在应用程序开发中,加载时间是不可避免的。从用户体验(UX)的角度来看,最重要的事情是向用户表明正在进行加载。一种向用户传达数据正在加载的常用方法是,在近似加载内容类型的形状上显示一种带有闪光动画的铬色。

以下动画展示了应用程序的行为

Gif showing the UI loading

本指南首先定义并定位了内容小部件。在右下角还有一个浮动操作按钮(FAB),它在加载模式和已加载模式之间切换,以便您可以轻松验证您的实现。

绘制闪光形状

#

此效果中的闪光形状独立于最终加载的实际内容。

因此,目标是尽可能准确地显示代表最终内容的形状。

在内容具有清晰边界的情况下,显示准确的形状很容易。例如,在本指南中,有一些圆形图像和一些圆角矩形图像。您可以绘制精确匹配这些图像轮廓的形状。

另一方面,请考虑圆角矩形图像下方出现的文本。在文本加载之前,您不知道会有多少行文本。因此,试图为每一行文本绘制一个矩形是没有意义的。相反,在数据加载时,您绘制几个非常细的圆角矩形来表示将要出现的文本。形状和大小不完全匹配,但没关系。

从屏幕顶部的圆形列表项开始。确保每个 CircleListItem 小部件在图像加载时显示一个带有颜色的圆。

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

  @override
  Widget build(BuildContext context) {
    return Padding(
      padding: const EdgeInsets.symmetric(horizontal: 8, vertical: 8),
      child: Container(
        width: 54,
        height: 54,
        decoration: const BoxDecoration(
          color: Colors.black,
          shape: BoxShape.circle,
        ),
        child: ClipOval(
          child: Image.network(
            'https://docs.fluttercn.cn/cookbook'
            '/img-files/effects/split-check/Avatar1.jpg',
            fit: BoxFit.cover,
          ),
        ),
      ),
    );
  }
}

只要您的小部件显示某种形状,您就可以应用此指南中的闪光效果。

CircleListItem 小部件类似,请确保 CardListItem 小部件在图像将要显示的位置显示一个颜色。此外,在 CardListItem 小部件中,根据当前加载状态在文本和矩形的显示之间切换。

dart
class CardListItem extends StatelessWidget {
  const CardListItem({super.key, required this.isLoading});

  final bool isLoading;

  @override
  Widget build(BuildContext context) {
    return Padding(
      padding: const EdgeInsets.symmetric(horizontal: 24, vertical: 16),
      child: Column(
        crossAxisAlignment: CrossAxisAlignment.start,
        children: [_buildImage(), const SizedBox(height: 16), _buildText()],
      ),
    );
  }

  Widget _buildImage() {
    return AspectRatio(
      aspectRatio: 16 / 9,
      child: Container(
        width: double.infinity,
        decoration: BoxDecoration(
          color: Colors.black,
          borderRadius: BorderRadius.circular(16),
        ),
        child: ClipRRect(
          borderRadius: BorderRadius.circular(16),
          child: Image.network(
            'https://docs.fluttercn.cn/cookbook'
            '/img-files/effects/split-check/Food1.jpg',
            fit: BoxFit.cover,
          ),
        ),
      ),
    );
  }

  Widget _buildText() {
    if (isLoading) {
      return Column(
        crossAxisAlignment: CrossAxisAlignment.start,
        children: [
          Container(
            width: double.infinity,
            height: 24,
            decoration: BoxDecoration(
              color: Colors.black,
              borderRadius: BorderRadius.circular(16),
            ),
          ),
          const SizedBox(height: 16),
          Container(
            width: 250,
            height: 24,
            decoration: BoxDecoration(
              color: Colors.black,
              borderRadius: BorderRadius.circular(16),
            ),
          ),
        ],
      );
    } else {
      return const Padding(
        padding: EdgeInsets.symmetric(horizontal: 8),
        child: Text(
          'Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do '
          'eiusmod tempor incididunt ut labore et dolore magna aliqua.',
        ),
      );
    }
  }
}

您的 UI 现在会根据是加载中还是已加载而有所不同。通过暂时注释掉图像 URL,您可以看到 UI 的两种渲染方式。

Gif showing the shimmer animation

下一个目标是用一种看起来像闪光的渐变来绘制所有彩色区域。

绘制闪光渐变

#

本指南中实现效果的关键是使用一个名为 ShaderMask 的小部件。ShaderMask 小部件顾名思义,会将一个着色器应用于其子项,但仅限于子项已经绘制了某些内容的区域。例如,您将仅将着色器应用于您之前配置的黑色形状。

