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创建下载按钮

Contents

应用充满了执行长时间运行行为的按钮。 例如，一个按钮可能会触发下载， 这将启动下载过程，随着时间的推移接收数据， 然后提供对已下载资源的访问。 向用户显示长时间运行进程的进度很有帮助，按钮本身就是提供此反馈的好地方。 在这个食谱中， 你将构建一个下载按钮，它会根据应用程序下载的状态转换多个可视状态。

下面的动画显示了应用程序的行为：

下载按钮循环遍历其阶段

定义一个新的无状态小部件

#

你的按钮小部件需要随着时间的推移改变其外观。 因此，你需要使用自定义无状态小部件来实现你的按钮。

定义一个名为 DownloadButton 的新的无状态小部件。

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

#

下载按钮根据下载状态更改其形状。 在 notDownloaded 和 downloaded 状态下，按钮显示一个灰色的圆角矩形。 在 fetchingDownload 和 downloading 状态下，按钮显示一个透明的圆圈。

根据当前的 DownloadStatus，使用 ShapeDecoration 构建一个 AnimatedContainer，该装饰显示圆角矩形或圆圈。

考虑在一个单独的 Stateless 小部件中定义形状的 widget 树，以便主 build() 方法保持简单，允许添加后续内容。与其创建一个返回小部件的函数，例如 Widget _buildSomething() {}，始终更倾向于创建一个 StatelessWidget 或 StatefulWidget，这更高效。更多关于此的考虑可以在文档或 Flutter YouTube 频道中的专用视频中找到。

目前，AnimatedContainer 子项只是一个 SizedBox，因为我们将在另一个步骤中返回它。

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(),
    );
  }
}

你可能想知道，鉴于它不可见，为什么需要一个透明圆圈的 ShapeDecoration 小部件。 不可见圆圈的目的是协调所需的动画。AnimatedContainer 以圆角矩形开始。当 DownloadStatus 更改为 fetchingDownload 时，AnimatedContainer 需要从圆角矩形动画到圆圈，然后在动画进行时淡出。实现此动画的唯一方法是定义圆角矩形的起始形状和圆圈的结束形状。但是，你不想显示最终的圆圈，因此你将其设置为透明，这会导致动画淡出。

显示按钮文本

#

在 notDownloaded 阶段，DownloadButton 显示 GET；在 downloaded 阶段，显示 OPEN；在两者之间不显示文本。

添加小部件以在每个下载阶段显示文本，并在两者之间动画化文本的不透明度。将文本小部件树添加为按钮包装器小部件中 AnimatedContainer 的子项。

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,
                ),
          ),
        ),
      ),
    );
  }
}

在获取下载时显示微调器

#

在 fetchingDownload 阶段，DownloadButton 显示一个径向微调器。此微调器从 notDownloaded 阶段淡入，并从 fetchingDownload 阶段淡出。

实现一个位于按钮形状顶部的径向微调器，并在适当的时候淡入和淡出。

我们已删除 ButtonShapeWidget 的构造函数，以保持对它的构建方法和我们添加的 Stack 小部件的关注。

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,
            ),
          ),
        ),
      ],
    ),
  );
}

在下载过程中显示进度和停止按钮

#

fetchingDownload 阶段之后是 downloading 阶段。 在 downloading 阶段，DownloadButton 将径向进度微调器替换为增长的径向进度条。DownloadButton 还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。

向 DownloadButton 小部件添加一个进度属性，然后更新进度显示，以便在 downloading 阶段切换到径向进度条。

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

#

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

为回调添加小部件属性，以启动下载、取消下载和打开下载。

最后，使用 GestureDetector 小部件包装 DownloadButton 的现有小部件树，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！你已经拥有了一个按钮，它可以根据按钮所处的阶段更改显示：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。

交互式示例

#

运行应用程序：

  • 点击GET按钮启动模拟下载。
  • 按钮更改为进度指示器以模拟正在进行的下载。
  • 模拟下载完成后，按钮转换为OPEN，表示应用程序已准备好让用户打开已下载的资源。
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('打开应用 ${index + 1}'),
      ),
    );
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: const Text('应用')),
      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(
        '应用 ${index + 1}',
        overflow: TextOverflow.ellipsis,
        style: theme.textTheme.titleLarge,
      ),
      subtitle: Text(
        'Lorem ipsum dolor #${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();

    // 等待一秒钟以模拟获取时间。
    await Future<void>.delayed(const Duration(seconds: 1));

    // 如果用户选择取消下载，则停止模拟。
    if (!_isDownloading) {
      return;
    }

    // 切换到下载阶段。
    _downloadStatus = DownloadStatus.downloading;
    notifyListeners();

    const downloadProgressStops = [0.0, 0.15, 0.45, 0.8, 1.0];
    for (final stop in downloadProgressStops) {
      // 等待一秒钟以模拟不同的下载速度。
      await Future<void>.delayed(const Duration(seconds: 1));

      // 如果用户选择取消下载，则停止模拟。
      if (!_isDownloading) {
        return;
      }

      // 更新下载进度。
      _progress = stop;
      notifyListeners();
    }

    // 等待一秒钟以模拟最终延迟。
    await Future<void>.delayed(const Duration(seconds: 1));

    // 如果用户选择取消下载，则停止模拟。
    if (!_isDownloading) {
      return;
    }

    // 切换到已下载状态，完成模拟。
    _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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