肆、Create a scrolling parallax effect

參見:Create a scrolling parallax effect

當我們在滾動時，圖像的滾動速度會比視窗的其餘部分稍慢，而這種速度不一致會產生前景與後景的視差效果，而接下來的範例中，我們會介紹如何用自定義的滾動模式與設置來達成滾動時，讓背景影像的位置會根據滾動位置而變化，從而產生視差效果。

一、元件佈局區塊

入口點與佈景主題設定：

MyApp Class是應用程式的root元件，繼承自 StatelessWidget。
設定了暗黑主題，並將應用程式欄的背景顏色設為 darkBlue。

ExampleParallax class：

是一個 StatelessWidget，用於建立包含捲動清單的單一子視圖。
使用 SingleChildScrollView 建立一個垂直捲動的列，列中包含了多個 LocationListItem 子元件。

LocationListItem class：

表示捲動清單中的每個項目，包括一個背景圖像和相關資訊。
它接受三個必需的參數：imageUrl（圖像連結）、name（名稱）和 country（國家）。
使用 Padding、AspectRatio 和 Stack 組合來建立每個項目的版面。
Stack包含了背景圖像、漸層效果和標題資訊。

ParallaxFlowDelegate class：

是一個自訂的 FlowDelegate，用於實現滾動視差效果。
透過監聽滾動事件，它計算每個列表項目的位置，以及應用背景圖像的垂直滾動效果。

paintChildren 方法用於繪製背景圖像，並根據滾動百分比的高度來定位。

Parallax class 和 RenderParallax class：

用於封裝捲動視差效果的自訂 Widget 和 RenderObject。
Parallax 會將背景圖像作為子部件傳遞給 RenderParallax。
RenderParallax 處理特定的背景影像的繪製邏輯，根據捲動位置計算垂直偏移，以實現滾動視差效果。

二、資料區塊

Location 資料模型：

Location class表示一個地點，包括名稱、國家和圖像連結。
locations: 一個包含多個 Location 實例的列表，每個實例代表一個不同的地點。

三、使用範例

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) {
    // 計算此列表在視窗中的位置
    final scrollableBox = scrollable.context.findRenderObject() as RenderBox;
    final listItemBox = listItemContext.findRenderObject() as RenderBox;
    final listItemOffset = listItemBox.localToGlobal(
        listItemBox.size.centerLeft(Offset.zero),
        ancestor: scrollableBox);

     // 決定此列表項目在清單中可捲動區域的百分比位置
    final viewportDimension = scrollable.position.viewportDimension;
    final scrollFraction =
        (listItemOffset.dy / viewportDimension).clamp(0.0, 1.0);

    // 基於滾動百分比計算背景的垂直對齊方式
    final verticalAlignment = Alignment(0.0, scrollFraction * 2 - 1);

    // 將背景對齊轉換為像素偏移以用於繪畫
    final backgroundSize =
        (backgroundImageKey.currentContext!.findRenderObject() as RenderBox)
            .size;
    final listItemSize = context.size;
    final childRect =
        verticalAlignment.inscribe(backgroundSize, Offset.zero & listItemSize);

    // 繪製背景
    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;

    // 強制背景佔據所有可用寬度，然後根據圖像的長寬比縮放其高度
    final background = child!;
    final backgroundImageConstraints =
        BoxConstraints.tightFor(width: size.width);
    background.layout(backgroundImageConstraints, parentUsesSize: true);

    // 設定背景的局部偏移量為零。
    (background.parentData as ParallaxParentData).offset = Offset.zero;
  }

  @override
  void paint(PaintingContext context, Offset offset) {
    // 取得可滾動區域大小
    final viewportDimension = scrollable.position.viewportDimension;

    // 計算該列表項的全域位置
    final scrollableBox = scrollable.context.findRenderObject() as RenderBox;
    final backgroundOffset =
        localToGlobal(size.centerLeft(Offset.zero), ancestor: scrollableBox);

    // 決定此清單項目在可捲動區域內的百分比位置。
    final scrollFraction =
        (backgroundOffset.dy / viewportDimension).clamp(0.0, 1.0);

    // 根據滾動百分比計算背景的垂直對齊方式。
    final verticalAlignment = Alignment(0.0, scrollFraction * 2 - 1);

    // 將背景對齊轉換為像素偏移以用於繪畫
    final background = child!;
    final backgroundSize = background.size;
    final listItemSize = size;
    final childRect =
        verticalAlignment.inscribe(backgroundSize, Offset.zero & listItemSize);

    // 繪製背景
    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/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',
  ),
];

輸出結果

透過滾動到不同位置，我們可以看到圖片呈現的方向皆不太一樣(可以觀察Gardens By The Bay的圖片)，大家可以嘗試用dartpad執行，動態滑動過程中的視差效果會更明顯!這樣我們就可以完成一個世界七大奇景圖片的視差滾動效果Demo!