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当你在应用中滚动卡片列表（例如包含图片）时，你可能会注意到这些图片似乎比屏幕的其他部分滚动得更慢。这看起来几乎就像列表中的卡片在前景中，但图片本身却远在背景中。这种效果称为视差。

在本教程中，你将通过构建一个卡片列表（带圆角，包含一些文本）来创建视差效果。每张卡片还包含一张图片。当卡片向上滑动时，每张卡片中的图片都会向下滑动。

以下动画展示了应用程序的行为

Parallax scrolling

创建列表以容纳视差项目

#

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 的新无状态小部件。在 ParallaxRecipe 中，使用 SingleChildScrollView 和 Column 构建一个构成列表的小部件树。

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

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

显示带文本和静态图像的项目

#

每个列表项都显示一张圆角矩形背景图片，代表世界上七个地点之一。背景图片上方叠加着地点名称及其国家，位于左下角。背景图片和文本之间是深色渐变，这提高了文本在背景下的可读性。

实现一个名为 LocationListItem 的无状态小部件，它由前面提到的视觉元素组成。目前，使用静态 Image 小部件作为背景。稍后，你将用视差版本替换该小部件。

dart
@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 小部件中。

dart
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,
            ),
        ],
      ),
    );
  }
}

你现在有一个典型的可滚动卡片列表，显示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。

实现视差效果

#

视差滚动效果是通过将背景图片略微推向与列表其余部分相反的方向来实现的。当列表项向上滑动时，每个背景图片会略微向下滑动。相反，当列表项向下滑动时，每个背景图片会略微向上滑动。在视觉上，这导致了视差效果。

视差效果取决于列表项在其祖先 Scrollable 中的当前位置。随着列表项滚动位置的变化，列表项背景图片的位置也必须随之变化。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可用。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 的小部件，它专门用于在小部件绘制之前立即控制子小部件的变换。换句话说，你可以拦截绘制阶段并控制子小部件的重新定位，随心所欲。

用 Flow 小部件包裹你的背景 Image 小部件。

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

引入一个名为 ParallaxFlowDelegate 的新 FlowDelegate。

dart
Widget _buildParallaxBackground(BuildContext context) {
  return Flow(
    delegate: ParallaxFlowDelegate(),
    children: [Image.network(imageUrl, fit: BoxFit.cover)],
  );
}
dart
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 控制其子元素的尺寸以及这些子元素的绘制位置。在这种情况下，你的 Flow 小部件只有一个子元素：背景图片。该图片必须与 Flow 小部件一样宽。

为你的背景图片子元素返回严格的宽度约束。

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

你的背景图片现在尺寸合适，但你仍然需要根据每个背景图片的滚动位置计算其垂直位置，然后绘制它。

计算背景图片所需位置需要三个关键信息：

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

要查找 Scrollable 的边界，请将 ScrollableState 传递给你的 FlowDelegate。

要查找单个列表项的边界，请将列表项的 BuildContext 传递给你的 FlowDelegate。

要查找背景图片的最终尺寸，请为你的 Image 小部件分配一个 GlobalKey，然后将该 GlobalKey 传递给你的 FlowDelegate。

使这些信息可供 ParallaxFlowDelegate 使用。

dart
@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),
      ],
    );
  }
}
dart
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。

dart
@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。

dart
@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计算其距离 Scrollable 顶部的高度百分比。滚动区域顶部的列表项应产生 0%，滚动区域底部的列表项应产生 100%。

dart
@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,
  );
  // ···
}

使用滚动百分比计算 Alignment。在 0% 时，你需要 Alignment(0.0, -1.0)，在 100% 时，你需要 Alignment(0.0, 1.0)。这些坐标分别对应顶部对齐和底部对齐。

dart
@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，生成一个 Rect，它决定了背景图片的定位位置。

dart
@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，绘制具有所需平移变换的背景图片。正是这种随时间变化的变换为你提供了视差效果。

dart
@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,
  );
}

你需要最后一个细节来实现视差效果。当输入改变时，ParallaxFlowDelegate 会重新绘制，但 ParallaxFlowDelegate 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。

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

恭喜！你现在有了一个带有视差滚动背景图片的卡片列表。

互动示例

#

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.fluttercn.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'),
];

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