Handling HTML Element Events in MFC applications - A simple alternative approach

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Introduction

The conventional approach to sink events when hosting a WebBrowser control involves using the MSHTML Element Events2 Interface. It requires retrieving a pointer to the element of interest of the page,  retrieving a pointer to the connection point container, finding the required connection point, advising the connection point to start receiving events, implementing IDispatch::Invoke to handle any fired event and closing the connections when no longer needed.

This article illustrates a simple alternative technique to intercept and handle, in a MFC application, HTML element events generated in a hosted WebBrowser Control. It also shows how to use xml objects to pack event information to the hosting application, and how to extract this information to implement an event handler.

Implementation

Before navigation occurs in a WebBrowser control, a BeforeNavigate2 message is fired. VC++ 6.0 MFC Class Wizard, automatically adds the appropriate OnBeforeNavigate2 member function to handle this event.

Two of the six parameters taken by OnBeforeNavigate2, are useful to intercept DHTML events: VARIANT FAR* URL that contains the url address to be navigated to, and BOOL FAR* Cancel, that cancels navigation before it occurs.

In a WebBrowser's HTML page, the VBScript property location.href of the browser document's object, sets the entire URL as a string. When a given URL is set in the HTML script event handler, the corresponding string is passed as the second parameter of the member function OnBeforeNavigate2 of the hosting application.

In order to intercept and handle HTML element events, the href property should contain, instead of a regular URL address, a string with the necessary information of the DHTML event fired in the Browser control.

In most cases, the information required by the hosting application consists of the html element's id , and an associated value. This information is packed in a xml string having the form "<element idevent='001'>value</element>".

Once this value is passed to OnBeforeNavigate2, the parameter Cancel is set to TRUE to avoid navigation. The following two code snippets illustrate these ideas.

<SCRIPT ID = "clientEventHandlersVBS" LANGUAGE="VBScript">
<!--

dim strIndex

Sub myelement_onmouseover()
    strIndex = "007"
    strXML = "<element idevent='" & strIndex & "'>" &_<BR> myelement.innerText & "</element>"
    location.href = "Event:" & strXML
End Sub

--> 
</SCRIPT>

// OnBeforeNavigate2 Event

handler void CDlgDHTMLEventsDlg::OnBeforeNavigate2(LPDISPATCH pDisp,
                                                   VARIANT FAR* URL,
                                                   VARIANT FAR* Flags,
                                                   VARIANT FAR* TargetFrameName,
                                                   VARIANT FAR* PostData,
                                                   VARIANT FAR* Headers,
                                                   BOOL FAR* Cancel) 
{
    CString strURL(URL->bstrVal);
    CString strXML(strURL.Right(strURL.GetLength()-6));
    strXML.Replace("%20", " ");

    if(strURL == _T("about:blank"))
        *Cancel = FALSE;
    else
    {
        ExtractXMLInfo(strXML);
        *Cancel = TRUE;
    }
}

The sample application

dlgDHTMLEvents is a dialog based MFC application. The dialog hosts a WebBrowser control that shows a HTML page having these DHTML objects: two INPUT elements, two BUTTONS, one <h1> text element and one IMAGE object. When some predetermined events are fired by each one of these elements, its id and associated value are passed to the MFC dialog app.

Since xml is used to convey DHTML event information, the Microsoft XML parser should be imported into the MFC project. To do it, add this line to StdAfx.h (If not installed, it can be downloaded from http://www.msdn.microsoft.com/downloads/default.asp ):

#import "Msxml3.dll" named_guids raw_interfaces_only

To add ATL support (CComBSTR types are used), include this header file in StdAfx.h:

#include <atlbase.h> 

The HTML script is stored as a resource of the project (ID: IDR_HTML). The script's event handlers are included in the section <SCRIPT ID = "clientEventHandlersVBS" LANGUAGE="VBScript">of this resource.

The page is loaded into the Browser using the IPersistStreamInit interface and associated methods, of the CWebBrowser2 class. The member function LoadHTMLPage(CString strBaseAddress) implements this operation. The strBaseAddress parameter sets the baseline URL for the graphical resources used in the  HTML page.

The member variable m_strBaseAddress contains this parameter. In the sample application, this value is set, in the dialog constructor, to _T("c:\\dlgDHTMLEvents\\res"). Please modify according to your path.

Before a pointer to IPersistStreamInit can be obtained, Mshtml.dll has to be loaded (MSHTML is the IE rendering engine and parser for HTML). For this reason, the initial navigation of the browser is set to the valid  "about:blank" URL. LoadHTMLPage() is called from the event handler OnDocumentComplete(), provided m_bolLoaded is false.

The member function ExtractXMLInfo(CString strXML) converts the string parameter strXML, received from the Web browser control, to a valid xml MSXML2::IXMLDOMDocument* pxmlDoc object. To obtain a valid pointer to this interface, CoCreateInstance() is called as follows:

hr = CoCreateInstance( MSXML2::CLSID_DOMDocument, 
                       NULL, 
                       CLSCTX_INPROC_SERVER, 
                       MSXML2::IID_IXMLDOMDocument, 
                       (void**)&pxmlDoc);

After obtaining this pointer, the parameter strXML, which has the form _T("<element idevent='001'>value</element>"), is loaded and navigated to obtain the attribute node MSXML2::IXMLDOMNode* pattrNode, and the value node MSXML2::IXMLDOMNode* pNode. The event id and its associated value, are passed as parameters to the event handler function EventHandler(INT nEvent, CString strValue). The static controls m_stcEvent and m_stcValue display in the dialog, the DHTML event description and the associated value.

In summary, the steps involved for intercepting and handling HTML events are:

1. Declare any desired event handlers in the HTML script.
2. In each event handler, set the location.href property to the xml string that contains the values to be passed to the MFC host application.     
3. Call the function that loads the HTML document stream from the event handler OnDocumentComplete().
4. Implement OnBeforeNavigate2().
5. Extract HTML element event information from the received xml string.
6. Use this information in the MFC event handler function.

With minor modifications, these steps can be used for applications that utilizes CHtmlView classes.

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