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应用程序中充满了执行耗时操作的按钮。例如，按钮可能会触发下载，这会启动一个下载过程，随时间接收数据，然后提供对下载资产的访问。向用户显示耗时操作的进度很有帮助，而按钮本身就是提供此反馈的好地方。在本教程中，您将构建一个下载按钮，它会根据应用程序下载的状态，在多个视觉状态之间进行转换。

以下动画展示了应用程序的行为

The download button cycles through its stages

定义一个新的无状态组件

#

您的按钮组件需要随时间改变其外观。因此，您需要使用自定义的无状态组件来实现您的按钮。

定义一个新的无状态组件，名为 DownloadButton。

dart
@immutable
class DownloadButton extends StatelessWidget {
  const DownloadButton({super.key});

  @override
  Widget build(BuildContext context) {
    // TODO:
    return const SizedBox();
  }
}

定义按钮可能的视觉状态

#

下载按钮的视觉呈现基于给定的下载状态。定义下载的可能状态，然后更新 DownloadButton 以接受一个 DownloadStatus 和一个 Duration，用于指定按钮从一个状态动画到另一个状态所需的时间。

dart
enum DownloadStatus { notDownloaded, fetchingDownload, downloading, downloaded }

@immutable
class DownloadButton extends StatelessWidget {
  const DownloadButton({
    super.key,
    required this.status,
    this.transitionDuration = const Duration(milliseconds: 500),
  });

  final DownloadStatus status;
  final Duration transitionDuration;

  @override
  Widget build(BuildContext context) {
    // TODO: We'll add more to this later.
    return const SizedBox();
  }
}

显示按钮形状

#

下载按钮会根据下载状态改变其形状。在 notDownloaded 和 downloaded 状态下，按钮显示为灰色圆角矩形。在 fetchingDownload 和 downloading 状态下，按钮显示为透明圆形。

根据当前的 DownloadStatus，构建一个 AnimatedContainer，其中包含一个 ShapeDecoration，用于显示圆角矩形或圆形。

考虑将形状的组件树定义在一个独立的 Stateless 组件中，以便主 build() 方法保持简洁，从而方便后续添加内容。与其创建一个返回组件的函数，例如 Widget _buildSomething() {}，不如始终优先创建性能更好的 StatelessWidget 或 StatefulWidget。有关此内容的更多考虑，请参阅文档或 Flutter YouTube 频道中的专用视频。

目前，AnimatedContainer 的子组件只是一个 SizedBox，因为我们将在下一步中再次处理它。

dart
@immutable
class DownloadButton extends StatelessWidget {
  const DownloadButton({
    super.key,
    required this.status,
    this.transitionDuration = const Duration(milliseconds: 500),
  });

  final DownloadStatus status;
  final Duration transitionDuration;

  bool get _isDownloading => status == DownloadStatus.downloading;

  bool get _isFetching => status == DownloadStatus.fetchingDownload;

  bool get _isDownloaded => status == DownloadStatus.downloaded;

  @override
  Widget build(BuildContext context) {
    return ButtonShapeWidget(
      transitionDuration: transitionDuration,
      isDownloaded: _isDownloaded,
      isDownloading: _isDownloading,
      isFetching: _isFetching,
    );
  }
}

@immutable
class ButtonShapeWidget extends StatelessWidget {
  const ButtonShapeWidget({
    super.key,
    required this.isDownloading,
    required this.isDownloaded,
    required this.isFetching,
    required this.transitionDuration,
  });

  final bool isDownloading;
  final bool isDownloaded;
  final bool isFetching;
  final Duration transitionDuration;

  @override
  Widget build(BuildContext context) {
    final ShapeDecoration shape;
    if (isDownloading || isFetching) {
      shape = const ShapeDecoration(
        shape: CircleBorder(),
        color: Colors.transparent,
      );
    } else {
      shape = const ShapeDecoration(
        shape: StadiumBorder(),
        color: CupertinoColors.lightBackgroundGray,
      );
    }

    return AnimatedContainer(
      duration: transitionDuration,
      curve: Curves.ease,
      width: double.infinity,
      decoration: shape,
      child: const SizedBox(),
    );
  }
}

