Flutter实现资源下载断点续传的示例代码-移动开发

Flutter实现资源下载断点续传的示例代码目录协议梳理实现步骤写在最后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

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

1.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});
}

2.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;
  }
}

3.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);
    }
  }
}