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,最重要的是向用户展示正在加载。一种流行的向用户传达数据正在加载的方法是,在模拟加载内容类型的形状上,显示带有微光动画的浏览器颜色。

以下动画展示了应用程序的行为

Gif showing the UI loading

本教程从已定义和定位的内容组件开始。右下角还有一个浮动操作按钮(FAB),可在加载模式和已加载模式之间切换,以便您轻松验证您的实现。

绘制微光形状

#

在此效果中产生微光的形状与最终加载的实际内容无关。

因此,目标是尽可能准确地显示代表最终内容的形状。

在内容具有明确边界的情况下,显示准确的形状很容易。例如,在本教程中,有一些圆形图片和一些圆角矩形图片。您可以绘制与这些图片的轮廓精确匹配的形状。

另一方面,考虑圆角矩形图片下方出现的文本。在文本加载之前,您不会知道有多少行文本。因此,尝试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,您会绘制几个非常薄的圆角矩形来表示将要出现的文本。形状和大小并不完全匹配,但这没关系。

从屏幕顶部的圆形列表项开始。确保每个 CircleListItem 组件在图片加载时显示一个带颜色的圆形。

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.flutterdart.cn/cookbook'
            '/img-files/effects/split-check/Avatar1.jpg',
            fit: BoxFit.cover,
          ),
        ),
      ),
    );
  }
}

只要您的组件显示某种形状,您就可以在本教程中应用微光效果。

CircleListItem 组件类似,确保 CardListItem 组件在图片将要出现的位置显示一种颜色。此外,在 CardListItem 组件中,根据当前的加载状态在文本和矩形显示之间切换。

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.flutterdart.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 现在根据是加载中还是已加载而呈现不同。通过暂时注释掉图片 URL,您可以看到 UI 的两种渲染方式。

Gif showing the shimmer animation

下一个目标是使用看起来像微光的单一渐变绘制所有着色区域。

绘制微光渐变

#

本教程中实现效果的关键是使用一个名为 ShaderMask 的组件。顾名思义,ShaderMask 组件会对其子组件应用着色器,但仅限于子组件已绘制内容的区域。例如,您将仅对之前配置的黑色形状应用着色器。

定义一个将应用于微光形状的浏览器颜色线性渐变。

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组件 ShimmerLoading,它使用 ShaderMask 包裹给定的 child 组件。将 ShaderMask 组件配置为以 srcATopblendMode 应用微光渐变作为着色器。srcATop 混合模式将 child 组件绘制的任何颜色替换为着色器颜色。

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,
    );
  }
}

ShimmerLoading 组件包裹您的 CircleListItem 组件。

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

ShimmerLoading 组件包裹您的 CardListItem 组件。

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

当您的形状正在加载时,它们现在会显示从 shaderCallback 返回的微光渐变。

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 组件和每个 CardListItem 组件都显示了渐变的新版本。对于本教程,整个屏幕应该看起来像一个大的闪烁表面。您将在下一步解决此问题。

绘制一个大微光效果

#

要在屏幕上绘制一个大的微光效果,每个 ShimmerLoading 组件需要根据该 ShimmerLoading 组件在屏幕上的位置绘制相同的全屏渐变。

更精确地说,与其假设微光应该占据整个屏幕,不如存在一个共享微光的区域。也许该区域占据了整个屏幕,或者不占据。在 Flutter 中解决此类问题的方法是定义另一个组件,它在组件树中位于所有 ShimmerLoading 组件的上方,并将其命名为 Shimmer。然后,每个 ShimmerLoading 组件获取对 Shimmer 祖先的引用,并请求要显示的大小和渐变。

定义一个新的有状态组件 Shimmer,它接受一个 LinearGradient 并为子代提供对其 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,以便提供对 linearGradientShimmerStateRenderBox 大小的访问,并查找子代在 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();
  }
}

Shimmer 组件包裹您屏幕上的所有内容。

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 组件中使用 Shimmer 组件来绘制共享渐变。

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 组件现在显示一个共享渐变,它占据了 Shimmer 组件内的所有空间。

动画微光效果

#

微光渐变需要移动,才能呈现出闪烁的光泽效果。

LinearGradient 有一个名为 transform 的属性,可用于变换渐变的外观,例如,水平移动它。transform 属性接受一个 GradientTransform 实例。

定义一个名为 _SlidingGradientTransform 的类,它实现 GradientTransform 以实现水平滑动的外观。

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,以创建运动的外观。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,
  ),
);

渐变现在会动了,但您的各个 ShimmerLoading 组件不会随着渐变的变化而重绘自身。因此,看起来什么都没有发生。

_shimmerControllerShimmerState 作为 Listenable 暴露出来。

dart
Listenable get shimmerChanges => _shimmerController;

ShimmerLoading 中,监听祖先 ShimmerStateshimmerChanges 属性的变化,并重绘微光渐变。

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!您现在拥有一个全屏、动画微光效果,它会随着内容的加载而开启和关闭。

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.flutterdart.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.flutterdart.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.',
        ),
      );
    }
  }
}