您可能想知道为什么一个透明的圆形（既然它是不可见的）还需要一个 ShapeDecoration 组件。这个不可见圆形的目的是协调所需的动画。AnimatedContainer 最初是一个圆角矩形。当 DownloadStatus 变为 fetchingDownload 时，AnimatedContainer 需要从圆角矩形动画到圆形，然后随着动画的进行而逐渐淡出。实现此动画的唯一方法是同时定义圆角矩形的起始形状和圆形的结束形状。但是，您不希望最终的圆形可见，因此您将其设置为透明，这会导致动画逐渐淡出。

显示按钮文本

#

DownloadButton 在 notDownloaded 阶段显示 GET，在 downloaded 阶段显示 OPEN，在这些阶段之间不显示文本。

在每个下载阶段添加显示文本的组件，并在其间动画化文本的不透明度。将文本组件树作为按钮包装组件中 AnimatedContainer 的子组件。

dart
@immutable
class ButtonShapeWidget extends StatelessWidget {
  const ButtonShapeWidget({
    super.key,
    required this.isDownloading,
    required this.isDownloaded,
    required this.isFetching,
    required this.transitionDuration,
  });

  final bool isDownloading;
  final bool isDownloaded;
  final bool isFetching;
  final Duration transitionDuration;

  @override
  Widget build(BuildContext context) {
    final ShapeDecoration shape;
    if (isDownloading || isFetching) {
      shape = const ShapeDecoration(
        shape: CircleBorder(),
        color: Colors.transparent,
      );
    } else {
      shape = const ShapeDecoration(
        shape: StadiumBorder(),
        color: CupertinoColors.lightBackgroundGray,
      );
    }

    return AnimatedContainer(
      duration: transitionDuration,
      curve: Curves.ease,
      width: double.infinity,
      decoration: shape,
      child: Padding(
        padding: const EdgeInsets.symmetric(vertical: 6),
        child: AnimatedOpacity(
          duration: transitionDuration,
          opacity: isDownloading || isFetching ? 0.0 : 1.0,
          curve: Curves.ease,
          child: Text(
            isDownloaded ? 'OPEN' : 'GET',
            textAlign: TextAlign.center,
            style: Theme.of(context).textTheme.labelLarge?.copyWith(
              fontWeight: FontWeight.bold,
              color: CupertinoColors.activeBlue,
            ),
          ),
        ),
      ),
    );
  }
}

在获取下载时显示加载指示器

#

在 fetchingDownload 阶段，DownloadButton 显示一个径向加载指示器。该加载指示器从 notDownloaded 阶段淡入，并淡出到 fetchingDownload 阶段。

实现一个径向加载指示器，它位于按钮形状的上方，并在适当的时候淡入和淡出。

我们已移除 ButtonShapeWidget 的构造函数，以将焦点保持在其 build 方法和我们添加的 Stack 组件上。

dart
@override
Widget build(BuildContext context) {
  return GestureDetector(
    onTap: _onPressed,
    child: Stack(
      children: [
        ButtonShapeWidget(
          transitionDuration: transitionDuration,
          isDownloaded: _isDownloaded,
          isDownloading: _isDownloading,
          isFetching: _isFetching,
        ),
        Positioned.fill(
          child: AnimatedOpacity(
            duration: transitionDuration,
            opacity: _isDownloading || _isFetching ? 1.0 : 0.0,
            curve: Curves.ease,
            child: ProgressIndicatorWidget(
              downloadProgress: downloadProgress,
              isDownloading: _isDownloading,
              isFetching: _isFetching,
            ),
          ),
        ),
      ],
    ),
  );
}

在下载时显示进度和停止按钮

#

fetchingDownload 阶段之后是 downloading 阶段。在 downloading 阶段，DownloadButton 将径向加载指示器替换为一个增长的径向进度条。DownloadButton 还会显示一个停止按钮图标，以便用户可以取消正在进行的下载。

