Flutter Bloc异步函数调用emit触发断言错误问题排查

你在Bloc的定时任务中调用emit时触发了_AssertionError，错误定位在periodicFetch方法的emit(WalletListState(wallets: wallets!))语句。

相关代码 #

Future<void> fetchInitialData(FetchInitialDataEvent event, Emitter<WalletState> emit) async {
    if (wallets != null) {
      emit(WalletListState(wallets: wallets!));
    } else {
      walletsModel = await getWallets();
      assetsModel = await getSupportedAssets();
      cmc = await getCmc();
      currencies = await getCurrencies();

      fetchAddress();

      wallets = walletEngine.buildWallet(assetsModel, walletsModel, cmc);
      if (wallets != null && configEngine.filteredMarkets == null && configEngine.orgMarkets != null) {
        configEngine.filteredMarkets = walletEngine.filterMarket(wallets!, configEngine.orgMarkets!);
        openSocket.init(configEngine.filteredMarkets!);
      }
      emit(WalletListState(wallets: wallets!));
    }

    if (currencies != null) {
      listenTickerStream(currencies!, coins);
    }
    startWalletScheduler(emit);
  }

startWalletScheduler(Emitter<WalletState> emit) {
    if (walletScheduler?.isActive != true) {
      Duration interval = Duration(seconds: walletIntervalSeconds);
      walletScheduler = Timer.periodic(interval, (timer) async {
        await periodicFetch(emit);
      });
    }
  }

  Future<void> periodicFetch(Emitter<WalletState> emit) async {
    walletsModel = await getWallets();
    wallets = walletEngine.buildWallet(assetsModel, walletsModel, cmc);
    emit(WalletListState(wallets: wallets!));
  }

Future<WalletsResponse?> getWallets() async {
    final result = await repo.wallets(cancelToken: cancelToken);

    return result.fold((error) {
      return Future.value(null);
    }, (resp) {
      return resp;
    });
  }

错误栈 #

_AssertionError._doThrowNew (dart:core-patch/errors_patch.dart:51:61)
I/flutter ( 8440): #1 _AssertionError._throwNew (dart:core-patch/errors_patch.dart:40:5)
I/flutter ( 8440): #2 _Emitter.call (package:bloc/src/emitter.dart:114:7)
I/flutter ( 8440): #3 WalletBloc.periodicFetch (package:drip_mobile/src/core/wallet/biz/wallet_bloc.dart:122:9)
I/flutter ( 8440): #4 WalletBloc.startWalletScheduler.<anonymous closure> (package:drip_mobile/src/core/wallet/biz/wallet_bloc.dart:114:9)

问题原因 #

Bloc的Emitter实例是绑定到当前事件处理周期的，当fetchInitialData事件处理完成后，这个Emitter会被标记为无效。你把该Emitter传递给定时任务并在回调中使用，此时原事件周期已结束，调用emit就会触发断言错误。

解决方法 #

不要跨周期复用Emitter，而是通过Bloc的add方法发送新事件，让Bloc的标准事件处理流程来更新状态：

修改步骤 #

1. 定义新事件用于触发钱包刷新：

class RefreshWalletsEvent extends WalletEvent {}

2. 在Bloc中添加该事件的处理逻辑（以Bloc v8+的on语法为例）：

on<RefreshWalletsEvent>((event, emit) async {
  walletsModel = await getWallets();
  wallets = walletEngine.buildWallet(assetsModel, walletsModel, cmc);
  emit(WalletListState(wallets: wallets!));
});

3. 修改定时任务逻辑，改为发送事件而非直接调用emit：

void startWalletScheduler() {
  if (walletScheduler?.isActive != true) {
    Duration interval = Duration(seconds: walletIntervalSeconds);
    walletScheduler = Timer.periodic(interval, (timer) {
      add(RefreshWalletsEvent());
    });
  }
}

4. 调整fetchInitialData中的调用，不再传递emit：

// 替换原startWalletScheduler(emit);
startWalletScheduler();

异步函数中高效使用emit的建议 #

• 禁止跨事件周期复用Emitter：Emitter仅在当前事件处理的生命周期内有效，事件处理完成后必须丢弃。
• 通过事件触发状态更新：定时任务、异步回调等脱离当前事件周期的操作，一律通过add方法发送新事件，让Bloc标准流程处理状态变更。
• 状态更新逻辑封装到事件处理中：所有emit操作应该放在事件处理回调内，保证状态流的可追踪性和一致性。
• 旧版Bloc使用async*流：如果使用Bloc v7及以下版本，在mapEventToState中用async*配合yield发送状态，天然适配异步场景。

内容的提问来源于stack exchange，提问作者Mohammed Safiq