在事件处理程序正常完成块错误后，如何解决发出被调用的问题？答案

【问题标题】：How to resolve emit was called after an event handler completed normally bloc error?在事件处理程序正常完成块错误后，如何解决发出被调用的问题？
【发布时间】：2021-12-02 08:57:48
【问题描述】：

我正在使用颤振块显示下载进度百分比，但我一直遇到这个问题。我认为问题出现在 onDone 方法中，但我不知道如何解决它。

错误：

发生了异常。 _AssertionError ('package:bloc/src/bloc.dart': 断言失败: line 137 pos 7: '!_isCompleted': 在事件处理程序正常完成后调用发出。这通常是由于事件处理程序中没有等待的未来。请确保使用事件处理程序等待所有异步操作并在调用 emit() 之前的异步操作之后使用 emit.isDone 确保事件处理程序尚未完成。

不好

 on<Event>((event, emit) {
    future.whenComplete(() => emit(...));
  });

好

  on<Event>((event, emit) async {
    await future.whenComplete(() => emit(...));
  });
)

代码：


import 'package:bloc/bloc.dart';
import 'package:download_progress_with_bloc/downlaod_file.dart';
import 'package:download_progress_with_bloc/download_event.dart';
import 'package:download_progress_with_bloc/download_state.dart';
import 'package:download_progress_with_bloc/permission_handler.dart';
import 'package:download_progress_with_bloc/store_book_repo.dart';
import 'package:http/http.dart' as http;

class DownloadBloc extends Bloc<DownloadEvent, DownloadState> {
  DownloadBloc({required this.storeBookRepo}) : super(DownloadInitial()) {
    on<DownloadStarted>(onStarted);
    on<DownloadProgressed>(onProgressed);
  }
  final StoreBookRepo storeBookRepo;

  http.StreamedResponse? response;
  // StreamController? _controller;
  int received = 0;
  List<int> bytes = [];
  int totalSize = 0;

  @override
  Future<void> close() {
    return super.close();
  }

  Future<void> onStarted(
      DownloadStarted event, Emitter<DownloadState> emit) async {
    try {
      await PermissionHandler.requestStoragePermission();
      response = await downloadFile();
      totalSize = response!.contentLength ?? 0;
      emit(DownloadInProgress(progress: received, totalSize: totalSize));
      response?.stream.asBroadcastStream().listen((value) async {
        received += value.length;
        bytes.addAll(value);
        add(DownloadProgressed(progress: received));
        print('received value is $received');
      }).onDone(
        () async {
          await storeBookRepo
              .storePdf(
                bytes.toString(),
                bookTitle: 'bookTitle',
              )
              .then((value) => emit(DownloadCompleted()));
          // emit(DownloadCompleted());
        },
      );
    } catch (e) {
      emit(DownlaodFailed(errorMessage: '$e'));
    }
  }

  void onProgressed(DownloadProgressed event, Emitter<DownloadState> emit) {
    emit(DownloadInProgress(progress: event.progress, totalSize: totalSize));
  }


}

【问题讨论】：

标签： flutter dart state bloc

【解决方案1】：

如果像这样将listen 重写为await for 会怎样？

Future<void> onStarted(
  DownloadStarted event,
  Emitter<DownloadState> emit,
) async {
  try {
    await PermissionHandler.requestStoragePermission();
    response = await downloadFile();
    totalSize = response!.contentLength ?? 0;

    emit(DownloadInProgress(
      progress: received,
      totalSize: totalSize,
    ));

    await for (final value in response?.stream) {
      received += value.length;
      bytes.addAll(value);

      add(DownloadProgressed(progress: received));
      print('received value is $received');
    }

    await storeBookRepo.storePdf(
      bytes.toString(),
      bookTitle: 'bookTitle',
    );

    emit(DownloadCompleted());
  } catch (e) {
    emit(DownlaodFailed(errorMessage: '$e'));
  }
}

【讨论】：