grpc C++异步调用的例子

代码主要是基于grpc官方examples/cpp/helloworld项目改的, 对helloworld的做了改进，可以方便的增加多个rpc调用接口

关于增加rpc接口的方法做如下介绍:

    1. 修改proto文件，增加新接口 如增加新接口 在Greeter里面增加 rpc UploadPlateIn(PlateInRequest) returns (PlateInReply) {}

并增加PlateInRequest 和 PlateInReply数据

syntax = "proto3";

option java_multiple_files = true;
option java_package = "io.grpc.examples.helloworld";
option java_outer_classname = "HelloWorldProto";
option objc_class_prefix = "HLW";

package helloworld;

// The greeting service definition.
service Greeter {
  // Sends a greeting
  rpc SayHello (HelloRequest) returns (HelloReply) {}
  rpc UploadPlateIn(PlateInRequest) returns (PlateInReply) {}
}

enum PlateType {
  TEMPORARY = 0;
  MONTHLY_PAYMENT = 1;
  OTHER_TYPE = 2;
}

// The request message containing the user's name.
message HelloRequest {
  string name = 1;
}

// The response message containing the greetings
message HelloReply {
  string message = 1;
}

message PlateInRequest {
  string plate = 1;
}
message PlateInReply {
  string     plate = 1;
  PlateType  plate_type = 2;
}

2. 这里我吧官方例子中greeter_async_server.cc中的CallData改成了模版函数，并且该模版函数继承自CallDataBase接口类(为了暴露接口Proceed() 方便处理不同rpc接口的处理函数的调用)

greeter_async_server.h的代码如下：

#include <memory>
#include <iostream>
#include <string>
#include <thread>

#include <grpcpp/grpcpp.h>
#include <grpc/support/log.h>

#ifdef BAZEL_BUILD
#include "examples/protos/helloworld.grpc.pb.h"
#else
#include "helloworld.grpc.pb.h"
#endif

using grpc::Server;
using grpc::ServerAsyncResponseWriter;
using grpc::ServerBuilder;
using grpc::ServerContext;
using grpc::ServerCompletionQueue;
using grpc::Status;
using helloworld::HelloRequest;
using helloworld::HelloReply;
using helloworld::Greeter;

using helloworld::PlateInRequest;
using helloworld::PlateInReply;

class ServerImpl final {
  public:
    ~ServerImpl() {
      server_->Shutdown();
      // Always shutdown the completion queue after the server.
      cq_->Shutdown();
    }

    // There is no shutdown handling in this code.
    void Run();

 private:
    // This can be run in multiple threads if needed.
    void HandleRpcs();

    std::unique_ptr<ServerCompletionQueue> cq_;
    Greeter::AsyncService service_;
    std::unique_ptr<Server> server_;
};

class CallDataBase
{
  public:
    virtual void Proceed() = 0;
    virtual ~CallDataBase() {}
};
// Class encompasing the state and logic needed to serve a request.
template<class req, class rep>
class CallData : public CallDataBase
{
  public:
    // Take in the "service" instance (in this case representing an asynchronous
    // server) and the completion queue "cq" used for asynchronous communication
    // with the gRPC runtime.
    CallData(Greeter::AsyncService* service, ServerCompletionQueue* cq)
        : service_(service), cq_(cq), responder_(