创建照片滤镜轮播

众所周知，照片加上滤镜会更好看。在本食谱中，您将构建一个可滚动的滤镜选择轮播。

以下动画展示了应用程序的行为

本食谱从已经就位的照片和滤镜开始。滤镜使用 Image 小部件的 color 和 colorBlendMode 属性应用。

选定的滤镜圆圈显示在选择环内。此外，在可用滤镜后面有一个深色渐变，这有助于滤镜与您选择的任何照片之间的对比度。

创建一个名为 FilterSelector 的新有状态小部件，您将使用它来实现选择器。

@immutable
class FilterSelector extends StatefulWidget {
  const FilterSelector({
    super.key,
  });

  @override
  State<FilterSelector> createState() => _FilterSelectorState();
}

class _FilterSelectorState extends State<FilterSelector> {
  @override
  Widget build(BuildContext context) {
    return const SizedBox();
  }
}

将 FilterSelector 小部件添加到现有的小部件树中。将 FilterSelector 小部件放置在照片的顶部，底部居中。

Stack(
  children: [
    Positioned.fill(
      child: _buildPhotoWithFilter(),
    ),
    const Positioned(
      left: 0.0,
      right: 0.0,
      bottom: 0.0,
      child: FilterSelector(),
    ),
  ],
),

在 FilterSelector 小部件中，使用 Stack 小部件在深色渐变之上显示一个选择器环。

class _FilterSelectorState extends State<FilterSelector> {
  static const _filtersPerScreen = 5;
  static const _viewportFractionPerItem = 1.0 / _filtersPerScreen;

  @override
  Widget build(BuildContext context) {
    return LayoutBuilder(
      builder: (context, constraints) {
        final itemSize = constraints.maxWidth * _viewportFractionPerItem;

        return Stack(
          alignment: Alignment.bottomCenter,
          children: [
            _buildShadowGradient(itemSize),
            _buildSelectionRing(itemSize),
          ],
        );
      },
    );
  }

  Widget _buildShadowGradient(double itemSize) {
    return SizedBox(
      height: itemSize * 2 + widget.padding.vertical,
      child: const DecoratedBox(
        decoration: BoxDecoration(
          gradient: LinearGradient(
            begin: Alignment.topCenter,
            end: Alignment.bottomCenter,
            colors: [
              Colors.transparent,
              Colors.black,
            ],
          ),
        ),
        child: SizedBox.expand(),
      ),
    );
  }

  Widget _buildSelectionRing(double itemSize) {
    return IgnorePointer(
      child: Padding(
        padding: widget.padding,
        child: SizedBox(
          width: itemSize,
          height: itemSize,
          child: const DecoratedBox(
            decoration: BoxDecoration(
              shape: BoxShape.circle,
              border: Border.fromBorderSide(
                BorderSide(width: 6, color: Colors.white),
              ),
            ),
          ),
        ),
      ),
    );
  }
}

选择器圆圈的大小和背景渐变的大小取决于轮播中单个滤镜的大小，称为 itemSize。itemSize 取决于可用宽度。因此，使用 LayoutBuilder 小部件来确定可用空间，然后计算单个滤镜的 itemSize 大小。

选择器环包含一个 IgnorePointer 小部件，因为当添加轮播交互性时，选择器环不应干扰点击和拖动事件。

轮播中的每个滤镜项目都显示一个圆形图像，图像上应用了与关联滤镜颜色相对应的颜色。

定义一个名为 FilterItem 的新无状态小部件，它显示单个列表项。

@immutable
class FilterItem extends StatelessWidget {
  const FilterItem({
    super.key,
    required this.color,
    this.onFilterSelected,
  });

  final Color color;
  final VoidCallback? onFilterSelected;

  @override
  Widget build(BuildContext context) {
    return GestureDetector(
      onTap: onFilterSelected,
      child: AspectRatio(
        aspectRatio: 1.0,
        child: Padding(
          padding: const EdgeInsets.all(8),
          child: ClipOval(
            child: Image.network(
              'https://docs.fluttercn.cn/cookbook/img-files'
              '/effects/instagram-buttons/millennial-texture.jpg',
              color: color.withOpacity(0.5),
              colorBlendMode: BlendMode.hardLight,
            ),
          ),
        ),
      ),
    );
  }
}

