跳至主要內容

當你在應用程式中滾動一個卡片清單（例如包含圖片的卡片）時，你可能會注意到那些圖片看起來比螢幕上的其他內容滾動得更慢。這種效果幾乎讓人感覺清單中的卡片位於前景，而圖片本身則像是遠遠地位於背景。這種效果被稱為視差（parallax）。

在本教學中，你將透過建立一個卡片清單（每個卡片有圓角並包含一些文字）來製作視差效果。每個卡片同時也包含一張圖片。當卡片往螢幕上方滑動時，卡片內的圖片則會往下滑動。

下方動畫展示了應用程式的行為：

Parallax scrolling

建立一個用於存放視差項目的清單

#

若要顯示一個具有視差滾動效果的圖片清單，你必須先建立一個清單。

建立一個名為 ParallaxRecipe 的無狀態元件（StatelessWidget）。在 ParallaxRecipe 中，建立一個包含 SingleChildScrollView 和 Column 的元件樹，這樣就形成了一個清單。

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

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

顯示帶有文字和靜態圖片的項目

#

每個清單項目會顯示一個圓角矩形的背景圖片，代表世界上七個地點之一。

在該背景圖片的上方，堆疊著該地點的名稱及其國家，並定位在左下角。

背景圖片與文字之間有一層深色漸層，這有助於提升文字在背景上的可讀性。

請實作一個名為 LocationListItem 的無狀態元件（StatelessWidget），其內容包含上述所提到的視覺元素。

目前，請先使用靜態的 Image 元件作為背景。

稍後，你將會以視差（parallax）版本取代該元件。

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

接下來，將清單項目加入你的 ParallaxRecipe 元件（Widget）。

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,
            ),
        ],
      ),
    );
  }
}

你現在已經擁有一個典型、可滾動的卡片清單， 展示了世界上七個獨特的地點。 在下一步，你將為背景圖片加入視差（parallax）效果。

實作視差效果

#

視差滾動效果是透過將背景圖片 稍微往與清單其餘部分相反的方向推動來實現的。 當清單項目往螢幕上方滑動時，每個背景圖片會稍微往下滑動。 反之，當清單項目往螢幕下方滑動時， 每個背景圖片會稍微往上滑動。 從視覺上來看，這就產生了視差效果。

視差效果取決於清單項目在其父層 Scrollable 中的當前位置。 隨著清單項目的滾動位置改變，該項目的背景圖片位置也必須跟著改變。 這是一個有趣的問題。清單項目在 Scrollable 中的位置，直到 Flutter 的版面配置階段結束後才會得知。 這表示背景圖片的位置必須在繪製（paint）階段決定，而這個階段是在版面配置階段之後。 幸運的是，Flutter 提供了一個名為 Flow 的元件（Widget）， 它專門設計用來讓你在子元件繪製前立即控制其變換（transform）。 換句話說，你可以攔截繪製階段，並自行掌控子元件的位置調整。

將你的背景 Image 元件（Widget）包裹在 Flow 元件（Widget）中。

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

引入一個新的 FlowDelegate，名稱為 ParallaxFlowDelegate。

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 控制其子元件（children）的尺寸，以及這些子元件的繪製位置。在這個例子中，你的 Flow 元件（Widget）只有一個子元件：背景圖片。該圖片的寬度必須與 Flow 元件完全相同。

請為你的背景圖片子元件回傳緊縮（tight）的寬度約束條件。

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

你的背景圖片現在已經有正確的尺寸， 但你仍然需要根據每張背景圖片的滾動位置來計算其垂直位置，然後將其繪製出來。

要計算背景圖片的理想位置，有三個關鍵資訊你需要取得：

  • 祖先 Scrollable 的邊界（bounds）
  • 個別清單項目的邊界
  • 圖片縮放至適合清單項目後的尺寸

若要查詢 Scrollable 的邊界， 你需要將 ScrollableState 傳遞給 FlowDelegate。

若要查詢個別清單項目的邊界， 請將你的清單項目的 BuildContext 傳遞給 FlowDelegate。

若要查詢背景圖片的最終尺寸， 請為你的 Image 元件（Widget）指定一個 GlobalKey， 然後將該 GlobalKey 傳遞給 FlowDelegate。

請將這些資訊提供給 ParallaxFlowDelegate。

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;
}

現在已經擁有實作視差滾動（parallax scrolling）所需的所有資訊，請實作 shouldRepaint() 方法。

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

現在，來實作視差效果（parallax effect）的版面配置計算。

首先，計算一個清單項目在其祖先 Scrollable 內的像素位置。

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,
  );
}

使用清單項目的像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清單項目應產生 0%，而位於可滾動區域底部的清單項目則應產生 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,
  );
  // ···
}

使用滾動百分比來計算Alignment。 當為 0% 時，你需要Alignment(0.0, -1.0)， 而當為 100% 時，你需要Alignment(0.0, 1.0)。 這些座標分別對應到頂部和底部對齊。

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，結合清單項目的尺寸以及背景圖片的尺寸，來產生Rect，以決定背景圖片應該被定位在哪個位置。

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，以所需的平移轉換來繪製背景圖片。 隨著時間變化的這個轉換，就是產生視差（parallax）效果的關鍵。

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,
  );
}

你還需要最後一個細節來實現視差（parallax）效果。 當輸入（inputs）改變時，ParallaxFlowDelegate 會重新繪製（repaint）， 但當滾動位置改變時，ParallaxFlowDelegate 並不會每次都重新繪製。

將 ScrollableState 的 ScrollPosition 傳遞給 FlowDelegate 的父類別（superclass），這樣 FlowDelegate 就能在每次 ScrollPosition 改變時都重新繪製。

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

恭喜你！ 你現在已經擁有一個帶有視差（parallax）效果、 可滾動背景圖片的卡片清單。

互動範例

#

執行應用程式：

  • 上下滾動以觀察視差效果。
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.dev/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'),
];

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