监听 java 原生代码到 dart flutter 的回调方法

问：

我会与surepay集成，以在我的应用程序中添加付款 SurePay 使用 java 原生代码，就我而言，我想在我的 flutter 应用程序中使用此原生代码 一切都很好，钱取款成功，支付过程正确完成，但是有一个回调函数，我必须从中获取响应以确保支付过程成功或失败，并相应地告诉用户该怎么做。 就我而言，我无法监听本机代码中发生的变化 这是我的 java 原生代码

注意：我想在调用（sendAmount - getLastTransactionFromTerminal）方法后监听onUsbGetTransaction回调方法

public class MainActivity extends FlutterActivity implements Usb.OnReceivingData, Usb.OnConnectStatusChanges{


    private static final String CHANNEL = "tkafol/channel";
    private static final String MY_EVENT_CHANNEL_NAME = "tkafol/event";
    private final Usb ConnParam = new Usb();
    Usb.TransactionInformation model =new Usb.TransactionInformation();
    private EventChannel.EventSink usbTransactionEventSink;

    String transactionInfo ;


    @Override
    public void configureFlutterEngine(@NonNull FlutterEngine flutterEngine) {
        super.configureFlutterEngine(flutterEngine);
        // Setup a method channel to receive messages from Flutter
        ConnParam.setTimeout(1000 * 60 * 7);
        new EventChannel(flutterEngine.getDartExecutor().getBinaryMessenger(), MY_EVENT_CHANNEL_NAME)
                .setStreamHandler(new EventChannel.StreamHandler() {

                    @Override
                    public void onListen(Object arguments, EventChannel.EventSink eventSink) {
                        Log.d("Stream listen", "top listen");

                        Toast.makeText(getBaseContext(),"stream in native code listen",Toast.LENGTH_SHORT).show();

                        usbTransactionEventSink = eventSink;
                    }
                    
                    @Override
                    public void onCancel(Object arguments) {
                        Log.d("Stream cancel", "top cancel");
                        Toast.makeText(getBaseContext(),"stream in native code cancel",Toast.LENGTH_SHORT).show();
                        usbTransactionEventSink = null;
                    }
                });
        new MethodChannel(flutterEngine.getDartExecutor().getBinaryMessenger(), CHANNEL).setMethodCallHandler(
                (call, result) -> {
                    switch (call.method) {
                        case "sendAmount" :
                            String amount = call.argument("amount");
                            ConnParam.sendAmount(MainActivity.this,  amount);
                            //ConnParam.sendAmount(MainActivity.this, "10.25");
                            result.success(null);
                            break;
                        case "sendReconciliation" :
                            ConnParam.sendReconciliation(MainActivity.this);
                            result.success(null);
                            break;
                        case "getLastTransactionFromTerminal" :

                            ConnParam.getLastTransactionFromTerminal(MainActivity.this);
                            result.success( ConnParam.getLastTransactionFromTerminal(MainActivity.this));
                            break;
                            result.notImplemented();
                            break;
                    }
            });
    }

    // A method that calls the native code
//    private boolean callNativeMethod(String message) {
//        return true;
//    }
    @Override
    public void onUsbGetTransaction(Usb.TransactionInformation model) {
        Log.d("STREAM","now is null");
        Toast.makeText(getBaseContext(),"stream null now",Toast.LENGTH_SHORT).show();
        if (usbTransactionEventSink != null) {
            // Send the event data to Flutter
            Toast.makeText(getBaseContext(),"stream in native code",Toast.LENGTH_SHORT).show();
            String transactionInfo = model.toString();
            Map<String, String> arguments = new HashMap<>();
            arguments.put("transactionInfo", transactionInfo);

            new MethodChannel(Objects.requireNonNull(getFlutterEngine()).getDartExecutor().getBinaryMessenger(), CHANNEL)
                    .invokeMethod("onGetTransaction", arguments);
            usbTransactionEventSink.success(model.toString());
        }
        String transactionInfo = model.toString();
        Map<String, String> arguments = new HashMap<>();
        arguments.put("transactionInfo", transactionInfo);

        new MethodChannel(Objects.requireNonNull(getFlutterEngine()).getDartExecutor().getBinaryMessenger(), CHANNEL)
                .invokeMethod("onGetTransaction", arguments);
       // usbTransactionEventSink.success(model.toString());
    }


   
}

还有我用于监听回调方法的 Flutter 代码

final EventChannel _eventChannel =
      const EventChannel('tkafol/event');
StreamSubscription? _usbTransactionSubscription;
  listenToEvent() {
    showError("we stream before");
    _usbTransactionSubscription =
        _eventChannel.receiveBroadcastStream().listen((transactionInfo) {
      showError("we stream after");
      setState(() {
        responseDataUsb = "callback listener : " + transactionInfo.toString();
      });
    });
  }

我正在使用这个函数来监听本机代码中的事件

final EventChannel _eventChannel =
      const EventChannel('tkafol/event');
StreamSubscription? _usbTransactionSubscription;
  listenToEvent() {
    showError("we stream before");
    _usbTransactionSubscription =
        _eventChannel.receiveBroadcastStream().listen((transactionInfo) {
      showError("we stream after");
      setState(() {
        responseDataUsb = "callback listener : " + transactionInfo.toString();
      });
    });
  }

并在 initState 方法中调用它，但总体上没有得到任何结果