowDelegate();

  @override
  BoxConstraints getConstraintsForChild(int i, BoxConstraints constraints) {
    // TODO: We'll add more to this later.
  }

  @override
  void paintChildren(FlowPaintingContext context) {
    // TODO: We'll add more to this later.
  }

  @override
  bool shouldRepaint(covariant FlowDelegate oldDelegate) {
    // TODO: We'll add more to this later.
    return true;
  }
}

FlowDelegate 控制其子 widget 的大小以及子 widget 的绘制位置。在这种情况下，你的 Flow widget 只有一个子 widget：背景图片。该图片的宽度必须与 Flow widget 的宽度完全相同。

为你的背景图片子 widget 返回严格的宽度约束。

@override
BoxConstraints getConstraintsForChild(int i, BoxConstraints constraints) {
  return BoxConstraints.tightFor(width: constraints.maxWidth);
}

你的背景图片现在已正确调整大小，但你仍需要根据每个背景图片的滚动位置计算其垂直位置，然后进行绘制。

你需要计算背景图片所需位置的三个关键信息

• 祖先 Scrollable 的边界
• 单个列表项的边界
• 图片缩小以适应列表项后的尺寸

要查找 Scrollable 的边界，你需要将一个 ScrollableState 传入你的 FlowDelegate。

要查找单个列表项的边界，你需要将列表项的 BuildContext 传入你的 FlowDelegate。

要查找背景图片的最终尺寸，你需要为你的 Image widget 分配一个 GlobalKey，然后将该 GlobalKey 传入你的 FlowDelegate。

使这些信息对 ParallaxFlowDelegate 可用。

@immutable
class LocationListItem extends StatelessWidget {
  final GlobalKey _backgroundImageKey = GlobalKey();

  Widget _buildParallaxBackground(BuildContext context) {
    return Flow(
      delegate: ParallaxFlowDelegate(
        scrollable: Scrollable.of(context),
        listItemContext: context,
        backgroundImageKey: _backgroundImageKey,
      ),
      children: [
        Image.network(imageUrl, key: _backgroundImageKey, fit: BoxFit.cover),
      ],
    );
  }
}

class ParallaxFlowDelegate extends FlowDelegate {
  ParallaxFlowDelegate({
    required this.scrollable,
    required this.listItemContext,
    required this.backgroundImageKey,
  });

  final ScrollableState scrollable;
  final BuildContext listItemContext;
  final GlobalKey backgroundImageKey;
}

拥有实现视差滚动所需的所有信息后，实现 shouldRepaint() 方法。

@override
bool shouldRepaint(ParallaxFlowDelegate oldDelegate) {
  return scrollable != oldDelegate.scrollable ||
      listItemContext != oldDelegate.listItemContext ||
      backgroundImageKey != oldDelegate.backgroundImageKey;
}

现在，实现视差效果的布局计算。

首先，计算列表项在其祖先 Scrollable 中的像素位置。

@override
void paintChildren(FlowPaintingContext context) {
  // Calculate the position of this list item within the viewport.
  final scrollableBox = scrollable.context.findRenderObject() as RenderBox;
  final listItemBox = listItemContext.findRenderObject() as RenderBox;
  final listItemOffset = listItemBox.localToGlobal(
    listItemBox.size.centerLeft(Offset.zero),
    ancestor: scrollableBox,
  );
}

使用列表项的像素位置来计算其相对于 Scrollable 顶部的百分比。位于可滚动区域顶部的列表项应为 0%，位于底部的列表项应为 100%。

@override
void paintChildren(FlowPaintingContext context) {
  // Calculate the position of this list item within the viewport.
  final scrollableBox = scrollable.context.findRenderObject() as RenderBox;
  final listItemBox = listItemContext.findRenderObject() as RenderBox;
  final listItemOffset = listItemBox.localToGlobal(
    listItemBox.size.centerLeft(Offset.zero),
    ancestor: scrollableBox,
  );

  // Determine the percent position of this list item within the
  // scrollable area.
  final viewportDimension = scrollable.position.viewportDimension;
  final scrollFraction = (listItemOffset.dy / viewportDimension).clamp(
    0.0,
    1.0,
  );
  // ···
}

使用滚动百分比来计算 Alignment。在 0% 时，你希望得到 Alignment(0.0, -1.0)；在 100% 时，你希望得到 Alignment(0.0, 1.0)。这些坐标分别对应于顶部对齐和底部对齐。

