flutter eventChannel crash on ios

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flutter issue

crash

记录一次困扰了很久的 flutter event channel crash

EventChannel dart

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Stream<dynamic> receiveBroadcastStream([ dynamic arguments ]) {
    final MethodChannel methodChannel = MethodChannel(name, codec);
    StreamController<dynamic> controller;
    controller = StreamController<dynamic>.broadcast(onListen: () async {
      defaultBinaryMessenger.setMessageHandler(name, (ByteData reply) async {
        if (reply == null) {
          controller.close();
        } else {
          try {
            controller.add(codec.decodeEnvelope(reply));
          } on PlatformException catch (e) {
            controller.addError(e);
          }
        }
        return null;
      });
      try {
        await methodChannel.invokeMethod<void>('listen', arguments);
      } catch (exception, stack) {
        FlutterError.reportError(FlutterErrorDetails(
          exception: exception,
          stack: stack,
          library: 'services library',
          context: ErrorDescription('while activating platform stream on channel $name'),
        ));
      }
    }, onCancel: () async {
      defaultBinaryMessenger.setMessageHandler(name, null);
      try {
        await methodChannel.invokeMethod<void>('cancel', arguments);
      } catch (exception, stack) {
        FlutterError.reportError(FlutterErrorDetails(
          exception: exception,
          stack: stack,
          library: 'services library',
          context: ErrorDescription('while de-activating platform stream on channel $name'),
        ));
      }
    });
    return controller.stream;
  }

FlutterChannel.mm

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static void SetStreamHandlerMessageHandlerOnChannel(NSObject<FlutterStreamHandler>* handler,
                                                    NSString* name,
                                                    NSObject<FlutterBinaryMessenger>* messenger,
                                                    NSObject<FlutterMethodCodec>* codec) {
  __block FlutterEventSink currentSink = nil;
  FlutterBinaryMessageHandler messageHandler = ^(NSData* message, FlutterBinaryReply callback) {
    FlutterMethodCall* call = [codec decodeMethodCall:message];
    if ([call.method isEqual:@"listen"]) {
      if (currentSink) {
        FlutterError* error = [handler onCancelWithArguments:nil];
        if (error)
          NSLog(@"Failed to cancel existing stream: %@. %@ (%@)", error.code, error.message,
                error.details);
      }
      currentSink = ^(id event) {
        if (event == FlutterEndOfEventStream)
          [messenger sendOnChannel:name message:nil];
        else if ([event isKindOfClass:[FlutterError class]])
          [messenger sendOnChannel:name message:[codec encodeErrorEnvelope:(FlutterError*)event]];
        else
          [messenger sendOnChannel:name message:[codec encodeSuccessEnvelope:event]];
      };
      FlutterError* error = [handler onListenWithArguments:call.arguments eventSink:currentSink];
      if (error)
        callback([codec encodeErrorEnvelope:error]);
      else
        callback([codec encodeSuccessEnvelope:nil]);
    } else if ([call.method isEqual:@"cancel"]) {
      if (!currentSink) {
        callback(
            [codec encodeErrorEnvelope:[FlutterError errorWithCode:@"error"
                                                           message:@"No active stream to cancel"
                                                           details:nil]]);
        return;
      }
      currentSink = nil;
      FlutterError* error = [handler onCancelWithArguments:call.arguments];
      if (error)
        callback([codec encodeErrorEnvelope:error]);
      else
        callback([codec encodeSuccessEnvelope:nil]);
    } else {
      callback(nil);
    }
  };
  [messenger setMessageHandlerOnChannel:name binaryMessageHandler:messageHandler];
}

EventSink

正常结束stream流 eventSink(FlutterEndOfEventStream) ，异常结束stream流 eventSink(FlutterError) 都会回调执行 onCancel

参考

Flutter 与 Native(iOS) 通信原理
深入Flutter技术内幕:Platform Channel设计与实现

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