当用户拖动时，滤镜项目会向左和向右滚动。滚动需要某种 Scrollable 小部件。

您可能考虑使用水平 ListView 小部件，但 ListView 小部件将第一个元素放置在可用空间的开头，而不是选择器环所在的中心。

PageView 小部件更适合轮播。PageView 小部件从可用空间的中心布局其子项，并提供捕捉物理效果。捕捉物理效果是导致项目捕捉到中心的原因，无论用户在何处释放拖动。

配置您的窗口小部件树以腾出空间用于 PageView。

@override
Widget build(BuildContext context) {
  return LayoutBuilder(builder: (context, constraints) {
    final itemSize = constraints.maxWidth * _viewportFractionPerItem;

    return Stack(
      alignment: Alignment.bottomCenter,
      children: [
        _buildShadowGradient(itemSize),
        _buildCarousel(itemSize),
        _buildSelectionRing(itemSize),
      ],
    );
  });
}

Widget _buildCarousel(double itemSize) {
  return Container(
    height: itemSize,
    margin: widget.padding,
    child: PageView.builder(
      itemCount: widget.filters.length,
      itemBuilder: (context, index) {
        return const SizedBox();
      },
    ),
  );
}

根据给定的index，在PageView小部件内构建每个FilterItem小部件。

Color itemColor(int index) => widget.filters[index % widget.filters.length];

Widget _buildCarousel(double itemSize) {
  return Container(
    height: itemSize,
    margin: widget.padding,
    child: PageView.builder(
      itemCount: widget.filters.length,
      itemBuilder: (context, index) {
        return Center(
          child: FilterItem(
            color: itemColor(index),
            onFilterSelected: () {},
          ),
        );
      },
    ),
  );
}

PageView小部件显示所有FilterItem小部件，您可以左右拖动。但是，现在每个FilterItem小部件占据了整个屏幕宽度，并且每个FilterItem小部件都以相同的大小和不透明度显示。屏幕上应该有五个FilterItem小部件，并且FilterItem小部件需要随着它们远离屏幕中心而缩小和淡出。

这两个问题的解决方案是引入PageViewController。PageViewController的viewportFraction属性用于在屏幕上同时显示多个FilterItem小部件。在PageViewController更改时重建每个FilterItem小部件，允许您在用户滚动时更改每个FilterItem小部件的大小和不透明度。

创建一个PageViewController并将其连接到PageView小部件。

class _FilterSelectorState extends State<FilterSelector> {
  static const _filtersPerScreen = 5;
  static const _viewportFractionPerItem = 1.0 / _filtersPerScreen;

  late final PageController _controller;

  Color itemColor(int index) => widget.filters[index % widget.filters.length];

  @override
  void initState() {
    super.initState();
    _controller = PageController(
      viewportFraction: _viewportFractionPerItem,
    );
    _controller.addListener(_onPageChanged);
  }

  void _onPageChanged() {
    final page = (_controller.page ?? 0).round();
    widget.onFilterChanged(widget.filters[page]);
  }

  @override
  void dispose() {
    _controller.dispose();
    super.dispose();
  }

  Widget _buildCarousel(double itemSize) {
    return Container(
      height: itemSize,
      margin: widget.padding,
      child: PageView.builder(
        controller: _controller,
        itemCount: widget.filters.length,
        itemBuilder: (context, index) {
          return Center(
            child: FilterItem(
              color: itemColor(index),
              onFilterSelected: () {},
            ),
          );
        },
      ),
    );
  }
}

添加PageViewController后，屏幕上同时可见五个FilterItem小部件，并且照片过滤器在您滚动时会发生变化，但FilterItem小部件的大小仍然相同。

