如何解决Flutter Bloc事件处理正常结束后调用emit的报错？

算力阿峰

2026-9-30

Flutter Bloc下载进度功能报错修复方案

错误根因

你遇到的断言错误是因为onStarted事件处理函数没有等待下载流的完整处理流程就提前执行结束，Bloc标记该事件处理上下文已关闭，后续onDone回调中调用emit时触发了校验失败。

修复后的核心代码

Future<void> onStarted(
    DownloadStarted event, Emitter<DownloadState> emit) async {
  try {
    // 每次下载前重置状态，避免多次下载数据累加
    received = 0;
    bytes.clear();
    await PermissionHandler.requestStoragePermission();
    response = await downloadFile();
    totalSize = response!.contentLength ?? 0;
    // emit前加判断做双重防护
    if (!emit.isDone) {
      emit(DownloadInProgress(progress: received, totalSize: totalSize));
    }
    // 用await for监听流，保证整个流处理过程被等待
    await for (final value in response!.stream.asBroadcastStream()) {
      received += value.length;
      bytes.addAll(value);
      add(DownloadProgressed(progress: received));
      print('received value is $received');
    }
    // 流处理完成后执行存储逻辑
    await storeBookRepo.storePdf(
      bytes.toString(),
      bookTitle: 'bookTitle',
    );
    if (!emit.isDone) {
      emit(DownloadCompleted());
    }
  } catch (e) {
    if (!emit.isDone) {
      // 注意你原代码中状态类名拼写错误为DownlaodFailed，建议修正为DownloadFailed
      emit(DownloadFailed(errorMessage: '$e'));
    }
  }
}