Flutter资源下载如何实现断点续传？

作者：Karl_wei
链接：https://juejin.cn/post/7123965772132515877
来源：稀土掘金

在项目开发中，特别是C端的产品，资源下载实现断点续传是非常有必要的。今天我们不讲过多原理的知识，分享下简单实用的资源断点续传。

协议梳理

一般情况下，下载的功能模块，至少需要提供如下基础功能：资源下载、取消当前下载、资源是否下载成功、资源文件的大小、清除缓存文件。
而断点续传主要体现在取消当前下载后，再次下载时能在之前已下载的基础上继续下载。这个能极大程度的减少我们服务器的带宽损耗，而且还能为用户减少流量，避免重复下载，提高用户体验。
前置条件：资源必须支持断点续传。如何确定可否支持？看看你的服务器是否支持Range请求即可。

实现步骤

定好协议。我们用的http库是dio；通过校验md5检测文件缓存完整性；关于代码中的subDir，设计上认为资源会有多种：音频、视频、安装包等，每种资源分开目录进行存储。

import 'package:dio/dio.dart';

typedef ProgressCallBack = void Function(int count, int total);

typedef CancelTokenProvider = void Function(CancelToken cancelToken);

abstract class AssetRepositoryProtocol {
  /// 下载单一资源
  Future<String> downloadAsset(String url,
      {String? subDir,
      ProgressCallBack? onReceiveProgress,
      CancelTokenProvider? cancelTokenProvider,
      Function(String)? done,
      Function(Exception)? failed});

  /// 取消下载，Dio中通过CancelToken可控制
  void cancelDownload(CancelToken cancelToken);

  /// 获取文件的缓存地址
  Future<String?> filePathForAsset(String url, {String? subDir});

  /// 检查文件是否缓存成功，简单对比md5
  Future<String?> checkCachedSuccess(String url, {String? md5Str});

  /// 查看缓存文件的大小
  Future<int> cachedFileSize({String? subDir});

  /// 清除缓存
  Future<void> clearCache({String? subDir});
}

实现抽象协议，其中HttpManagerProtocol内部封装了dio的相关请求。

class AssetRepository implements AssetRepositoryProtocol {
  AssetRepository(this.httpManager);

  final HttpManagerProtocol httpManager;

  @override
  Future<String> downloadAsset(String url,
      {String? subDir,
      ProgressCallBack? onReceiveProgress,
      CancelTokenProvider? cancelTokenProvider,
      Function(String)? done,
      Function(Exception)? failed}) async {
    CancelToken cancelToken = CancelToken();
    if (cancelTokenProvider != null) {
      cancelTokenProvider(cancelToken);
    }

    final savePath = await _getSavePath(url, subDir: subDir);
    try {
      httpManager.downloadFile(
          url: url,
          savePath: savePath + '.temp',
          onReceiveProgress: onReceiveProgress,
          cancelToken: cancelToken,
          done: () {
            done?.call(savePath);
          },
          failed: (e) {
            print(e);
            failed?.call(e);
          });
      return savePath;
    } catch (e) {
      print(e);
      rethrow;
    }
  }

  @override
  void cancelDownload(CancelToken cancelToken) {
    try {
      if (!cancelToken.isCancelled) {
        cancelToken.cancel();
      }
    } catch (e) {
      print(e);
    }
  }

  @override
  Future<String?> filePathForAsset(String url, {String? subDir}) async {
    final path = await _getSavePath(url, subDir: subDir);
    final file = File(path);
    if (!(await file.exists())) {
      return null;
    }
    return path;
  }

  @override
  Future<String?> checkCachedSuccess(String url, {String? md5Str}) async {
    String? path = await _getSavePath(url, subDir: FileType.video.dirName);
    bool isCached = await File(path).exists();
    if (isCached && (md5Str != null && md5Str.isNotEmpty)) {
      // 存在但是md5验证不通过
      File(path).readAsBytes().then((Uint8List str) {
        if (md5.convert(str).toString() != md5Str) {
          path = null;
        }
      });
    } else if (isCached) {
      return path;
    } else {
      path = null;
    }
    return path;
  }

  @override
  Future<int> cachedFileSize({String? subDir}) async {
    final dir = await _getDir(subDir: subDir);
    if (!(await dir.exists())) {
      return 0;
    }

    int totalSize = 0;
    await for (var entity in dir.list(recursive: true)) {
      if (entity is File) {
        try {
          totalSize += await entity.length();
        } catch (e) {
          print('Get size of $entity failed with exception: $e');
        }
      }
    }

    return totalSize;
  }

  @override
  Future<void> clearCache({String? subDir}) async {
    final dir = await _getDir(subDir: subDir);
    if (!(await dir.exists())) {
      return;
    }
    dir.deleteSync(recursive: true);
  }

  Future<String> _getSavePath(String url, {String? subDir}) async {
    final saveDir = await _getDir(subDir: subDir);

    if (!saveDir.existsSync()) {
      saveDir.createSync(recursive: true);
    }

    final uri = Uri.parse(url);
    final fileName = uri.pathSegments.last;
    return saveDir.path + fileName;
  }

  Future<Directory> _getDir({String? subDir}) async {
    final cacheDir = await getTemporaryDirectory();
    late final Directory saveDir;
    if (subDir == null) {
      saveDir = cacheDir;
    } else {
      saveDir = Directory(cacheDir.path + '/$subDir/');
    }
    return saveDir;
  }
}

封装dio下载，实现资源断点续传。
这里的逻辑比较重点，首先未缓存100%的文件，我们以.temp后缀进行命名，在每次下载时检测下是否有.temp的文件，拿到其文件字节大小；传入在header中的range字段，服务器就会去解析需要从哪个位置继续下载；下载全部完成后，再把文件名改回正确的后缀即可。

final downloadDio = Dio();

Future<void> downloadFile({
  required String url,
  required String savePath,
  required CancelToken cancelToken,
  ProgressCallback? onReceiveProgress,
  void Function()? done,
  void Function(Exception)? failed,
}) async {
  int downloadStart = 0;
  File f = File(savePath);
  if (await f.exists()) {
    // 文件存在时拿到已下载的字节数
    downloadStart = f.lengthSync();
  }
  print("start: $downloadStart");
  try {
    var response = await downloadDio.get<ResponseBody>(
      url,
      options: Options(
        /// Receive response data as a stream
        responseType: ResponseType.stream,
        followRedirects: false,
        headers: {
          /// 加入range请求头，实现断点续传
          "range": "bytes=$downloadStart-",
        },
      ),
    );
    File file = File(savePath);
    RandomAccessFile raf = file.openSync(mode: FileMode.append);
    int received = downloadStart;
    int total = await _getContentLength(response);
    Stream<Uint8List> stream = response.data!.stream;
    StreamSubscription<Uint8List>? subscription;
    subscription = stream.listen(
      (data) {
        /// Write files must be synchronized
        raf.writeFromSync(data);
        received += data.length;
        onReceiveProgress?.call(received, total);
      },
      onDone: () async {
        file.rename(savePath.replaceAll('.temp', ''));
        await raf.close();
        done?.call();
      },
      onError: (e) async {
        await raf.close();
        failed?.call(e);
      },
      cancelOnError: true,
    );
    cancelToken.whenCancel.then((_) async {
      await subscription?.cancel();
      await raf.close();
    });
  } on DioError catch (error) {
    if (CancelToken.isCancel(error)) {
      print("Download cancelled");
    } else {
      failed?.call(error);
    }
  }
}