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目錄

建立下載按鈕

目錄

應用程式中充滿了執行長時間行為的按鈕。例如，按鈕可能會觸發下載，啟動下載程序，隨著時間推移接收資料，然後提供對已下載資產的存取權。向使用者顯示長時間執行的程序的進度很有幫助，而按鈕本身就是提供這種回饋的好地方。在此食譜中，您將建構一個下載按鈕，該按鈕會根據應用程式下載的狀態轉換為多個視覺狀態。

以下動畫顯示應用程式的行為

The download button cycles through its stages

定義一個新的無狀態 Widget

#

您的按鈕 Widget 需要隨著時間變更其外觀。因此，您需要使用自訂的無狀態 Widget 來實作您的按鈕。

定義一個名為 DownloadButton 的新無狀態 Widget。

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

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

定義按鈕可能的視覺狀態

#

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，以表示按鈕從一個狀態動畫轉換到另一個狀態所需的時間長度。

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();
  }
}

顯示按鈕的形狀

#

下載按鈕會根據下載狀態變更其形狀。按鈕在 notDownloaded 和 downloaded 狀態期間顯示灰色、圓角的矩形。按鈕在 fetchingDownload 和 downloading 狀態期間顯示透明圓形。

根據目前的 DownloadStatus，建構一個具有 ShapeDecoration 的 AnimatedContainer，該容器會顯示圓角矩形或圓形。

請考慮在一個獨立的 Stateless Widget 中定義形狀的 Widget 樹狀結構，以便主要 build() 方法保持簡單，以便後續新增內容。與其建立一個函式來傳回 Widget（例如 Widget _buildSomething() {}），不如始終優先建立效能更高的 StatelessWidget 或 StatefulWidget。關於這一點的更多考量可以在文件或 Flutter YouTube 頻道上的專屬影片中找到。

目前，AnimatedContainer 的子項只是一個 SizedBox，因為我們會在另一個步驟中再回到它。

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) {
    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(),
    );
  }
}

您可能想知道為什麼需要一個透明圓形的 ShapeDecoration Widget，因為它是不可見的。不可見圓形的目的是協調所需的動畫。AnimatedContainer 從圓角矩形開始。當 DownloadStatus 變更為 fetchingDownload 時，AnimatedContainer 需要從圓角矩形動畫轉換為圓形，然後在動畫發生時淡出。實作此動畫的唯一方法是同時定義圓角矩形的起始形狀和圓形的結束形狀。但是，您不希望最終的圓形可見，因此您將其設為透明，這會導致動畫淡出。

顯示按鈕文字

#

DownloadButton 在 notDownloaded 階段顯示 GET，在 downloaded 階段顯示 OPEN，並且在兩者之間不顯示文字。

新增 Widget 以在每個下載階段顯示文字，並在兩者之間為文字的不透明度新增動畫。將文字 Widget 樹狀結構新增為按鈕包裝 Widget 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) {
    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,
                ),
          ),
        ),
      ),
    );
  }
}

在獲取下載時顯示微調器

#

在 fetchingDownload 階段，DownloadButton 會顯示徑向微調器。此微調器從 notDownloaded 階段淡入，並淡出到 fetchingDownload 階段。

實作一個位於按鈕形狀頂部的徑向微調器，並在適當的時間淡入和淡出。

我們已移除 ButtonShapeWidget 的建構函式，以將焦點保持在其建構方法和我們已新增的 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,
            ),
          ),
        ),
      ],
    ),
  );
}

在下載時顯示進度和停止按鈕

#

fetchingDownload 階段之後是 downloading 階段。在 downloading 階段，DownloadButton 會將徑向進度微調器替換為增長的徑向進度條。DownloadButton 也會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。

在 DownloadButton Widget 中新增進度屬性，然後更新進度顯示，以便在 downloading 階段切換到徑向進度條。

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

新增按鈕點擊回呼

#

您的 DownloadButton 需要的最後一個細節是按鈕行為。當使用者點擊按鈕時，按鈕必須執行操作。

新增 Widget 屬性，以便回呼來啟動下載、取消下載和開啟下載。

最後，使用 GestureDetector Widget 包裝 DownloadButton 的現有 Widget 樹狀結構，並將點擊事件轉發到對應的回呼屬性。

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！您有一個按鈕，其顯示會根據按鈕所處的階段而變更：未下載、正在擷取下載、正在下載和已下載。現在，使用者可以點擊以啟動下載、點擊以取消正在進行的下載，以及點擊以開啟已完成的下載。

互動式範例

#

執行應用程式

  • 按一下 GET 按鈕以開始模擬下載。
  • 按鈕會變更為進度指示器以模擬正在進行的下載。
  • 當模擬下載完成時，按鈕會轉換為 OPEN，以指示應用程式已準備好讓使用者開啟已下載的資產。
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,
          );
        },
      ),
    );
  }
}

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