XMLRPC的C++代碼在下載后的ros_comm-noetic-develutilitiesxmlrpcpp路徑下。

還好，整個工程不算太大。XMLRPC分成客戶端和服務器端兩大部分。

咱們先看客戶端，主要代碼在XmlRpcClient.cpp文件里。

擒賊先擒王，XmlRpcClient.cpp文件中最核心的函數就是execute，用于執行遠程調用，代碼如下。

// Execute the named procedure on the remote server.
// Params should be an array of the arguments for the method.
// Returns true if the request was sent and a result received (although the result might be a fault).
bool XmlRpcClient::execute(const char* method, XmlRpcValue const& params, XmlRpcValue& result)
{
  XmlRpcUtil::log(1, "XmlRpcClient::execute: method %s (_connectionState %s).", method, connectionStateStr(_connectionState));


  // This is not a thread-safe operation, if you want to do multithreading, use separate
  // clients for each thread. If you want to protect yourself from multiple threads
  // accessing the same client, replace this code with a real mutex.
  if (_executing)
    return false;


  _executing = true;
  ClearFlagOnExit cf(_executing);


  _sendAttempts = 0;
  _isFault = false;


  if ( ! setupConnection())
    return false;


  if ( ! generateRequest(method, params))
    return false;


  result.clear();
  double msTime = -1.0;   // Process until exit is called
  _disp.work(msTime);


  if (_connectionState != IDLE || ! parseResponse(result)) {
    _header = "";
    return false;
  }


  // close() if server does not supports HTTP1.1
  // otherwise, reusing the socket to write leads to a SIGPIPE because
  // the remote server could shut down the corresponding socket.
  if (_header.find("HTTP/1.1 200 OK", 0, 15) != 0) {
    close();
  }


  XmlRpcUtil::log(1, "XmlRpcClient::execute: method %s completed.", method);
  _header = "";
  _response = "";
  return true;
}

它首先調用setupConnection()函數與服務器端建立連接。

連接成功后，調用generateRequest()函數生成發送請求報文。

XMLRPC請求報文的頭部又交給generateHeader()函數做了，代碼如下。

// Prepend http headers
std::string XmlRpcClient::generateHeader(size_t length) const
{
  std::string header = 
    "POST " + _uri + " HTTP/1.1rn"
    "User-Agent: ";
  header += XMLRPC_VERSION;
  header += "rnHost: ";
  header += _host;


  char buff[40];
  std::snprintf(buff,40,":%drn", _port);


  header += buff;
  header += "Content-Type: text/xmlrnContent-length: ";


  std::snprintf(buff,40,"%zurnrn", length);
  return header + buff;
}

主體部分則先將遠程調用的方法和參數變成XML格式，generateRequest()函數再將頭部和主體組合成完整的報文，如下：

std::string header = generateHeader(body.length());
_request = header + body;

把報文發給服務器后，就開始靜靜地等待。

一旦接收到服務器返回的報文后，就調用parseResponse函數解析報文數據，也就是把XML格式變成純凈的數據格式。

我們發現，XMLRPC使用了socket功能實現客戶端和服務器通信。

我們搜索socket這個單詞，發現它原始的意思是插座。這非常形象，建立連接實現通信就像把插頭插入插座。

雖說XMLRPC也是ROS的一部分，但它畢竟只是一個基礎功能，我們會用即可，暫時不去探究其實現細節，