@override
void paintChildren(FlowPaintingContext context) {
  // Calculate the position of this list item within the viewport.
  final scrollableBox = scrollable.context.findRenderObject() as RenderBox;
  final listItemBox = listItemContext.findRenderObject() as RenderBox;
  final listItemOffset = listItemBox.localToGlobal(
    listItemBox.size.centerLeft(Offset.zero),
    ancestor: scrollableBox,
  );

  // Determine the percent position of this list item within the
  // scrollable area.
  final viewportDimension = scrollable.position.viewportDimension;
  final scrollFraction = (listItemOffset.dy / viewportDimension).clamp(
    0.0,
    1.0,
  );

  // Calculate the vertical alignment of the background
  // based on the scroll percent.
  final verticalAlignment = Alignment(0.0, scrollFraction * 2 - 1);
}

使用 verticalAlignment，以及列表项的尺寸和背景图片的尺寸，来生成一个 Rect，该 Rect 决定了背景图片的定位。

@override
void paintChildren(FlowPaintingContext context) {
  // Calculate the position of this list item within the viewport.
  final scrollableBox = scrollable.context.findRenderObject() as RenderBox;
  final listItemBox = listItemContext.findRenderObject() as RenderBox;
  final listItemOffset = listItemBox.localToGlobal(
    listItemBox.size.centerLeft(Offset.zero),
    ancestor: scrollableBox,
  );

  // Determine the percent position of this list item within the
  // scrollable area.
  final viewportDimension = scrollable.position.viewportDimension;
  final scrollFraction = (listItemOffset.dy / viewportDimension).clamp(
    0.0,
    1.0,
  );

  // Calculate the vertical alignment of the background
  // based on the scroll percent.
  final verticalAlignment = Alignment(0.0, scrollFraction * 2 - 1);

  // Convert the background alignment into a pixel offset for
  // painting purposes.
  final backgroundSize =
      (backgroundImageKey.currentContext!.findRenderObject() as RenderBox)
          .size;
  final listItemSize = context.size;
  final childRect = verticalAlignment.inscribe(
    backgroundSize,
    Offset.zero & listItemSize,
  );
}

使用 childRect，以所需的平移变换绘制背景图片。正是这种随时间变化的变换为你带来了视差效果。

@override
void paintChildren(FlowPaintingContext context) {
  // Calculate the position of this list item within the viewport.
  final scrollableBox = scrollable.context.findRenderObject() as RenderBox;
  final listItemBox = listItemContext.findRenderObject() as RenderBox;
  final listItemOffset = listItemBox.localToGlobal(
    listItemBox.size.centerLeft(Offset.zero),
    ancestor: scrollableBox,
  );

  // Determine the percent position of this list item within the
  // scrollable area.
  final viewportDimension = scrollable.position.viewportDimension;
  final scrollFraction = (listItemOffset.dy / viewportDimension).clamp(
    0.0,
    1.0,
  );

  // Calculate the vertical alignment of the background
  // based on the scroll percent.
  final verticalAlignment = Alignment(0.0, scrollFraction * 2 - 1);

  // Convert the background alignment into a pixel offset for
  // painting purposes.
  final backgroundSize =
      (backgroundImageKey.currentContext!.findRenderObject() as RenderBox)
          .size;
  final listItemSize = context.size;
  final childRect = verticalAlignment.inscribe(
    backgroundSize,
    Offset.zero & listItemSize,
  );

  // Paint the background.
  context.paintChild(
    0,
    transform: Transform.translate(
      offset: Offset(0.0, childRect.top),
    ).transform,
  );
}

要实现视差效果，你还需要一个最后的细节。当输入改变时，ParallaxFlowDelegate 会重新绘制，但它不会在每次滚动位置改变时都重新绘制。

将 ScrollableState 的 ScrollPosition 传递给 FlowDelegate 超类，以便 FlowDelegate 在每次 ScrollPosition 改变时都重新绘制。

class ParallaxFlowDelegate extends FlowDelegate {
  ParallaxFlowDelegate({
    required this.scrollable,
    required this.listItemContext,
    required this.backgroundImageKey,
  }) : super(repaint: scrollable.position);
}

恭喜！你现在拥有了一个带有视差滚动背景图片的卡片列表。

运行应用

• 上下滚动以观察视差效果。

import 'package:flutter/material.dart';
import 'package:flutter/rendering.dart';

const Color darkBlue = Color.fromARGB(255, 18, 32, 47);

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

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: ThemeData.dark().copyWith(scaffoldBackgroundColor: darkBlue),
      debugShowCheckedModeBanner: false,
      home: const Scaffold(body: Center(child: ExampleParallax())),
    );
  }
}

class ExampleParallax extends StatelessWidget {
  const ExampleParallax({super.key});