将每个FilterItem小部件包装在AnimatedBuilder中，以在滚动位置更改时更改每个FilterItem小部件的视觉属性。

Widget _buildCarousel(double itemSize) {
  return Container(
    height: itemSize,
    margin: widget.padding,
    child: PageView.builder(
      controller: _controller,
      itemCount: widget.filters.length,
      itemBuilder: (context, index) {
        return Center(
          child: AnimatedBuilder(
            animation: _controller,
            builder: (context, child) {
              return FilterItem(
                color: itemColor(index),
                onFilterSelected: () => {},
              );
            },
          ),
        );
      },
    ),
  );
}

AnimatedBuilder小部件在_controller更改其滚动位置时每次都会重建。这些重建允许您在用户拖动时更改FilterItem的大小和不透明度。

在AnimatedBuilder中为每个FilterItem小部件计算适当的比例和不透明度，并应用这些值。

Widget _buildCarousel(double itemSize) {
  return Container(
    height: itemSize,
    margin: widget.padding,
    child: PageView.builder(
      controller: _controller,
      itemCount: widget.filters.length,
      itemBuilder: (context, index) {
        return Center(
          child: AnimatedBuilder(
            animation: _controller,
            builder: (context, child) {
              if (!_controller.hasClients ||
                  !_controller.position.hasContentDimensions) {
                // The PageViewController isn't connected to the
                // PageView widget yet. Return an empty box.
                return const SizedBox();
              }

              // The integer index of the current page,
              // 0, 1, 2, 3, and so on
              final selectedIndex = _controller.page!.roundToDouble();

              // The fractional amount that the current filter
              // is dragged to the left or right, for example, 0.25 when
              // the current filter is dragged 25% to the left.
              final pageScrollAmount = _controller.page! - selectedIndex;

              // The page-distance of a filter just before it
              // moves off-screen.
              const maxScrollDistance = _filtersPerScreen / 2;

              // The page-distance of this filter item from the
              // currently selected filter item.
              final pageDistanceFromSelected =
                  (selectedIndex - index + pageScrollAmount).abs();

              // The distance of this filter item from the
              // center of the carousel as a percentage, that is, where the selector
              // ring sits.
              final percentFromCenter =
                  1.0 - pageDistanceFromSelected / maxScrollDistance;

              final itemScale = 0.5 + (percentFromCenter * 0.5);
              final opacity = 0.25 + (percentFromCenter * 0.75);

              return Transform.scale(
                scale: itemScale,
                child: Opacity(
                  opacity: opacity,
                  child: FilterItem(
                    color: itemColor(index),
                    onFilterSelected: () => () {},
                  ),
                ),
              );
            },
          ),
        );
      },
    ),
  );
}

每个FilterItem小部件现在随着它远离屏幕中心而缩小和淡出。

添加一个方法，在点击FilterItem小部件时更改所选过滤器。

void _onFilterTapped(int index) {
  _controller.animateToPage(
    index,
    duration: const Duration(milliseconds: 450),
    curve: Curves.ease,
  );
}

配置每个FilterItem小部件，使其在点击时调用_onFilterTapped。

FilterItem(
  color: itemColor(index),
  onFilterSelected: () => _onFilterTapped,
),

恭喜！您现在拥有一个可拖动、可点击的照片过滤器轮播。

import 'dart:math' as math;

import 'package:flutter/material.dart';
import 'package:flutter/rendering.dart' show ViewportOffset;

void main() {
  runApp(
    const MaterialApp(
      home: ExampleInstagramFilterSelection(),
      debugShowCheckedModeBanner: false,
    ),
  );
}

@immutable
class ExampleInstagramFilterSelection extends StatefulWidget {
  const ExampleInstagramFilterSelection({super.key});

