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