Implementing XMLHTTPRequest onReadyStateChange in C++

Environment: Developed in (but not restricted to) VC++ .NET

I recently needed to support the asynchronous version of the send() operation of the XMLHTTPRequest object that's included in Microsoft's XML 4.0 library. To achieve this, I chose to leverage the "onreadystatechange" property to signal when the send() operation was completed and data was ready to be received.

IXMLHTTPRequest provides the "onreadystatechange" property to allow the caller to monitor and react to the state changes of an asynchronous send() invocation. The documentation for this property states that "onreadystatechange" is "not readily accessible" in C++. However, it turns out to be rather simple to implement using ATL templates.

Architecture

To receive onreadystatechange events, I created a class called CXMLHTTPEvent that implements the IDispEventSimpleImpl template. IDispEventSimpleImpl provides the minimum framework to capture events returned by XMLHTTPRequest.

class CXMLHTTPEvent : public IDispEventSimpleImpl</*nID =*/ 1,
      CXMLHTTPEvent, &__uuidof(MSXML::XMLDOMDocumentEvents)>

I then added a SINK_ENTRY_INFO to the SINK_MAP to capture the XMLHTTPRequest events.

BEGIN_SINK_MAP(CXMLHTTPEvent)
  SINK_ENTRY_INFO(/*nID =*/ 1, __uuidof(MSXML::
                                        XMLDOMDocumentEvents),
                          /*dispid =*/ 0, OnReadyStateChange,
                          &OnEventInfo)
END_SINK_MAP()

When an event is captured, I then notify the main process of the state change via a Windows message or function callback.

// State change call back handler
void __stdcall CXMLHTTPEvent::OnReadyStateChange ( )
{
  ATLTRACE(L"CXMLHTTPEvent: ReadyStateChange = %i \n",
           m_spRequest->readyState);

  if (m_pCallBack)
    m_pCallBack->OnReadyStateChange(m_spRequest->GetreadyState());

  if (m_hwndPostWindow)
    ::PostMessage(m_hwndPostWindow, WM_XMLHTTP_READYSTATE_CHANGE,
                  0, MAKELPARAM(m_spRequest->GetreadyState(), 0));
}

Using CXMLHTTPEvent

After creating the XMLHTTPRequest object, create an instance of the CXMLHTTPEvent prior to invoking XMLHTTPRequest operations. This will connect XMLHTTPRequest with how you'd like your application to be notified about the state changes.

void CXMLHTTPCallBackDlg::OnBnClickedButton1()
{
  // Create the XMLHTTPRequest
  m_spXMLHTTPRequest.CreateInstance(L"Msxml2.XMLHTTP.4.0");

  ...

  // Use WM_MESSAGE pump
  m_pXMLHTTPEvent = new CXMLHTTPEvent(m_spXMLHTTPRequest,
                                      GetSafeHwnd());

  ...

  // Use Function Pointer CallBack
  m_pXMLHTTPEvent = new CXMLHTTPEvent(m_spXMLHTTPRequest,
                                      NULL, this);

  ..

  // Open the async connection
    m_spXMLHTTPRequest->open("GET", m_sURL.AllocSysString(),
                              VARIANT_TRUE);

  // Send the async request
  m_spXMLHTTPRequest->send();
}

If you are using the message pump to get notifications, add a message handler and map entry for the WM_XMLHTTP_READYSTATE_CHANGE message. This message will be sent by CXMLHTTPEvent when XMLHTTPRequest signals a ready state change.

BEGIN_MESSAGE_MAP(CXMLHTTPCallBackDlg, CDialog)
  ...
  ON_MESSAGE(WM_XMLHTTP_READYSTATE_CHANGE,
             OnReadyStateChange2)
  ...
END_MESSAGE_MAP()

When WM_XMLHTTP_READYSTATE_CHANGE is sent, the lParam will contain the ReadyState value.

LRESULT CXMLHTTPCallBackDlg::OnReadyStateChange2(WPARAM wParam,
                                                 LPARAM lParam)
{
  UINT nState = (UINT)lParam;
  ...
  return 0;
}

If you're using the callback method, make sure the target class for events inherits from the CXMLHTTPEventCallBack class and implements the OnReadyStateChange(long lReadyState) method.

  class CXMLHTTPCallBackDlg : public CDialog, CXMLHTTPEventCallBack

  ...

  void CXMLHTTPCallBackDlg::OnReadyStateChange(long lReadyState)
  {
    ...
  }

Downloads

Download demo project - 23 Kb
Download source - 27 Kb


Comments

  • A good knowledge sharing, thank you!

    Posted by Legacy on 08/10/2003 12:00am

    Originally posted by: TW

    TQ!

    Reply
Leave a Comment
  • Your email address will not be published. All fields are required.

Top White Papers and Webcasts

  • U.S. companies are desperately trying to recruit and hire skilled software engineers and developers, but there is simply not enough quality talent to go around. Tiempo Development is a nearshore software development company. Our headquarters are in AZ, but we are a pioneer and leader in outsourcing to Mexico, based on our three software development centers there. We have a proven process and we are experts at providing our customers with powerful solutions. We transform ideas into reality.

  • When individual departments procure cloud service for their own use, they usually don't consider the hazardous organization-wide implications. Read this paper to learn best practices for setting up an internal, IT-based cloud brokerage function that service the entire organization. Find out how this approach enables you to retain top-down visibility and control of network security and manage the impact of cloud traffic on your WAN.

Most Popular Programming Stories

More for Developers

RSS Feeds

Thanks for your registration, follow us on our social networks to keep up-to-date