  @override
  State<ExampleInstagramFilterSelection> createState() =>
      _ExampleInstagramFilterSelectionState();
}

class _ExampleInstagramFilterSelectionState
    extends State<ExampleInstagramFilterSelection> {
  final _filters = [
    Colors.white,
    ...List.generate(
      Colors.primaries.length,
      (index) => Colors.primaries[(index * 4) % Colors.primaries.length],
    )
  ];

  final _filterColor = ValueNotifier<Color>(Colors.white);

  void _onFilterChanged(Color value) {
    _filterColor.value = value;
  }

  @override
  Widget build(BuildContext context) {
    return Material(
      color: Colors.black,
      child: Stack(
        children: [
          Positioned.fill(
            child: _buildPhotoWithFilter(),
          ),
          Positioned(
            left: 0.0,
            right: 0.0,
            bottom: 0.0,
            child: _buildFilterSelector(),
          ),
        ],
      ),
    );
  }

  Widget _buildPhotoWithFilter() {
    return ValueListenableBuilder(
      valueListenable: _filterColor,
      builder: (context, color, child) {
        return Image.network(
          'https://docs.fluttercn.cn/cookbook/img-files'
          '/effects/instagram-buttons/millennial-dude.jpg',
          color: color.withOpacity(0.5),
          colorBlendMode: BlendMode.color,
          fit: BoxFit.cover,
        );
      },
    );
  }

  Widget _buildFilterSelector() {
    return FilterSelector(
      onFilterChanged: _onFilterChanged,
      filters: _filters,
    );
  }
}

@immutable
class FilterSelector extends StatefulWidget {
  const FilterSelector({
    super.key,
    required this.filters,
    required this.onFilterChanged,
    this.padding = const EdgeInsets.symmetric(vertical: 24),
  });

  final List<Color> filters;
  final void Function(Color selectedColor) onFilterChanged;
  final EdgeInsets padding;

  @override
  State<FilterSelector> createState() => _FilterSelectorState();
}

class _FilterSelectorState extends State<FilterSelector> {
  static const _filtersPerScreen = 5;
  static const _viewportFractionPerItem = 1.0 / _filtersPerScreen;

  late final PageController _controller;
  late int _page;

  int get filterCount => widget.filters.length;

  Color itemColor(int index) => widget.filters[index % filterCount];

  @override
  void initState() {
    super.initState();
    _page = 0;
    _controller = PageController(
      initialPage: _page,
      viewportFraction: _viewportFractionPerItem,
    );
    _controller.addListener(_onPageChanged);
  }

  void _onPageChanged() {
    final page = (_controller.page ?? 0).round();
    if (page != _page) {
      _page = page;
      widget.onFilterChanged(widget.filters[page]);
    }
  }

  void _onFilterTapped(int index) {
    _controller.animateToPage(
      index,
      duration: const Duration(milliseconds: 450),
      curve: Curves.ease,
    );
  }

  @override
  void dispose() {
    _controller.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return Scrollable(
      controller: _controller,
      axisDirection: AxisDirection.right,
      physics: const PageScrollPhysics(),
      viewportBuilder: (context, viewportOffset) {
        return LayoutBuilder(
          builder: (context, constraints) {
            final itemSize = constraints.maxWidth * _viewportFractionPerItem;
            viewportOffset
              ..applyViewportDimension(constraints.maxWidth)
              ..applyContentDimensions(0.0, itemSize * (filterCount - 1));

            return Stack(
              alignment: Alignment.bottomCenter,
              children: [
                _buildShadowGradient(itemSize),
                _buildCarousel(
                  viewportOffset: viewportOffset,
                  itemSize: itemSize,
                ),
                _buildSelectionRing(itemSize),
              ],
            );
          },
        );
      },
    );
  }

  Widget _buildShadowGradient(double itemSize) {
    return SizedBox(
      height: itemSize * 2 + widget.padding.vertical,
      child: const DecoratedBox(
        decoration: BoxDecoration(
          gradient: LinearGradient(
            begin: Alignment.topCenter,
            end: Alignment.bottomCenter,
            colors: [
              Colors.transparent,
              Colors.black,
            ],
          ),
        ),
        child: SizedBox.expand(),
      ),
    );
  }

