ros入门(3)—如何添加一个srv

添加srv文件

srv和msg有很多是想通的，特别是在package.xml和CmakeLists.txt的修改中，不同的就是目录和后缀的写法(srv，msg)。
首先在srv目录下创建一个SrvTutorial.srv，

int64 a
int64 b
---
int64 result

后续在编译时，roscpp会将该srv文件转换为头文件SrvTutorial.h SrvTutorialRequest.h SrvTutorialResponse.h，大概格式如下，定义了SrvTutorial这个class，内部有请求和返回的消息Request和Response。

namespace my_package
{
struct SrvTutorial
{
  class Request
  {
    ...
  };

  class Response
  {
    ...
  };

  Request request;
  Response response;
}; 
}

The Request class provides the input to the service. The Response class is returned to the client as the service’s output.

client

对于client，利用NodeHandle去调用service，

ros::NodeHandle nh;
ros::ServiceClient client = nh.serviceClient<my_package::SrvTutorial>("my_service_name");
my_package::SrvTutorial srv;
...
if (client.call(srv))
{
  ...
}

其中serviceClient模板函数为如下所示，

  template<class Service>
  ServiceClient serviceClient(const std::string& service_name, bool persistent = false, 
                              const M_string& header_values = M_string())
  {
    ServiceClientOptions ops;
    ops.template init<Service>(service_name, persistent, header_values);
    return serviceClient(ops);
  }

server

In roscpp you provide a service by creating a ros::ServiceServerthrough the ros::NodeHandle::advertiseService() method.
其中advertiseService模板函数为，

template<class MReq, class MRes>
ros::ServiceServer nh.advertiseService(const std::string& service, <callback>);

其中，callback可以为functions，class methods，functor objects，这里只描述函数指针，

bool callback(std_srvs::Empty::Request& request, std_srvs::Empty::Response& response)
{
  return true;
}

ros::ServiceServer service = nh.advertiseService("my_service", callback);
ros::spin();

when using functor objects you must explicitly specify the request and response types as template arguments, because the compiler cannot deduce them in this case.在使用函数指针和class时，调用advertiseService函数可以不用添加模板参数，编译器会自动推导，但是对functor来说你必须指明，如下所示

ros::ServiceServer srv = nh.advertiseService<std_srvs::Empty::Request, std_srvs::Empty::Response>("my_service", Foo());