定义一个铬色线性渐变,该渐变将应用于闪光形状。

dart
const _shimmerGradient = LinearGradient(
  colors: [Color(0xFFEBEBF4), Color(0xFFF4F4F4), Color(0xFFEBEBF4)],
  stops: [0.1, 0.3, 0.4],
  begin: Alignment(-1.0, -0.3),
  end: Alignment(1.0, 0.3),
  tileMode: TileMode.clamp,
);

定义一个新的有状态小部件 ShimmerLoading,它用 ShaderMask 包装给定的 child 小部件。将 ShaderMask 小部件配置为将闪光渐变作为着色器应用,并将 blendMode 设置为 srcATopsrcATop 混合模式会用着色器颜色替换您 child 小部件绘制的任何颜色。

dart
class ShimmerLoading extends StatefulWidget {
  const ShimmerLoading({
    super.key,
    required this.isLoading,
    required this.child,
  });

  final bool isLoading;
  final Widget child;

  @override
  State<ShimmerLoading> createState() => _ShimmerLoadingState();
}

class _ShimmerLoadingState extends State<ShimmerLoading> {
  @override
  Widget build(BuildContext context) {
    if (!widget.isLoading) {
      return widget.child;
    }

    return ShaderMask(
      blendMode: BlendMode.srcATop,
      shaderCallback: (bounds) {
        return _shimmerGradient.createShader(bounds);
      },
      child: widget.child,
    );
  }
}

将您的 CircleListItem 小部件包装在 ShimmerLoading 小部件中。

dart
Widget _buildTopRowItem() {
  return ShimmerLoading(isLoading: _isLoading, child: const CircleListItem());
}

将您的 CardListItem 小部件包装在 ShimmerLoading 小部件中。

dart
Widget _buildListItem() {
  return ShimmerLoading(
    isLoading: _isLoading,
    child: CardListItem(isLoading: _isLoading),
  );
}

当您的形状正在加载时,它们现在显示从 shaderCallback 返回的闪光渐变。

这是朝着正确方向迈出的一大步,但这种渐变显示存在一个问题。每个 CircleListItem 小部件和每个 CardListItem 小部件都会显示一个新版本的渐变。对于本指南,整个屏幕应该看起来像一个大的闪光表面。您将在下一步解决此问题。

绘制一个大的闪光

#

要绘制一个大的闪光覆盖整个屏幕,每个 ShimmerLoading 小部件都需要根据该 ShimmerLoading 小部件在屏幕上的位置绘制相同的全屏渐变。

更准确地说,与其假设闪光应该占据整个屏幕,不如应该有一个共享闪光的区域。也许该区域占据整个屏幕,或者不占据。在 Flutter 中解决此类问题的最佳方法是定义另一个小部件,它位于小部件树中所有 ShimmerLoading 小部件的上方,并将其命名为 Shimmer。然后,每个 ShimmerLoading 小部件都会获得对 Shimmer 父级组件的引用,并请求要显示的所需大小和渐变。

定义一个名为 Shimmer 的新有状态小部件,它接受一个 LinearGradient 并向后代提供对其 State 对象的访问权限。

dart
class Shimmer extends StatefulWidget {
  static ShimmerState? of(BuildContext context) {
    return context.findAncestorStateOfType<ShimmerState>();
  }

  const Shimmer({super.key, required this.linearGradient, this.child});

  final LinearGradient linearGradient;
  final Widget? child;

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

class ShimmerState extends State<Shimmer> {
  @override
  Widget build(BuildContext context) {
    return widget.child ?? const SizedBox();
  }
}

ShimmerState 类添加方法,以便访问 linearGradientShimmerStateRenderBox 的大小,以及查找后代在 ShimmerStateRenderBox 中的位置。

dart
class ShimmerState extends State<Shimmer> {
  Gradient get gradient => LinearGradient(
    colors: widget.linearGradient.colors,
    stops: widget.linearGradient.stops,
    begin: widget.linearGradient.begin,
    end: widget.linearGradient.end,
  );

  bool get isSized =>
      (context.findRenderObject() as RenderBox?)?.hasSize ?? false;

  Size get size => (context.findRenderObject() as RenderBox).size;

