建立滾動視差效果

當你滾動應用中的卡片列表（例如包含影像的卡片）時，可能會注意到這些影像的滾動速度似乎比螢幕的其他部分要慢。這看起來就像列表中的卡片位於前景，而影像本身則處於遙遠的背景中。這種效果被稱為視差。

在本指南中，你將透過構建一個卡片列表（帶有圓角幷包含一些文字）來建立視差效果。每張卡片還包含一張影像。當卡片在螢幕上向上滑動時，每張卡片內的影像會向下滑動。

以下動畫展示了應用程式的行為

要展示視差滾動影像列表，首先必須展示一個列表。

建立一個名為 ParallaxRecipe 的新無狀態元件（stateless widget）。在 ParallaxRecipe 中，構建一個包含 SingleChildScrollView 和 Column 的元件樹，從而形成一個列表。

class ParallaxRecipe extends StatelessWidget {
  const ParallaxRecipe({super.key});
​
  @override
  Widget build(BuildContext context) {
    return const SingleChildScrollView(child: Column(children: []));
  }
}

每個列表項都顯示一張圓角矩形的背景圖，代表世界上的七個地點之一。在那張背景圖之上，堆疊顯示了該地點的名稱及其所屬國家，並定位在左下角。在背景影像和文字之間添加了深色漸變，以提高文字在背景上的可讀性。

實現一個名為 LocationListItem 的無狀態元件，由上述視覺元素組成。目前，請使用靜態的 Image 元件作為背景。稍後，你將使用視差版本替換該元件。

@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),
          ),
        ],
      ),
    );
  }
}

接下來，將列表項新增到你的 ParallaxRecipe 元件中。

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,
            ),
        ],
      ),
    );
  }
}

現在你已經擁有了一個典型的、可滾動的卡片列表，展示了世界上的七個獨特地點。下一步，你將為背景影像新增視差效果。

視差滾動效果是透過將背景影像向列表其餘部分的相反方向略微推動來實現的。當列表項在螢幕上向上滑動時，每個背景影像會略微向下滑動。相反，當列表項在螢幕上向下滑動時，每個背景影像會略微向上滑動。在視覺上，這產生了視差。

視差效果取決於列表項在其祖先 Scrollable 中的當前位置。隨著列表項滾動位置的變化，列表項背景影像的位置也必須隨之改變。這是一個有趣的難題。在 Flutter 的佈局階段完成之前，無法獲取列表項在 Scrollable 中的位置。這意味著背景影像的位置必須在佈局階段之後的繪製階段確定。幸運的是，Flutter 提供了一個名為 Flow 的元件，它專門用於讓你在元件繪製前立即控制子元件的變換。換句話說，你可以攔截繪製階段，並獲取控制權，根據需要重新定位你的子元件。

使用 Flow 元件包裹你的背景 Image 元件。

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

引入一個名為 ParallaxFlowDelegate 的新 FlowDelegate。

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 控制其子元件的大小以及它們的繪製位置。在這種情況下，你的 Flow 元件只有一個子元件：背景影像。該影像的寬度必須與 Flow 元件完全一致。

為你的背景影像子元件返回緊密約束（tight constraints）的寬度。

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

你的背景影像現在大小調整得當了，但你仍然需要根據每個背景影像的滾動位置計算其垂直位置，然後將其繪製出來。

計算背景影像目標位置需要三個關鍵資訊：

• 祖先 Scrollable 的邊界。
• 單個列表項的邊界。
• 影像縮小以適應列表項後的大小。

要查詢 Scrollable 的邊界，請將 ScrollableState 傳入你的 FlowDelegate。

要查詢單個列表項的邊界，請將列表項的 BuildContext 傳入你的 FlowDelegate。

要查詢背景影像的最終大小，請為你的 Image 元件分配一個 GlobalKey，然後將該 GlobalKey 傳入你的 FlowDelegate。

將這些資訊提供給 ParallaxFlowDelegate。

@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;
}

掌握了實現視差滾動所需的所有資訊後，實現 shouldRepaint() 方法。

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

現在，實現視差效果的佈局計算。

首先，計算列表項在其祖先 Scrollable 中的畫素位置。

@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,
  );
}

使用列表項的畫素位置來計算其距離 Scrollable頂部的百分比。位於可滾動區域頂部的列表項應產生 0%，位於底部的列表項應產生 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,
  );
  // ···
}

使用滾動百分比來計算 Alignment。在 0% 時，你希望得到 Alignment(0.0, -1.0)，而在 100% 時，你希望得到 Alignment(0.0, 1.0)。這些座標分別對應頂部和底部對齊。

@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，連同列表項的大小和背景影像的大小，生成一個 Rect，確定背景影像應放置的位置。

@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,
  );
​
  // 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，以所需的平移變換繪製背景影像。正是這種隨時間變化的變換為你帶來了視差效果。

@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,
  );
}

要實現視差效果，還需要最後一個細節。ParallaxFlowDelegate 會在輸入發生變化時重新繪製，但它不會在每次滾動位置變化時都自動重新繪製。

將 ScrollableState 的 ScrollPosition 傳遞給 FlowDelegate 父類，以便 FlowDelegate 在每次 ScrollPosition 發生變化時都能重新繪製。

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

恭喜！你現在擁有了一個帶有視差滾動背景影像的卡片列表。

執行應用

• 上下滾動即可觀察視差效果。

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.flutter.club.tw/assets/images/exercise/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'),
];