  @override
  Widget build(BuildContext context) {
    return SingleChildScrollView(
      child: Column(
        children: [
          for (final location in locations)
            LocationListItem(
              imageUrl: location.imageUrl,
              name: location.name,
              country: location.place,
            ),
        ],
      ),
    );
  }
}

class LocationListItem extends StatelessWidget {
  LocationListItem({
    super.key,
    required this.imageUrl,
    required this.name,
    required this.country,
  });

  final String imageUrl;
  final String name;
  final String country;
  final GlobalKey _backgroundImageKey = GlobalKey();

  @override
  Widget build(BuildContext context) {
    return Padding(
      padding: const EdgeInsets.symmetric(horizontal: 24, vertical: 16),
      child: AspectRatio(
        aspectRatio: 16 / 9,
        child: ClipRRect(
          borderRadius: BorderRadius.circular(16),
          child: Stack(
            children: [
              _buildParallaxBackground(context),
              _buildGradient(),
              _buildTitleAndSubtitle(),
            ],
          ),
        ),
      ),
    );
  }

  Widget _buildParallaxBackground(BuildContext context) {
    return Flow(
      delegate: ParallaxFlowDelegate(
        scrollable: Scrollable.of(context),
        listItemContext: context,
        backgroundImageKey: _backgroundImageKey,
      ),
      children: [
        Image.network(imageUrl, key: _backgroundImageKey, fit: BoxFit.cover),
      ],
    );
  }

  Widget _buildGradient() {
    return Positioned.fill(
      child: DecoratedBox(
        decoration: BoxDecoration(
          gradient: LinearGradient(
            colors: [Colors.transparent, Colors.black.withValues(alpha: 0.7)],
            begin: Alignment.topCenter,
            end: Alignment.bottomCenter,
            stops: const [0.6, 0.95],
          ),
        ),
      ),
    );
  }

  Widget _buildTitleAndSubtitle() {
    return Positioned(
      left: 20,
      bottom: 20,
      child: Column(
        mainAxisSize: MainAxisSize.min,
        crossAxisAlignment: CrossAxisAlignment.start,
        children: [
          Text(
            name,
            style: const TextStyle(
              color: Colors.white,
              fontSize: 20,
              fontWeight: FontWeight.bold,
            ),
          ),
          Text(
            country,
            style: const TextStyle(color: Colors.white, fontSize: 14),
          ),
        ],
      ),
    );
  }
}

class ParallaxFlowDelegate extends FlowDelegate {
  ParallaxFlowDelegate({
    required this.scrollable,
    required this.listItemContext,
    required this.backgroundImageKey,
  }) : super(repaint: scrollable.position);


  final ScrollableState scrollable;
  final BuildContext listItemContext;
  final GlobalKey backgroundImageKey;

  @override
  BoxConstraints getConstraintsForChild(int i, BoxConstraints constraints) {
    return BoxConstraints.tightFor(width: constraints.maxWidth);
  }

  @override
  void paintChildren(FlowPaintingContext context) {
    // Calculate the position of this list item within the viewport.
    final scrollableBox = scrollable.context.findRenderObject() as RenderBox;
    final listItemBox = listItemContext.findRenderObject() as RenderBox;
    final listItemOffset = listItemBox.localToGlobal(
      listItemBox.size.centerLeft(Offset.zero),
      ancestor: scrollableBox,
    );

    // Determine the percent position of this list item within the
    // scrollable area.
    final viewportDimension = scrollable.position.viewportDimension;
    final scrollFraction = (listItemOffset.dy / viewportDimension).clamp(
      0.0,
      1.0,
    );

    // Calculate the vertical alignment of the background
    // based on the scroll percent.
    final verticalAlignment = Alignment(0.0, scrollFraction * 2 - 1);

    // Convert the background alignment into a pixel offset for
    // painting purposes.
    final backgroundSize =
        (backgroundImageKey.currentContext!.findRenderObject() as RenderBox)
            .size;
    final listItemSize = context.size;
    final childRect = verticalAlignment.inscribe(
      backgroundSize,
      Offset.zero & listItemSize,
    );

    // Paint the background.
    context.paintChild(
      0,
      transform: Transform.translate(
        offset: Offset(0.0, childRect.top),
      ).transform,
    );
  }

  @override
  bool shouldRepaint(ParallaxFlowDelegate oldDelegate) {
    return scrollable != oldDelegate.scrollable ||
        listItemContext != oldDelegate.listItemContext ||
        backgroundImageKey != oldDelegate.backgroundImageKey;
  }