为 DownloadButton 组件添加一个进度属性，然后在 downloading 阶段更新进度显示以切换到径向进度条。

接下来，在径向进度条的中心添加一个停止按钮图标。

dart
@override
Widget build(BuildContext context) {
  return GestureDetector(
    onTap: _onPressed,
    child: Stack(
      children: [
        ButtonShapeWidget(
          transitionDuration: transitionDuration,
          isDownloaded: _isDownloaded,
          isDownloading: _isDownloading,
          isFetching: _isFetching,
        ),
        Positioned.fill(
          child: AnimatedOpacity(
            duration: transitionDuration,
            opacity: _isDownloading || _isFetching ? 1.0 : 0.0,
            curve: Curves.ease,
            child: Stack(
              alignment: Alignment.center,
              children: [
                ProgressIndicatorWidget(
                  downloadProgress: downloadProgress,
                  isDownloading: _isDownloading,
                  isFetching: _isFetching,
                ),
                if (_isDownloading)
                  const Icon(
                    Icons.stop,
                    size: 14.0,
                    color: CupertinoColors.activeBlue,
                  ),
              ],
            ),
          ),
        ),
      ],
    ),
  );
}

添加按钮点击回调

#

您的 DownloadButton 所需的最后一个细节是按钮行为。当用户点击按钮时，按钮必须执行相应操作。

为开始下载、取消下载和打开下载的回调添加组件属性。

最后，使用 GestureDetector 组件包装 DownloadButton 现有的组件树，并将点击事件转发到相应的回调属性。

dart
@immutable
class DownloadButton extends StatelessWidget {
  const DownloadButton({
    super.key,
    required this.status,
    this.downloadProgress = 0,
    required this.onDownload,
    required this.onCancel,
    required this.onOpen,
    this.transitionDuration = const Duration(milliseconds: 500),
  });

  final DownloadStatus status;
  final double downloadProgress;
  final VoidCallback onDownload;
  final VoidCallback onCancel;
  final VoidCallback onOpen;
  final Duration transitionDuration;

  bool get _isDownloading => status == DownloadStatus.downloading;

  bool get _isFetching => status == DownloadStatus.fetchingDownload;

  bool get _isDownloaded => status == DownloadStatus.downloaded;

  void _onPressed() {
    switch (status) {
      case DownloadStatus.notDownloaded:
        onDownload();
      case DownloadStatus.fetchingDownload:
        // do nothing.
        break;
      case DownloadStatus.downloading:
        onCancel();
      case DownloadStatus.downloaded:
        onOpen();
    }
  }

  @override
  Widget build(BuildContext context) {
    return GestureDetector(
      onTap: _onPressed,
      child: const Stack(
        children: [
          /* ButtonShapeWidget and progress indicator */
        ],
      ),
    );
  }
}

恭喜！您现在拥有一个按钮，它会根据所处的阶段改变其显示：未下载、正在获取下载、正在下载和已下载。现在，用户可以点击开始下载、点击取消正在进行的下载，以及点击打开已完成的下载。

互动示例

#

运行应用

  • 点击 GET 按钮以启动模拟下载。
  • 按钮变为进度指示器，以模拟正在进行的下载。
  • 当模拟下载完成时，按钮转换为 OPEN，表示应用程序已准备好让用户打开下载的资产。
import 'package:flutter/cupertino.dart';
import 'package:flutter/material.dart';

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

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

  @override
  State<ExampleCupertinoDownloadButton> createState() =>
      _ExampleCupertinoDownloadButtonState();
}

class _ExampleCupertinoDownloadButtonState
    extends State<ExampleCupertinoDownloadButton> {
  late final List<DownloadController> _downloadControllers;

  @override
  void initState() {
    super.initState();
    _downloadControllers = List<DownloadController>.generate(
      20,
      (index) => SimulatedDownloadController(
        onOpenDownload: () {
          _openDownload(index);
        },
      ),
    );
  }

  void _openDownload(int index) {
    ScaffoldMessenger.of(
      context,
    ).showSnackBar(SnackBar(content: Text('Open App ${index + 1}')));
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: const Text('Apps')),
      body: ListView.separated(
        itemCount: _downloadControllers.length,
        separatorBuilder: (_, _) => const Divider(),
        itemBuilder: _buildListItem,
      ),
    );
  }

