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创建滚动视差效果

内容

当您在应用中滚动卡片列表（例如包含图像的卡片）时，您可能会注意到这些图像的滚动速度比屏幕上的其他内容慢。它看起来几乎像是列表中的卡片位于前景，但图像本身位于远处的背景中。这种效果称为视差。

在本菜谱中，您通过构建卡片列表（带有圆角，包含一些文本）来创建视差效果。每张卡片还包含一个图像。当卡片向上滑动屏幕时，每张卡片内的图像向下滑动。

以下动画显示了应用的行为

Parallax scrolling

创建列表以保存视差项目

#

要显示视差滚动图像列表，您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，构建一个包含SingleChildScrollView和Column的小部件树，它形成一个列表。

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

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

显示带有文本和静态图像的项目

#

每个列表项显示一个圆角矩形背景图像，表示世界上七个位置之一。叠加在该背景图像之上的是位置名称及其国家/地区，位于左下方。在背景图像和文本之间是深色渐变，这提高了文本在背景上的可读性。

实现一个名为LocationListItem的无状态小部件，它由前面提到的视觉效果组成。目前，对背景使用静态Image小部件。稍后，您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.withOpacity(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，将列表项添加到您的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,
            ),
        ],
      ),
    );
  }
}

您现在拥有一个典型的可滚动卡片列表，显示了世界上七个独特的位置。在下一步中，您将向背景图像添加视差效果。

实现视差效果

#

视差滚动效果是通过将背景图像稍微推向与列表其余部分相反的方向来实现的。当列表项向上滑动屏幕时，每个背景图像会稍微向下滑动。相反，当列表项向下滑动屏幕时，每个背景图像会稍微向上滑动。从视觉上看，这会导致视差。

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发生变化，列表项背景图像的位置也必须发生变化。这是一个有趣的解决问题的方法。列表项在Scrollable中的位置在Flutter的布局阶段完成之前不可用。这意味着背景图像的位置必须在绘制阶段确定，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，该小部件专门设计用于在小部件绘制之前立即控制子小部件的转换。换句话说，您可以拦截绘制阶段并控制以根据需要重新定位子小部件。

使用Flow小部件包装您的背景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控制其子项的大小以及这些子项的绘制位置。在本例中，您的Flow小部件只有一个子项：背景图像。该图像必须与Flow小部件一样宽。

为您的背景图像子项返回严格的宽度约束。

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

您的背景图像现在已正确调整大小，但您仍然需要根据其滚动位置计算每个背景图像的垂直位置，然后绘制它。

计算背景图像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
  • 图像缩小以适合列表项后的尺寸

要查找Scrollable的边界，您可以将ScrollableState传递到您的FlowDelegate中。

要查找单个列表项的边界，请将列表项的BuildContext传递到您的FlowDelegate中。

要查找背景图像的最终大小，请将GlobalKey分配到您的Image小部件，然后将该GlobalKey传递到您的FlowDelegate中。

使此信息可供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来计算其在Scrollable顶部的百分比。位于可滚动区域顶部的列表项应产生0%，位于可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，您希望Alignment(0.0, -1.0)，在100%时，您希望Alignment(0.0, 1.0)。这些坐标分别对应于顶部和底部对齐。

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以及列表项的大小和背景图像的大小来生成一个Rect，该Rect确定背景图像应放置的位置。

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，使用所需的平移变换绘制背景图像。随着时间的推移，正是这种变换为您提供了视差效果。

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,
  );
}

实现视差效果还需要一个最终细节。当输入发生变化时，ParallaxFlowDelegate会重新绘制，但ParallaxFlowDelegate不会在每次滚动位置发生变化时都重新绘制。

将ScrollableState的ScrollPosition传递到FlowDelegate超类，以便FlowDelegate在ScrollPosition每次发生变化时都重新绘制。

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

恭喜！您现在拥有一个带有视差、滚动背景图像的卡片列表。

交互式示例

#

运行应用

  • 向上和向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.withOpacity(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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