内容
description bug_report

创建下载按钮

内容

应用程序中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产的访问权限。向用户显示长时间运行过程的进度非常有用，按钮本身是提供此反馈的理想位置。在本食谱中，您将构建一个下载按钮，该按钮根据应用程序下载的状态在多个视觉状态之间转换。

以下动画显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小部件

#

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

定义一个名为 DownloadButton 的新无状态小部件。

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

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

定义按钮的可能视觉状态

#

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 DownloadStatus 和 Duration，以确定按钮从一个状态动画到另一个状态需要多长时间。

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改变形状。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形。

根据当前的 `DownloadStatus`，构建一个带有 `ShapeDecoration` 的 `AnimatedContainer`，以显示圆角矩形或圆形。

考虑将形状的 widget 树定义在一个单独的 `Stateless` widget 中，这样主 `build()` 方法保持简单，允许添加后续内容。不要创建返回 widget 的函数，例如 `Widget _buildSomething() {}`，始终优先创建 `StatelessWidget` 或 `StatefulWidget`，这更有效。有关此方面的更多注意事项，请参阅 文档 或 Flutter YouTube 频道 中的专门视频。

目前，`AnimatedContainer` 的子元素只是一个 `SizedBox`，因为我们将在后面的步骤中对其进行处理。

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) {
    var shape = const ShapeDecoration(
      shape: StadiumBorder(),
      color: CupertinoColors.lightBackgroundGray,
    );

    if (isDownloading || isFetching) {
      shape = ShapeDecoration(
        shape: const CircleBorder(),
        color: Colors.white.withOpacity(0),
      );
    }

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

你可能会好奇为什么需要一个 `ShapeDecoration` widget 来显示一个透明的圆形，因为它不可见。不可见圆形的目的是协调所需的动画。`AnimatedContainer` 从一个圆角矩形开始。当 `DownloadStatus` 更改为 `fetchingDownload` 时，`AnimatedContainer` 需要从圆角矩形动画到圆形，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开始形状和圆形的结束形状。但是，你不希望最终的圆形可见，因此将其设置为透明，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`，其他阶段不显示任何文本。

添加 widget 以在每个下载阶段显示文本，并在文本之间动画文本的不透明度。将文本 widget 树添加为按钮包装器 widget 中 `AnimatedContainer` 的子元素。

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) {
    var shape = const ShapeDecoration(
      shape: StadiumBorder(),
      color: CupertinoColors.lightBackgroundGray,
    );

    if (isDownloading || isFetching) {
      shape = ShapeDecoration(
        shape: const CircleBorder(),
        color: Colors.white.withOpacity(0),
      );
    }

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

在获取下载时显示一个加载器

#

在 `fetchingDownload` 阶段，`DownloadButton` 显示一个径向旋转器。此旋转器从 `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旋转器，并在适当的时间淡入淡出。

我们已删除 `ButtonShapeWidget` 的构造函数，以将重点放在其 build 方法和我们添加的 `Stack` widget 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,
            ),
          ),
        ),
      ],
    ),
  );
}

在下载过程中显示进度和停止按钮

#

在 fetchingDownload 阶段之后是 downloading 阶段。在 downloading 阶段，DownloadButton 会用一个不断增长的径向进度条替换径向进度指示器。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。

在 DownloadButton 小部件中添加一个进度属性，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 需要做的最后一个细节是按钮行为。当用户点击按钮时，按钮必须执行一些操作。

添加小部件属性，用于回调以启动下载、取消下载和打开下载。

最后，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！您现在拥有一个按钮，它会根据按钮所处的阶段更改其显示：未下载、正在获取下载、正在下载和已下载。现在，用户可以点击启动下载，点击取消正在进行的下载，点击打开已完成的下载。

交互式示例

#

运行应用程序

  • 点击 获取 按钮以启动模拟下载。
  • 按钮会更改为进度指示器，以模拟正在进行的下载。
  • 模拟下载完成后，按钮会过渡到 打开，以指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) {
    var shape = const ShapeDecoration(
      shape: StadiumBorder(),
      color: CupertinoColors.lightBackgroundGray,
    );

    if (isDownloading || isFetching) {
      shape = ShapeDecoration(
        shape: const CircleBorder(),
        color: Colors.white.withOpacity(0),
      );
    }

    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.white.withOpacity(0),
            valueColor: AlwaysStoppedAnimation(isFetching
                ? CupertinoColors.lightBackgroundGray
                : CupertinoColors.activeBlue),
            strokeWidth: 2,
            value: isFetching ? null : progress,
          );
        },
      ),
    );
  }
}