}

class Parallax extends SingleChildRenderObjectWidget {
  const Parallax({super.key, required Widget background})
    : super(child: background);

  @override
  RenderObject createRenderObject(BuildContext context) {
    return RenderParallax(scrollable: Scrollable.of(context));
  }

  @override
  void updateRenderObject(
    BuildContext context,
    covariant RenderParallax renderObject,
  ) {
    renderObject.scrollable = Scrollable.of(context);
  }
}

class ParallaxParentData extends ContainerBoxParentData<RenderBox> {}

class RenderParallax extends RenderBox
    with RenderObjectWithChildMixin<RenderBox>, RenderProxyBoxMixin {
  RenderParallax({required ScrollableState scrollable})
    : _scrollable = scrollable;

  ScrollableState _scrollable;

  ScrollableState get scrollable => _scrollable;

  set scrollable(ScrollableState value) {
    if (value != _scrollable) {
      if (attached) {
        _scrollable.position.removeListener(markNeedsLayout);
      }
      _scrollable = value;
      if (attached) {
        _scrollable.position.addListener(markNeedsLayout);
      }
    }
  }

  @override
  void attach(covariant PipelineOwner owner) {
    super.attach(owner);
    _scrollable.position.addListener(markNeedsLayout);
  }

  @override
  void detach() {
    _scrollable.position.removeListener(markNeedsLayout);
    super.detach();
  }

  @override
  void setupParentData(covariant RenderObject child) {
    if (child.parentData is! ParallaxParentData) {
      child.parentData = ParallaxParentData();
    }
  }

  @override
  void performLayout() {
    size = constraints.biggest;

    // Force the background to take up all available width
    // and then scale its height based on the image's aspect ratio.
    final background = child!;
    final backgroundImageConstraints = BoxConstraints.tightFor(
      width: size.width,
    );
    background.layout(backgroundImageConstraints, parentUsesSize: true);

    // Set the background's local offset, which is zero.
    (background.parentData as ParallaxParentData).offset = Offset.zero;
  }

  @override
  void paint(PaintingContext context, Offset offset) {
    // Get the size of the scrollable area.
    final viewportDimension = scrollable.position.viewportDimension;

    // Calculate the global position of this list item.
    final scrollableBox = scrollable.context.findRenderObject() as RenderBox;
    final backgroundOffset = localToGlobal(
      size.centerLeft(Offset.zero),
      ancestor: scrollableBox,
    );

    // Determine the percent position of this list item within the
    // scrollable area.
    final scrollFraction = (backgroundOffset.dy / viewportDimension).clamp(
      0.0,
      1.0,
    );

    // Calculate the vertical alignment of the background
    // based on the scroll percent.
    final verticalAlignment = Alignment(0.0, scrollFraction * 2 - 1);

    // Convert the background alignment into a pixel offset for
    // painting purposes.
    final background = child!;
    final backgroundSize = background.size;
    final listItemSize = size;
    final childRect = verticalAlignment.inscribe(
      backgroundSize,
      Offset.zero & listItemSize,
    );

    // Paint the background.
    context.paintChild(
      background,
      (background.parentData as ParallaxParentData).offset +
          offset +
          Offset(0.0, childRect.top),
    );
  }
}

class Location {
  const Location({
    required this.name,
    required this.place,
    required this.imageUrl,
  });

  final String name;
  final String place;
  final String imageUrl;
}

const urlPrefix =
    'https://docs.flutterdart.cn/cookbook/img-files/effects/parallax';
const locations = [
  Location(
    name: 'Mount Rushmore',
    place: 'U.S.A',
    imageUrl: '$urlPrefix/01-mount-rushmore.jpg',
  ),
  Location(
    name: 'Gardens By The Bay',
    place: 'Singapore',
    imageUrl: '$urlPrefix/02-singapore.jpg',
  ),
  Location(
    name: 'Machu Picchu',
    place: 'Peru',
    imageUrl: '$urlPrefix/03-machu-picchu.jpg',
  ),
  Location(
    name: 'Vitznau',
    place: 'Switzerland',
    imageUrl: '$urlPrefix/04-vitznau.jpg',
  ),
  Location(
    name: 'Bali',
    place: 'Indonesia',
    imageUrl: '$urlPrefix/05-bali.jpg',
  ),
  Location(
    name: 'Mexico City',
    place: 'Mexico',
    imageUrl: '$urlPrefix/06-mexico-city.jpg',
  ),
  Location(name: 'Cairo', place: 'Egypt', imageUrl: '$urlPrefix/07-cairo.jpg'),
];