Vehicle

The example simulates a vehicle dashboard that displays data sent by a gRPC server.

The example code has the following components:

• vehicle_client Qt gRPC client application that uses the qt_add_protobuf() and qt_add_grpc() CMake functions for message and service Qt code generation.
• vehicle_server application that calls C++ gRPC plugin for generating server code and implementing simple server logic.

Both components use generated messages from the protobuf schemas described in the files vehicleservice.proto and navigationservice.proto.

Vehicle service:

message SpeedMsg {
    int32 speed = 1;
}

message RpmMsg {
    int32 rpm = 1;
}

message AutonomyMsg {
    int32 autonomy = 1;
}

service VehicleService {
    rpc getSpeedStream(google.protobuf.Empty) returns (stream SpeedMsg) {}
    rpc getRpmStream(google.protobuf.Empty) returns (stream RpmMsg) {}
    rpc getAutonomy(google.protobuf.Empty) returns (AutonomyMsg) {}
}

Navigation service:

enum DirectionEnum {
    RIGHT = 0;
    LEFT = 1;
    STRAIGHT = 2;
    BACKWARD = 3;
}

message NavigationMsg {
    int32 totalDistance = 1;
    int32 traveledDistance = 2;
    DirectionEnum direction = 3;
    string street = 4;
}

service NavigationService {
    rpc getNavigationStream(google.protobuf.Empty) returns (stream NavigationMsg) {}
}

The VehicleManager C++ singleton uses two QThread instances to communicate with the server in parallel. The threads have different gRPC connection types. In this example, there are two types:

• Server streaming RPCs For example, the speed stream of the vehicle thread. It uses two callback functions: QGrpcServerStream::messageReceived and QGrpcOperation::finished

  Empty speedRequest;
  m_streamSpeed = m_client->getSpeedStream(speedRequest);
  
  connect(m_streamSpeed.get(), &QGrpcServerStream::messageReceived, this, [this]() {
      if (const auto speedResponse = m_streamSpeed->read<SpeedMsg>()) {
          emit speedChanged(speedResponse->speed());
      }
  });
  
  connect(
      m_streamSpeed.get(), &QGrpcServerStream::finished, this,
      [this](const QGrpcStatus &status) {
          if (!status.isOk()) {
              auto error = QString("Stream error fetching speed %1 (%2)")
                               .arg(status.message())
                               .arg(QVariant::fromValue(status.code()).toString());
              emit connectionError(error);
              qWarning() << error;
              return;
          }
      },
      Qt::SingleShotConnection);

• Unary RPCs The RPC getAutonomy operation is a unary RPC. It returns a single response. It is only connected to the QGrpcOperation::finished signal.

  Empty autonomyRequest;
  std::unique_ptr<QGrpcCallReply> autonomyReply = m_client->getAutonomy(autonomyRequest);
  const auto *autonomyReplyPtr = autonomyReply.get();
  connect(
      autonomyReplyPtr, &QGrpcCallReply::finished, this,
      [this, autonomyReply = std::move(autonomyReply)](const QGrpcStatus &status) {
          if (!status.isOk()) {
              auto error = QString("Call error fetching autonomy %1 (%2)")
                               .arg(status.message())
                               .arg(QVariant::fromValue(status.code()).toString());
              emit connectionError(error);
              qWarning() << error;
              return;
          }
  
          if (const auto autonomyMsg = autonomyReply->read<AutonomyMsg>()) {
              emit autonomyChanged(autonomyMsg->autonomy());
          }
      },
      Qt::SingleShotConnection);

The client main window interface is defined in the Main.qml file. It uses QML Connections type in order to connect to the signals of the VehicleManager C++ singleton to custom slots:

    Connections {
        target: VehicleManager

        // This slot will be executed when the VehicleManager::totalDistanceChanged
        // signal is emitted
        function onTotalDistanceChanged(distance: int): void {
            root.totalDistance = distance;
        }

        function onSpeedChanged(speed: int): void {
            root.speed = speed;
        }

        function onRpmChanged(rpm: int): void {
            root.rpm = rpm;
        }

        function onTraveledDistanceChanged(distance: int): void {
            root.traveledDistance = distance;
        }

        function onDirectionChanged(direction: int): void {
            if (direction == VehicleManager.RIGHT) {
                root.directionImageSource = "qrc:/direction_right.svg";
            } else if (direction == VehicleManager.LEFT) {
                root.directionImageSource =  "qrc:/direction_left.svg";
            } else if (direction == VehicleManager.STRAIGHT) {
                root.directionImageSource =  "qrc:/direction_straight.svg";
            } else {
                root.directionImageSource =  "";
            }
        }

After receiving a response, the client window updates the UI with the received data. This way, messages can be received in different threads and be sent to the client UI in a thread-safe way thanks to the signals.

Example project @ code.qt.io