  Widget _buildListItem(BuildContext context, int index) {
    final theme = Theme.of(context);
    final downloadController = _downloadControllers[index];

    return ListTile(
      leading: const DemoAppIcon(),
      title: Text(
        'App ${index + 1}',
        overflow: TextOverflow.ellipsis,
        style: theme.textTheme.titleLarge,
      ),
      subtitle: Text(
        'Lorem ipsum dolor #${index + 1}',
        overflow: TextOverflow.ellipsis,
        style: theme.textTheme.bodySmall,
      ),
      trailing: SizedBox(
        width: 96,
        child: AnimatedBuilder(
          animation: downloadController,
          builder: (context, child) {
            return DownloadButton(
              status: downloadController.downloadStatus,
              downloadProgress: downloadController.progress,
              onDownload: downloadController.startDownload,
              onCancel: downloadController.stopDownload,
              onOpen: downloadController.openDownload,
            );
          },
        ),
      ),
    );
  }
}

@immutable
class DemoAppIcon extends StatelessWidget {
  const DemoAppIcon({super.key});

  @override
  Widget build(BuildContext context) {
    return const AspectRatio(
      aspectRatio: 1,
      child: FittedBox(
        child: SizedBox(
          width: 80,
          height: 80,
          child: DecoratedBox(
            decoration: BoxDecoration(
              gradient: LinearGradient(colors: [Colors.red, Colors.blue]),
              borderRadius: BorderRadius.all(Radius.circular(20)),
            ),
            child: Center(
              child: Icon(Icons.ac_unit, color: Colors.white, size: 40),
            ),
          ),
        ),
      ),
    );
  }
}

enum DownloadStatus { notDownloaded, fetchingDownload, downloading, downloaded }

abstract class DownloadController implements ChangeNotifier {
  DownloadStatus get downloadStatus;
  double get progress;

  void startDownload();
  void stopDownload();
  void openDownload();
}

class SimulatedDownloadController extends DownloadController
    with ChangeNotifier {
  SimulatedDownloadController({
    DownloadStatus downloadStatus = DownloadStatus.notDownloaded,
    double progress = 0.0,
    required VoidCallback onOpenDownload,
  }) : _downloadStatus = downloadStatus,
       _progress = progress,
       _onOpenDownload = onOpenDownload;

  DownloadStatus _downloadStatus;
  @override
  DownloadStatus get downloadStatus => _downloadStatus;

  double _progress;
  @override
  double get progress => _progress;

  final VoidCallback _onOpenDownload;

  bool _isDownloading = false;

  @override
  void startDownload() {
    if (downloadStatus == DownloadStatus.notDownloaded) {
      _doSimulatedDownload();
    }
  }

  @override
  void stopDownload() {
    if (_isDownloading) {
      _isDownloading = false;
      _downloadStatus = DownloadStatus.notDownloaded;
      _progress = 0.0;
      notifyListeners();
    }
  }

  @override
  void openDownload() {
    if (downloadStatus == DownloadStatus.downloaded) {
      _onOpenDownload();
    }
  }

  Future<void> _doSimulatedDownload() async {
    _isDownloading = true;
    _downloadStatus = DownloadStatus.fetchingDownload;
    notifyListeners();

    // Wait a second to simulate fetch time.
    await Future<void>.delayed(const Duration(seconds: 1));

    // If the user chose to cancel the download, stop the simulation.
    if (!_isDownloading) {
      return;
    }

    // Shift to the downloading phase.
    _downloadStatus = DownloadStatus.downloading;
    notifyListeners();

    const downloadProgressStops = [0.0, 0.15, 0.45, 0.8, 1.0];
    for (final stop in downloadProgressStops) {
      // Wait a second to simulate varying download speeds.
      await Future<void>.delayed(const Duration(seconds: 1));

      // If the user chose to cancel the download, stop the simulation.
      if (!_isDownloading) {
        return;
      }

      // Update the download progress.
      _progress = stop;
      notifyListeners();
    }

    // Wait a second to simulate a final delay.
    await Future<void>.delayed(const Duration(seconds: 1));

    // If the user chose to cancel the download, stop the simulation.
    if (!_isDownloading) {
      return;
    }

    // Shift to the downloaded state, completing the simulation.
    _downloadStatus = DownloadStatus.downloaded;
    _isDownloading = false;
    notifyListeners();
  }
}

