Flutter视频加密致应用卡顿，使用isolate遇参数不可发送异常求助

问题情况 #

开发Flutter视频应用，支持用户观看、下载视频。为防止已下载视频被分享，需要对下载后的视频加密存储，但加密过程会让应用UI冻结约10秒。尝试过多种加密方法都没解决卡顿问题，听说用compute方法能搞定，但一开始没实现成功；后来试Isolate功能时，又碰到「Illegal argument in isolate message: object is unsendable」异常，最终通过把加密逻辑拆成独立静态函数解决了所有问题。

原问题代码 #

Future<void> downloadVideo(String url) async {
  var appDocDir = await getExternalStorageDirectory();
  String appDocPath = appDocDir!.path;
  String filePath = '$appDocPath/Videos/${url.split('/').last}';

  try {
    var request = await HttpClient().getUrl(Uri.parse(url));
    var response = await request.close();
    List<int> bytes = [];
    int downloaded = 0;
    int total = response.contentLength!;
    response.listen(
      (List<int> newBytes) {
        bytes.addAll(newBytes);
        downloaded += newBytes.length;
        downloadProgress.value = downloaded / total;
        print("Download progress: ${downloadProgress.value}");
      },
      onDone: () async {
        // 生成加密密钥和IV
        final key = encrypt.Key.fromSecureRandom(32);
        final iv = encrypt.IV.fromSecureRandom(16);

        // 加密视频字节数据
        final encrypter =
            encrypt.Encrypter(encrypt.AES(key, mode: encrypt.AESMode.cbc));
        final encryptedBytes = encrypter.encryptBytes(bytes, iv: iv).bytes;

        // 安全存储密钥和IV
        final storage = FlutterSecureStorage();
        await storage.write(key: 'key_$filePath', value: key.base64);
        await storage.write(key: 'iv_$filePath', value: iv.base64);

        // 保存加密后的视频
        File file = File(filePath);
        await file.writeAsBytes(encryptedBytes);
        print("Download completed: $filePath");
      },
      onError: (e) {
        print("Error downloading video: $e");
      },
      cancelOnError: true,
    );
  } catch (e) {
    print("Error downloading video: $e");
  }
}

问题原因 #

1. UI冻结：原代码里的加密逻辑在onDone回调中执行，属于主线程任务。视频文件字节量较大时，加密操作会占用主线程资源，导致UI无法响应。
2. Isolate传参异常：直接在Isolate中使用Key、IV这类对象时，因为它们不属于可跨Isolate传递的序列化类型，所以触发了参数非法的异常。

解决代码 #

第一步：抽离静态加密函数 #

把加密逻辑单独写成静态函数，确保参数和返回值都是可序列化的基础类型：

// 独立静态加密函数，仅处理纯数据转换，避免传递不可序列化对象
static Map<String, dynamic> encryptVideoBytes(List<int> rawBytes) {
  final key = encrypt.Key.fromSecureRandom(32);
  final iv = encrypt.IV.fromSecureRandom(16);
  final encrypter = encrypt.Encrypter(encrypt.AES(key, mode: encrypt.AESMode.cbc));
  final encryptedBytes = encrypter.encryptBytes(rawBytes, iv: iv).bytes;

  return {
    'encryptedBytes': encryptedBytes,
    'keyBase64': key.base64,
    'ivBase64': iv.base64,
  };
}

第二步：修改下载函数使用compute调用 #

在onDone回调中用compute执行加密逻辑，把加密任务移到后台Isolate：

Future<void> downloadVideo(String url) async {
  var appDocDir = await getExternalStorageDirectory();
  String appDocPath = appDocDir!.path;
  String filePath = '$appDocPath/Videos/${url.split('/').last}';

  try {
    var request = await HttpClient().getUrl(Uri.parse(url));
    var response = await request.close();
    List<int> bytes = [];
    int downloaded = 0;
    int total = response.contentLength!;
    response.listen(
      (List<int> newBytes) {
        bytes.addAll(newBytes);
        downloaded += newBytes.length;
        downloadProgress.value = downloaded / total;
        print("Download progress: ${downloadProgress.value}");
      },
      onDone: () async {
        // 用compute在后台Isolate执行加密，避免阻塞主线程
        final encryptionResult = await compute(encryptVideoBytes, bytes);

        final encryptedBytes = encryptionResult['encryptedBytes'] as List<int>;
        final keyBase64 = encryptionResult['keyBase64'] as String;
        final ivBase64 = encryptionResult['ivBase64'] as String;

        // 安全存储密钥和IV
        final storage = FlutterSecureStorage();
        await storage.write(key: 'key_$filePath', value: keyBase64);
        await storage.write(key: 'iv_$filePath', value: ivBase64);

        // 保存加密后的视频
        File file = File(filePath);
        await file.writeAsBytes(encryptedBytes);
        print("Download completed: $filePath");
      },
      onError: (e) {
        print("Error downloading video: $e");
      },
      cancelOnError: true,
    );
  } catch (e) {
    print("Error downloading video: $e");
  }
}

核心说明 #

• compute是Flutter封装好的Isolate工具，自动创建、管理后台Isolate，不用手动处理Isolate的通信和销毁。
• 静态函数里只处理纯数据转换，把Key、IV转成base64字符串返回，避免跨Isolate传递不可序列化对象。
• 加密任务移到后台后，主线程只处理UI和轻量操作，彻底解决了UI冻结问题。

内容的提问来源于stack exchange，提问作者Alex Mact