  Offset getDescendantOffset({
    required RenderBox descendant,
    Offset offset = Offset.zero,
  }) {
    final shimmerBox = context.findRenderObject() as RenderBox;
    return descendant.localToGlobal(offset, ancestor: shimmerBox);
  }

  @override
  Widget build(BuildContext context) {
    return widget.child ?? const SizedBox();
  }
}

将您屏幕上的所有内容都包装在 Shimmer 小部件中。

dart
class _ExampleUiLoadingAnimationState extends State<ExampleUiLoadingAnimation> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Shimmer(
        linearGradient: _shimmerGradient,
        child: ListView(
          // ListView Contents
        ),
      ),
    );
  }
}

在您的 ShimmerLoading 小部件中使用 Shimmer 小部件来绘制共享渐变。

dart
class _ShimmerLoadingState extends State<ShimmerLoading> {
  @override
  Widget build(BuildContext context) {
    if (!widget.isLoading) {
      return widget.child;
    }

    // Collect ancestor shimmer information.
    final shimmer = Shimmer.of(context)!;
    if (!shimmer.isSized) {
      // The ancestor Shimmer widget isn't laid
      // out yet. Return an empty box.
      return const SizedBox();
    }
    final shimmerSize = shimmer.size;
    final gradient = shimmer.gradient;
    final offsetWithinShimmer = shimmer.getDescendantOffset(
      descendant: context.findRenderObject() as RenderBox,
    );

    return ShaderMask(
      blendMode: BlendMode.srcATop,
      shaderCallback: (bounds) {
        return gradient.createShader(
          Rect.fromLTWH(
            -offsetWithinShimmer.dx,
            -offsetWithinShimmer.dy,
            shimmerSize.width,
            shimmerSize.height,
          ),
        );
      },
      child: widget.child,
    );
  }
}

您的 ShimmerLoading 小部件现在显示一个共享渐变,该渐变占据 Shimmer 小部件内的所有空间。

动画闪光

#

闪光渐变需要移动,以产生闪光的效果。

LinearGradient 有一个名为 transform 的属性,可用于转换渐变的外观,例如使其水平移动。transform 属性接受一个 GradientTransform 实例。

定义一个名为 _SlidingGradientTransform 的类,它实现 GradientTransform 来实现水平滑动效果。

dart
class _SlidingGradientTransform extends GradientTransform {
  const _SlidingGradientTransform({required this.slidePercent});

  final double slidePercent;

  @override
  Matrix4? transform(Rect bounds, {TextDirection? textDirection}) {
    return Matrix4.translationValues(bounds.width * slidePercent, 0.0, 0.0);
  }
}

渐变滑动百分比会随时间变化,以产生运动效果。要更改百分比,请在 ShimmerState 类中配置一个 AnimationController

dart
class ShimmerState extends State<Shimmer> with SingleTickerProviderStateMixin {
  late AnimationController _shimmerController;

  @override
  void initState() {
    super.initState();

    _shimmerController = AnimationController.unbounded(vsync: this)
      ..repeat(min: -0.5, max: 1.5, period: const Duration(milliseconds: 1000));
  }

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

通过使用 _shimmerControllervalue 作为 slidePercent,将 _SlidingGradientTransform 应用于 gradient

dart
LinearGradient get gradient => LinearGradient(
  colors: widget.linearGradient.colors,
  stops: widget.linearGradient.stops,
  begin: widget.linearGradient.begin,
  end: widget.linearGradient.end,
  transform: _SlidingGradientTransform(
    slidePercent: _shimmerController.value,
  ),
);

渐变现在会动画,但您的各个 ShimmerLoading 小部件不会随着渐变的改变而重绘。因此,看起来什么都没发生。

_shimmerController 作为 ListenableShimmerState 中公开。

dart
Listenable get shimmerChanges => _shimmerController;

ShimmerLoading 中,监听对父级 ShimmerStateshimmerChanges 属性的更改,并重绘闪光渐变。

dart
class _ShimmerLoadingState extends State<ShimmerLoading> {
  Listenable? _shimmerChanges;

  @override
  void didChangeDependencies() {
    super.didChangeDependencies();
    if (_shimmerChanges != null) {
      _shimmerChanges!.removeListener(_onShimmerChange);
    }
    _shimmerChanges = Shimmer.of(context)?.shimmerChanges;
    if (_shimmerChanges != null) {
      _shimmerChanges!.addListener(_onShimmerChange);
    }
  }

