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應用程式中充滿了執行長時間執行行為的按鈕。例如，一個按鈕可能會觸發一次下載，啟動一個下載程序，隨著時間的推移接收資料，然後提供對已下載資源的訪問。向用戶顯示長時間執行進度的狀態很有幫助，而按鈕本身是提供此反饋的好地方。在本食譜中，您將構建一個下載按鈕，該按鈕根據應用程式下載的狀態在多個視覺狀態之間過渡。

以下動畫展示了應用程式的行為

The download button cycles through its stages

定義一個新的無狀態小部件

#

您的按鈕小部件需要隨時間改變其外觀。因此，您需要使用自定義無狀態小部件來實現您的按鈕。

定義一個名為 DownloadButton 的新無狀態小部件。

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

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

定義按鈕的可能視覺狀態

#

下載按鈕的視覺呈現基於給定的下載狀態。定義下載的可能狀態，然後更新 DownloadButton 以接受 DownloadStatus 和 Duration，以指定按鈕在狀態之間動畫所需的時間。

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();
  }
}

顯示按鈕形狀

#

下載按鈕根據下載狀態改變其形狀。在 notDownloaded 和 downloaded 狀態期間，按鈕顯示一個灰色圓角矩形。在 fetchingDownload 和 downloading 狀態期間，按鈕顯示一個透明圓。

根據當前的 DownloadStatus，構建一個帶有 ShapeDecoration 的 AnimatedContainer，該 ShapeDecoration 顯示一個圓角矩形或一個圓。

考慮將形狀的小部件樹定義在一個單獨的 Stateless 小部件中，以便主 build() 方法保持簡單，從而可以進行後續的新增。不要建立返回小部件的函式，如 Widget _buildSomething() {}，始終優先建立 StatelessWidget 或 StatefulWidget，這更具效能。更多關於這方面的考慮可以在 文件 或 Flutter YouTube 頻道 的一個專用影片中找到。

目前，AnimatedContainer 的子項只是一個 SizedBox，因為我們將在另一個步驟中再來處理它。

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(),
    );
  }
}

您可能想知道為什麼對於一個透明圓需要一個 ShapeDecoration 小部件，因為它實際上是不可見的。不可見圓的目的是為了編排所需的動畫。AnimatedContainer 開始時是一個圓角矩形。當 DownloadStatus 更改為 fetchingDownload 時，AnimatedContainer 需要從圓角矩形動畫到一個圓，然後在動畫進行時淡出。實現此動畫的唯一方法是定義圓角矩形的起始形狀和圓的結束形狀。但是，您不希望最終的圓可見，因此將其設為透明，這將導致一個動畫淡出。

顯示按鈕文字

#

DownloadButton 在 notDownloaded 階段顯示 GET，在 downloaded 階段顯示 OPEN，而在這兩個階段之間不顯示任何文字。

新增小部件以在每個下載階段顯示文字，並在兩者之間動畫文字的不透明度。將文字小部件樹新增為按鈕包裝器小部件中 AnimatedContainer 的子項。

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

在獲取下載時顯示載入指示器

#

在 fetchingDownload 階段，DownloadButton 顯示一個徑向載入指示器。該載入指示器會從 notDownloaded 階段淡入，並淡出到 fetchingDownload 階段。

實現一個徑向載入指示器，它位於按鈕形狀之上，並在適當的時候淡入淡出。

為了將重點放在 ButtonShapeWidget 的構建方法和我們新增的 Stack 小部件上，我們已經移除了 ButtonShapeWidget 的建構函式。

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

在下載時顯示進度和停止按鈕

#

在 fetchingDownload 階段之後是 downloading 階段。在 downloading 階段，DownloadButton 將徑向進度載入指示器替換為不斷增長的徑向進度條。DownloadButton 還顯示一個停止按鈕圖示，以便使用者可以取消正在進行的下載。

向 DownloadButton 小部件新增一個進度屬性，然後在 downloading 階段更新進度顯示以切換到徑向進度條。

接下來，在徑向進度條的中心新增一個停止按鈕圖示。

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

新增按鈕點選回撥

#

您的 DownloadButton 所需的最後一個細節是按鈕的行為。當用戶點選按鈕時，按鈕必須執行相應的操作。

為啟動下載、取消下載和開啟下載的回撥新增小部件屬性。

最後，用 GestureDetector 小部件包裝 DownloadButton 現有的 widget tree，並將點選事件轉發到相應的回撥屬性。

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！您已經擁有了一個按鈕，該按鈕根據其所處的階段（未下載、正在獲取下載、正在下載和已下載）改變其顯示。現在，使用者可以點選開始下載，點選取消正在進行的下載，以及點選開啟已完成的下載。

互動示例

#

執行應用

  • 點選 GET 按鈕以啟動模擬下載。
  • 按鈕會變成一個進度指示器，以模擬正在進行的下載。
  • 當模擬下載完成後，按鈕會過渡到 OPEN，以指示應用程式已準備好讓使用者開啟已下載的資源。
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('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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