Magic 8 Ball

Creating a HTTP2 connection between a Qt gRPC™ client and a C++ gRPC server.

Magic 8 ball sends a question to a server and displays the received answer:

"Magic 8 ball example screenshot"

The example code includes the following components:

magic8ball Qt gRPC client application that uses the qt_add_protobuf() and qt_add_grpc() CMake functions for message and service Qt code generation.
server application that calls C++ gRPC plugin for generating server code and implementing simple server logic.

Note: you need the C++ gRPC plugin installed. Find details here: Module prerequisites

Both components use generated messages from the protobuf schema described in the exampleservice.proto file:

syntax = "proto3";

package qtgrpc.examples;

message AnswerRequest {
    string question = 1;
}

message AnswerResponse {
    string message = 1;
}

service ExampleService {
    rpc answerMethod(AnswerRequest) returns (AnswerResponse) {}
}

The gRPC client is defined as a QML object which is available after the code is compiled.

    ExampleServiceClient {
        id: grpcClient
        channel: grpcChannel.channel
    }

The client service connects to the localhost with port 50051, which is specified in the gRPC channel options:

    GrpcHttp2Channel {
        id: grpcChannel
        hostUri: "http://localhost:50051"
        // Optionally, you can specify custom channel options here
        // options: GrpcChannelOptions {}
    }

And sends a request to the server part:

    function requestAnswer(question: string): void {
        ...
        root.answerReq.question = question;
        grpcClient.answerMethod(root.answerReq, finishCallback, errorCallback, grpcCallOptions);
    }

answerMethod is a gRPC method that the client calls. It has four parameters: the request object, a finish callback function, an error callback function and a GrpcCallOptions object.

Click the Ask button to send the request to the magic8ball server.

Note: You have to run the server in parallel with the client application.

The server application chooses a random answer from the list of answers and sends the data to the client's port. It also checks that the request contains a non empty field question. If the field is empty, it returns a StatusCode::INVALID_ARGUMENT

    grpc::Status answerMethod(grpc::ServerContext *, const AnswerRequest *request,
                              AnswerResponse *response) override
    {
        if (request->question().empty()) {
            std::cerr << "Question is empty" << std::endl;
            return grpc::Status(grpc::StatusCode::INVALID_ARGUMENT, "Question is empty");
        }
        std::cout << "Received question: " << request->question() << std::endl;

        response->set_message(getRandomAnswer());

        return grpc::Status();
    };

After receiving a response, the client application shows the answer.

Example project @ code.qt.io

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