  @override
  void dispose() {
    _shimmerChanges?.removeListener(_onShimmerChange);
    super.dispose();
  }

  void _onShimmerChange() {
    if (widget.isLoading) {
      setState(() {
        // Update the shimmer painting.
      });
    }
  }
  }

恭喜!您现在拥有了一个全屏、动画闪光效果,该效果会随着内容的加载而打开和关闭。

互动示例

#
import 'package:flutter/material.dart';

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

const _shimmerGradient = LinearGradient(
  colors: [Color(0xFFEBEBF4), Color(0xFFF4F4F4), Color(0xFFEBEBF4)],
  stops: [0.1, 0.3, 0.4],
  begin: Alignment(-1.0, -0.3),
  end: Alignment(1.0, 0.3),
  tileMode: TileMode.clamp,
);

class ExampleUiLoadingAnimation extends StatefulWidget {
  const ExampleUiLoadingAnimation({super.key});

  @override
  State<ExampleUiLoadingAnimation> createState() =>
      _ExampleUiLoadingAnimationState();
}

class _ExampleUiLoadingAnimationState extends State<ExampleUiLoadingAnimation> {
  bool _isLoading = true;

  void _toggleLoading() {
    setState(() {
      _isLoading = !_isLoading;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Shimmer(
        linearGradient: _shimmerGradient,
        child: ListView(
          physics: _isLoading ? const NeverScrollableScrollPhysics() : null,
          children: [
            const SizedBox(height: 16),
            _buildTopRowList(),
            const SizedBox(height: 16),
            _buildListItem(),
            _buildListItem(),
            _buildListItem(),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _toggleLoading,
        child: Icon(_isLoading ? Icons.hourglass_full : Icons.hourglass_bottom),
      ),
    );
  }

  Widget _buildTopRowList() {
    return SizedBox(
      height: 72,
      child: ListView(
        physics: _isLoading ? const NeverScrollableScrollPhysics() : null,
        scrollDirection: Axis.horizontal,
        shrinkWrap: true,
        children: [
          const SizedBox(width: 16),
          _buildTopRowItem(),
          _buildTopRowItem(),
          _buildTopRowItem(),
          _buildTopRowItem(),
          _buildTopRowItem(),
          _buildTopRowItem(),
        ],
      ),
    );
  }

  Widget _buildTopRowItem() {
    return ShimmerLoading(isLoading: _isLoading, child: const CircleListItem());
  }

  Widget _buildListItem() {
    return ShimmerLoading(
      isLoading: _isLoading,
      child: CardListItem(isLoading: _isLoading),
    );
  }
}

class Shimmer extends StatefulWidget {
  static ShimmerState? of(BuildContext context) {
    return context.findAncestorStateOfType<ShimmerState>();
  }

  const Shimmer({super.key, required this.linearGradient, this.child});

  final LinearGradient linearGradient;
  final Widget? child;

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

class ShimmerState extends State<Shimmer> with SingleTickerProviderStateMixin {
  late AnimationController _shimmerController;

  @override
  void initState() {
    super.initState();

    _shimmerController = AnimationController.unbounded(vsync: this)
      ..repeat(min: -0.5, max: 1.5, period: const Duration(milliseconds: 1000));
  }

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

  LinearGradient get gradient => LinearGradient(
    colors: widget.linearGradient.colors,
    stops: widget.linearGradient.stops,
    begin: widget.linearGradient.begin,
    end: widget.linearGradient.end,
    transform: _SlidingGradientTransform(
      slidePercent: _shimmerController.value,
    ),
  );

  bool get isSized =>
      (context.findRenderObject() as RenderBox?)?.hasSize ?? false;

  Size get size => (context.findRenderObject() as RenderBox).size;

  Offset getDescendantOffset({
    required RenderBox descendant,
    Offset offset = Offset.zero,
  }) {
    final shimmerBox = context.findRenderObject() as RenderBox?;
    return descendant.localToGlobal(offset, ancestor: shimmerBox);
  }

  Listenable get shimmerChanges => _shimmerController;

  @override
  Widget build(BuildContext context) {
    return widget.child ?? const SizedBox();
  }
}

class _SlidingGradientTransform extends GradientTransform {
  const _SlidingGradientTransform({required this.slidePercent});

  final double slidePercent;

  @override
  Matrix4? transform(Rect bounds, {TextDirection? textDirection}) {
    return Matrix4.translationValues(bounds.width * slidePercent, 0.0, 0.0);
  }
}

class ShimmerLoading extends StatefulWidget {
  const ShimmerLoading({
    super.key,
    required this.isLoading,
    required this.child,
  });

  final bool isLoading;
  final Widget child;