@immutable
class DownloadButton extends StatelessWidget {
  const DownloadButton({
    super.key,
    required this.status,
    this.downloadProgress = 0.0,
    required this.onDownload,
    required this.onCancel,
    required this.onOpen,
    this.transitionDuration = const Duration(milliseconds: 500),
  });

  final DownloadStatus status;
  final double downloadProgress;
  final VoidCallback onDownload;
  final VoidCallback onCancel;
  final VoidCallback onOpen;
  final Duration transitionDuration;

  bool get _isDownloading => status == DownloadStatus.downloading;

  bool get _isFetching => status == DownloadStatus.fetchingDownload;

  bool get _isDownloaded => status == DownloadStatus.downloaded;

  void _onPressed() {
    switch (status) {
      case DownloadStatus.notDownloaded:
        onDownload();
      case DownloadStatus.fetchingDownload:
        // do nothing.
        break;
      case DownloadStatus.downloading:
        onCancel();
      case DownloadStatus.downloaded:
        onOpen();
    }
  }

  @override
  Widget build(BuildContext context) {
    return GestureDetector(
      onTap: _onPressed,
      child: Stack(
        children: [
          ButtonShapeWidget(
            transitionDuration: transitionDuration,
            isDownloaded: _isDownloaded,
            isDownloading: _isDownloading,
            isFetching: _isFetching,
          ),
          Positioned.fill(
            child: AnimatedOpacity(
              duration: transitionDuration,
              opacity: _isDownloading || _isFetching ? 1.0 : 0.0,
              curve: Curves.ease,
              child: Stack(
                alignment: Alignment.center,
                children: [
                  ProgressIndicatorWidget(
                    downloadProgress: downloadProgress,
                    isDownloading: _isDownloading,
                    isFetching: _isFetching,
                  ),
                  if (_isDownloading)
                    const Icon(
                      Icons.stop,
                      size: 14,
                      color: CupertinoColors.activeBlue,
                    ),
                ],
              ),
            ),
          ),
        ],
      ),
    );
  }
}

@immutable
class ButtonShapeWidget extends StatelessWidget {
  const ButtonShapeWidget({
    super.key,
    required this.isDownloading,
    required this.isDownloaded,
    required this.isFetching,
    required this.transitionDuration,
  });

  final bool isDownloading;
  final bool isDownloaded;
  final bool isFetching;
  final Duration transitionDuration;

  @override
  Widget build(BuildContext context) {
    final ShapeDecoration shape;
    if (isDownloading || isFetching) {
      shape = const ShapeDecoration(
        shape: CircleBorder(),
        color: Colors.transparent,
      );
    } else {
      shape = const ShapeDecoration(
        shape: StadiumBorder(),
        color: CupertinoColors.lightBackgroundGray,
      );
    }

    return AnimatedContainer(
      duration: transitionDuration,
      curve: Curves.ease,
      width: double.infinity,
      decoration: shape,
      child: Padding(
        padding: const EdgeInsets.symmetric(vertical: 6),
        child: AnimatedOpacity(
          duration: transitionDuration,
          opacity: isDownloading || isFetching ? 0.0 : 1.0,
          curve: Curves.ease,
          child: Text(
            isDownloaded ? 'OPEN' : 'GET',
            textAlign: TextAlign.center,
            style: Theme.of(context).textTheme.labelLarge?.copyWith(
              fontWeight: FontWeight.bold,
              color: CupertinoColors.activeBlue,
            ),
          ),
        ),
      ),
    );
  }
}

@immutable
class ProgressIndicatorWidget extends StatelessWidget {
  const ProgressIndicatorWidget({
    super.key,
    required this.downloadProgress,
    required this.isDownloading,
    required this.isFetching,
  });

  final double downloadProgress;
  final bool isDownloading;
  final bool isFetching;

  @override
  Widget build(BuildContext context) {
    return AspectRatio(
      aspectRatio: 1,
      child: TweenAnimationBuilder<double>(
        tween: Tween(begin: 0, end: downloadProgress),
        duration: const Duration(milliseconds: 200),
        builder: (context, progress, child) {
          return CircularProgressIndicator(
            backgroundColor: isDownloading
                ? CupertinoColors.lightBackgroundGray
                : Colors.transparent,
            valueColor: AlwaysStoppedAnimation(
              isFetching
                  ? CupertinoColors.lightBackgroundGray
                  : CupertinoColors.activeBlue,
            ),
            strokeWidth: 2,
            value: isFetching ? null : progress,
          );
        },
      ),
    );
  }
}

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