Create a photo filter carousel

Everybody knows that a photo looks better with a filter. In this recipe, you build a scrollable, filter selection carousel.

The following animation shows the app’s behavior:

Photo Filter Carousel

This recipe begins with the photo and filters already in place. Filters are applied with the color and colorBlendMode properties of the Image widget].

Add a selector ring and dark gradient

The selected filter circle is displayed within a selector ring. Additionally, a dark gradient is behind the available filters, which helps the contrast between the filters and any photo that you choose.

Create a new stateful widget called FilterSelector that you’ll use to implement the selector.

@immutable
class FilterSelector extends StatefulWidget {
 const FilterSelector({
   Key? key,
 }) : super(key: key);

 @override
 _FilterSelectorState createState() => _FilterSelectorState();
}

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

Add the FilterSelector widget to the existing widget tree. Position the FilterSelector widget on top of the photo, at the bottom and centered.

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

Within the FilterSelector widget, display a selector ring on top of a dark gradient by using a Stack widget.

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: 6.0, color: Colors.white),
             ),
           ),
         ),
       ),
     ),
   );
 }
}

The size of the selector circle and the background gradient depends on the size of an individual filter in the carousel called itemSize. The itemSize depends on the available width. Therefore, a LayoutBuilder widget is used to determine the available space, and then you calculate the size of an individual filter’s itemSize.

The selector ring includes an IgnorePointer widget because when carousel interactivity is added, the selector ring shouldn’t interfere with tap and drag events.

Each filter item in the carousel displays a circular image with a color applied to the image that corresponds to the associated filter color.

Define a new stateless widget called FilterItem that displays a single list item.

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

 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(8.0),
         child: ClipOval(
           child: Image.network(
             'https://flutter.dev/docs/cookbook/img-files'
             '/effects/instagram-buttons/millenial-texture.jpg',
             color: color.withOpacity(0.5),
             colorBlendMode: BlendMode.hardLight,
           ),
         ),
       ),
     ),
   );
 }
}

Filter items scroll to the left and right as the user drags. Scrolling requires some kind of Scrollable widget.

You might consider using a horizontal ListView widget, but a ListView widget positions the first element at the beginning of the available space, not at the center, where your selector ring sits.

A PageView widget is better suited for a carousel. A PageView widget lays out its children from the center of the available space and provides snapping physics. Snapping physics is what causes an item to snap to the center, no matter where the user releases a drag.

Configure your widget tree to make space for the 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 SizedBox();
     },
   ),
 );
}

Build each FilterItem widget within the PageView widget based on the given index.

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: () {},
         ),
       );
     },
   ),
 );
}

The PageView widget displays all of the FilterItem widgets, and you can drag to the left and right. However, right now each FilterItem widget takes up the entire width of the screen, and each FilterItem widget is displayed at the same size and opacity. There should be five FilterItem widgets on the screen, and the FilterItem widgets need to shrink and fade as they move farther from the center of the screen.

The solution to both of these issues is to introduce a PageViewController. The PageViewController’s viewportFraction property is used to display multiple FilterItem widgets on the screen at the same time. Rebuilding each FilterItem widget as the PageViewController changes allows you to change each FilterItem widget’s size and opacity as the user scrolls.

Create a PageViewController and connect it to the PageView widget.

class _FilterSelectorState extends State<FilterSelector> {
 late final PageController _controller;

 @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: () {},
           ),
         );
       },
     ),
   );
 }
}

With the PageViewController added, five FilterItem widgets are visible on the screen at the same time, and the photo filter changes as you scroll, but the FilterItem widgets are still the same size.

Wrap each FilterItem widget with an AnimatedBuilder to change the visual properties of each FilterItem widget as the scroll position changes.

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: () => {},
              );
            },
          ),
        );
      },
    ),
  );
}

The AnimatedBuilder widget rebuilds every time the _controller changes its scroll position. These rebuilds allow you to change the FilterItem size and opacity as the user drags.

Calculate an appropriate scale and opacity for each FilterItem widget within the AnimatedBuilder and apply those values.

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 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.
              final 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: () => () {},
                  ),
                ),
              );
            },
          ),
        );
      },
    ),
  );
}

Each FilterItem widget now shrinks and fades away as it moves farther from the center of the screen.

Add a method to change the selected filter when a FilterItem widget is tapped.

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

Configure each FilterItem widget to invoke _onFilterTapped when tapped.

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

Congratulations! You now have a draggable, tappable photo filter carousel.

Interactive example

import 'dart:math' as math;

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

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

@immutable
class ExampleInstagramFilterSelection extends StatefulWidget {
  const ExampleInstagramFilterSelection({Key? key}) : super(key: key);

  @override
  _ExampleInstagramFilterSelectionState 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, value, child) {
        final color = value as Color;
        return Image.network(
          'https://flutter.dev/docs/cookbook/img-files/effects/instagram-buttons/millenial-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({
    Key? key,
    required this.filters,
    required this.onFilterChanged,
    this.padding = const EdgeInsets.symmetric(vertical: 24.0),
  }) : super(key: key);

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

  @override
  _FilterSelectorState 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: 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: 6.0, 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 {
  FilterItem({
    Key? key,
    required this.color,
    this.onFilterSelected,
  }) : super(key: key);

  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(8.0),
          child: ClipOval(
            child: Image.network(
              'https://flutter.dev/docs/cookbook/img-files/effects/instagram-buttons/millenial-texture.jpg',
              color: color.withOpacity(0.5),
              colorBlendMode: BlendMode.hardLight,
            ),
          ),
        ),
      ),
    );
  }
}