JavaScript Calls from C++

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Environment: VC++ 6.0, NT 4.0, Win2000, Win95/98

Introduction

Sometimes, when we are using the IE Browser Control inside of a C++ application, we need to access the HTML elements. We can do it by using standard COM objects such as IWebBrowser2, IHTMLDocument2, and so forth. By doing this, we easily can implement features such as click button, click anchor, get input string, get HTML text, and so on. Unfortunately, Microsoft did not provide similar objects for JavaScript. In any case, it is possible to make a control for the JavaScript object inside an HTML page by using a traditional COM approach. This article describes the class CWebPage that allows you to do it and a technique to call a JavaScript function from C++ code.

How to Do This

As the result of using the presented class, it will be easy to call any JavaScript function from C++ code. To implement this feature, we should get a pointer to the IHTMLDocument2 interface. If we are using the CHtmlView class from MFC, we can get one by using member function CHtmlView::GetHtmlDocument(). In the case of using the IWebBrowser or IWebBrowser2 components, the function get_Document will bring us the desired interface. Here is an example:

CComPtr<IDispatch> spDisp = CHtmlView::GetHtmlDocument();
m_webPage.SetDocument(spDisp);

The rest of the things will be done by the CWebPage class. Here is an example of a JavaScript call without parameters.

m_webPage.CallJScript("Welcome");

The example of the JavaScript call with two parameters will look like this:

m_webPage.CallJScript("Miltiply","2.34","3.32");

The Class Implementation

class CWebPage
{
public:
  CWebPage();
  virtual ~CWebPage();

  bool SetDocument(IDispatch* pDisp);
  LPDISPATCH GetHtmlDocument() const;
  const CString GetLastError() const;
  bool GetJScript(CComPtr<IDispatch>& spDisp);
  bool GetJScripts(CComPtr<IHTMLElementCollection>& spColl);
  CString ScanJScript(CString& strAText,CStringArray& args);

  bool CallJScript(const CString strFunc);
  bool CallJScript(const CString strFunc,const CString strArg1);
  bool CallJScript(const CString strFunc,const CString strArg1,
                   const CString strArg2);
  bool CallJScript(const CString strFunc,const CString strArg1,
                   const CString strArg2,const CString strArg3);
  bool CallJScript(const CString strFunc,const 
                         CStringArray& paramArray);

protected

  CComPtr<IHTMLDocument2> m_spDoc;

};

Calling Technique

The previously mentioned technique splits the following steps:

• Getting a pointer to the IHTMLDocument2 interface.
• Getting IDispatch for a JavaScript object in an HTML document.
• Getting DISPID for a given name of a JavaScript function.
• Putting parameters to the DISPPARAM structure.
• Calling a JavaScript function by using the Invoke method of the IDispatch interface.

Here is an example of getting a IDispatch pointer to the Java Scripts objects:

bool CWebPage::GetJScript(CComPtr<IDispatch>& spDisp)
{
  HRESULT hr = m_spDoc->get_Script(&spDisp);
  ATLASSERT(SUCCEEDED(hr));
  return SUCCEEDED(hr);
}

And here is the final function to call JavaScript:

CComVariant CWebPage::CallJScript(const CString strFunc,
                                  const CStringArray&
                                        paramArray)
{
  //Getting IDispatch for Java Script objects
  CComPtr<IDispatch> spScript;
  if(!GetJScript(spScript))
  {
    ShowError("Cannot GetScript");
    return false;
  }
  //Find dispid for given function in the object
  CComBSTR bstrMember(strFunc);
  DISPID dispid = NULL;
  HRESULT hr = spScript->GetIDsOfNames(IID_NULL,&bstrMember,1,
                         LOCALE_SYSTEM_DEFAULT,&dispid);
  if(FAILED(hr))
  {
    ShowError(GetSystemErrorMessage(hr));
    return false;
  }

  const int arraySize = paramArray.GetSize();
  //Putting parameters
  DISPPARAMS dispparams;
  memset(&dispparams, 0, sizeof dispparams);
  dispparams.cArgs      = arraySize;
  dispparams.rgvarg     = new VARIANT[dispparams.cArgs];
  dispparams.cNamedArgs = 0;
  
  for( int i = 0; i < arraySize; i++)
  {
    CComBSTR> bstr = paramArray.GetAt(arraySize - 1 - i);
              // back reading
    bstr.CopyTo(&dispparams.rgvarg[i].bstrVal);
    dispparams.rgvarg[i].vt = VT_BSTR;
  }
  EXCEPINFO excepInfo;
  memset(&excepInfo, 0, sizeof excepInfo);
  CComVariant vaResult;
  UINT nArgErr = (UINT)-1;      // initialize to invalid arg
  //Call JavaScript function
  hr = spScript->Invoke(dispid,IID_NULL,0,
                        DISPATCH_METHOD,&dispparams,
                        &vaResult,&excepInfo,&nArgErr);
  delete [] dispparams.rgvarg;
  if(FAILED(hr))
  {
    ShowError(GetSystemErrorMessage(hr));
    return false;
  }
  return vaResult;
}

Notes About the Demo

To call a JavaScript function from the demo, you should select the function in the tree of the left window. After this, press the ! button on the menu bar.

Downloads

Download demo project - 34 Kb
Download source - 3 Kb