  @override
  State<ShimmerLoading> createState() => _ShimmerLoadingState();
}

class _ShimmerLoadingState extends State<ShimmerLoading> {
  Listenable? _shimmerChanges;

  @override
  void didChangeDependencies() {
    super.didChangeDependencies();
    if (_shimmerChanges != null) {
      _shimmerChanges!.removeListener(_onShimmerChange);
    }
    _shimmerChanges = Shimmer.of(context)?.shimmerChanges;
    if (_shimmerChanges != null) {
      _shimmerChanges!.addListener(_onShimmerChange);
    }
  }

  @override
  void dispose() {
    _shimmerChanges?.removeListener(_onShimmerChange);
    super.dispose();
  }

  void _onShimmerChange() {
    if (widget.isLoading) {
      setState(() {
        // Update the shimmer painting.
      });
    }
  }

  @override
  Widget build(BuildContext context) {
    if (!widget.isLoading) {
      return widget.child;
    }

    // Collect ancestor shimmer info.
    final shimmer = Shimmer.of(context)!;
    if (!shimmer.isSized) {
      // The ancestor Shimmer widget has not laid
      // itself out yet. Return an empty box.
      return const SizedBox();
    }
    final shimmerSize = shimmer.size;
    final gradient = shimmer.gradient;
    final offsetWithinShimmer = shimmer.getDescendantOffset(
      descendant: context.findRenderObject() as RenderBox,
    );

    return ShaderMask(
      blendMode: BlendMode.srcATop,
      shaderCallback: (bounds) {
        return gradient.createShader(
          Rect.fromLTWH(
            -offsetWithinShimmer.dx,
            -offsetWithinShimmer.dy,
            shimmerSize.width,
            shimmerSize.height,
          ),
        );
      },
      child: widget.child,
    );
  }
}

//----------- List Items ---------
class CircleListItem extends StatelessWidget {
  const CircleListItem({super.key});
  @override
  Widget build(BuildContext context) {
    return Padding(
      padding: const EdgeInsets.symmetric(horizontal: 8, vertical: 8),
      child: Container(
        width: 54,
        height: 54,
        decoration: const BoxDecoration(
          color: Colors.black,
          shape: BoxShape.circle,
        ),
        child: ClipOval(
          child: Image.network(
            'https://docs.fluttercn.cn/cookbook'
            '/img-files/effects/split-check/Avatar1.jpg',
            fit: BoxFit.cover,
          ),
        ),
      ),
    );
  }
}

class CardListItem extends StatelessWidget {
  const CardListItem({super.key, required this.isLoading});

  final bool isLoading;

  @override
  Widget build(BuildContext context) {
    return Padding(
      padding: const EdgeInsets.symmetric(horizontal: 24, vertical: 16),
      child: Column(
        crossAxisAlignment: CrossAxisAlignment.start,
        children: [_buildImage(), const SizedBox(height: 16), _buildText()],
      ),
    );
  }

  Widget _buildImage() {
    return AspectRatio(
      aspectRatio: 16 / 9,
      child: Container(
        width: double.infinity,
        decoration: BoxDecoration(
          color: Colors.black,
          borderRadius: BorderRadius.circular(16),
        ),
        child: ClipRRect(
          borderRadius: BorderRadius.circular(16),
          child: Image.network(
            'https://docs.fluttercn.cn/cookbook'
            '/img-files/effects/split-check/Food1.jpg',
            fit: BoxFit.cover,
          ),
        ),
      ),
    );
  }

  Widget _buildText() {
    if (isLoading) {
      return Column(
        crossAxisAlignment: CrossAxisAlignment.start,
        children: [
          Container(
            width: double.infinity,
            height: 24,
            decoration: BoxDecoration(
              color: Colors.black,
              borderRadius: BorderRadius.circular(16),
            ),
          ),
          const SizedBox(height: 16),
          Container(
            width: 250,
            height: 24,
            decoration: BoxDecoration(
              color: Colors.black,
              borderRadius: BorderRadius.circular(16),
            ),
          ),
        ],
      );
    } else {
      return const Padding(
        padding: EdgeInsets.symmetric(horizontal: 8),
        child: Text(
          'Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do '
          'eiusmod tempor incididunt ut labore et dolore magna aliqua.',
        ),
      );
    }
  }
}