  Widget _buildCarousel({
    required ViewportOffset viewportOffset,
    required double itemSize,
  }) {
    return Container(
      height: itemSize,
      margin: widget.padding,
      child: Flow(
        delegate: CarouselFlowDelegate(
          viewportOffset: viewportOffset,
          filtersPerScreen: _filtersPerScreen,
        ),
        children: [
          for (int i = 0; i < filterCount; i++)
            FilterItem(
              onFilterSelected: () => _onFilterTapped(i),
              color: itemColor(i),
            ),
        ],
      ),
    );
  }

  Widget _buildSelectionRing(double itemSize) {
    return IgnorePointer(
      child: Padding(
        padding: widget.padding,
        child: SizedBox(
          width: itemSize,
          height: itemSize,
          child: const DecoratedBox(
            decoration: BoxDecoration(
              shape: BoxShape.circle,
              border: Border.fromBorderSide(
                BorderSide(width: 6, color: Colors.white),
              ),
            ),
          ),
        ),
      ),
    );
  }
}

class CarouselFlowDelegate extends FlowDelegate {
  CarouselFlowDelegate({
    required this.viewportOffset,
    required this.filtersPerScreen,
  }) : super(repaint: viewportOffset);

  final ViewportOffset viewportOffset;
  final int filtersPerScreen;

  @override
  void paintChildren(FlowPaintingContext context) {
    final count = context.childCount;

    // All available painting width
    final size = context.size.width;

    // The distance that a single item "page" takes up from the perspective
    // of the scroll paging system. We also use this size for the width and
    // height of a single item.
    final itemExtent = size / filtersPerScreen;

    // The current scroll position expressed as an item fraction, e.g., 0.0,
    // or 1.0, or 1.3, or 2.9, etc. A value of 1.3 indicates that item at
    // index 1 is active, and the user has scrolled 30% towards the item at
    // index 2.
    final active = viewportOffset.pixels / itemExtent;

    // Index of the first item we need to paint at this moment.
    // At most, we paint 3 items to the left of the active item.
    final min = math.max(0, active.floor() - 3).toInt();

    // Index of the last item we need to paint at this moment.
    // At most, we paint 3 items to the right of the active item.
    final max = math.min(count - 1, active.ceil() + 3).toInt();

    // Generate transforms for the visible items and sort by distance.
    for (var index = min; index <= max; index++) {
      final itemXFromCenter = itemExtent * index - viewportOffset.pixels;
      final percentFromCenter = 1.0 - (itemXFromCenter / (size / 2)).abs();
      final itemScale = 0.5 + (percentFromCenter * 0.5);
      final opacity = 0.25 + (percentFromCenter * 0.75);

      final itemTransform = Matrix4.identity()
        ..translate((size - itemExtent) / 2)
        ..translate(itemXFromCenter)
        ..translate(itemExtent / 2, itemExtent / 2)
        ..multiply(Matrix4.diagonal3Values(itemScale, itemScale, 1.0))
        ..translate(-itemExtent / 2, -itemExtent / 2);

      context.paintChild(
        index,
        transform: itemTransform,
        opacity: opacity,
      );
    }
  }

  @override
  bool shouldRepaint(covariant CarouselFlowDelegate oldDelegate) {
    return oldDelegate.viewportOffset != viewportOffset;
  }
}

@immutable
class FilterItem extends StatelessWidget {
  const FilterItem({
    super.key,
    required this.color,
    this.onFilterSelected,
  });

  final Color color;
  final VoidCallback? onFilterSelected;

  @override
  Widget build(BuildContext context) {
    return GestureDetector(
      onTap: onFilterSelected,
      child: AspectRatio(
        aspectRatio: 1.0,
        child: Padding(
          padding: const EdgeInsets.all(8),
          child: ClipOval(
            child: Image.network(
              'https://docs.fluttercn.cn/cookbook/img-files'
              '/effects/instagram-buttons/millennial-texture.jpg',
              color: color.withOpacity(0.5),
              colorBlendMode: BlendMode.hardLight,
            ),
          ),
        ),
      ),
    );
  }
}