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Once again I will review how I went about designing the plug-in and implementing its features. Along the way I will also cover problems encountered and how they were solved.

When I look at the plug-ins I can write, it seems to me that I will be able to take most of my existing article series and convert each as I go. It has been a good starting point for upgrading this library. If users of the class wish to see a specific plug-in created, they should visit my blog page and post a suggesstion there.

Other articles in the series

• An MFC extension library to enable DLL plug-in technology for your application using MESSAGE_MAPs - This is the main plug-in article, with an MDI Tab bar example
• MFC extension library - A plugin to handle owner drawn menus - A plug-in article which adds owner drawn menu support.

Developing the plug-in

As a starting point, I created an MFC extension DLL as described in the main article. From there I got hold of the source from my Multi Page Print Preview enhancements for MFC Doc/View applications article and started combining the two into a working project.

The first step in this procedure was to replace the existing CView::OnFilePrintPreview() function with our own. So, I created a plug-in to handle this:

I added a message map entry to process the existing ID_FILE_PRINT_PREVIEW message.

BEGIN_MESSAGE_MAP(CEnhancedPrintPreview, CPlugInMap)
    //{{AFX_MSG_MAP(CEnhancedPrintPreview)
    ON_COMMAND(ID_FILE_PRINT_PREVIEW, OnFilePrintPreview)
    //}}AFX_MSG_MAP
END_MESSAGE_MAP()

With the V1.2 upgrade to the library, I was able to return true for all CView derived classes so that we can replace their existing print preview functionality.

bool CEnhancedPrintPreview::IsPlugInFor(CRuntimeClass *pClass)
{
    if (pClass-&gtIsDerivedFrom(RUNTIME_CLASS(CView)))
    {
        // we are only a plug-in for any view class
        // as the view classes support the preview option
        return true;
    }
    return false;
}

For the Pre call for the message we suppress the standard implementation of it using SuppressThisMessage(), as we do not want multiple preview windows being displayed.

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void CEnhancedPrintPreview::OnFilePrintPreview()
{

    if (IsPreCall())
    {
        // were replacing the standard MFC print preview and replacing it with
        // our enhanced version.

        SuppressThisMessage();
        // replace the default print preview with ours!

        // In derived classes, implement special window handling here
        // Be sure to Unhook Frame Window close if hooked.
        
        // must not create this on the frame.  Must outlive this function
        CPrintPreviewState* pState = new CPrintPreviewState;
        
        // DoPrintPreview's return value does not necessarily indicate that
        // Print preview succeeded or failed, but rather what actions are necessary
        // at this point.  If DoPrintPreview returns TRUE, it means that
        // OnEndPrintPreview will be (or has already been) called and the
        // pState structure will be/has been deleted.
        // If DoPrintPreview returns FALSE, it means that OnEndPrintPreview
        // WILL NOT be called and that cleanup, including deleting pState
        // must be done here.
        CView *pView = static_cast(CView*)(m_pPlugInFor);
        
        if (!pView->DoPrintPreview(IDD_PREVIEW, pView, 
           RUNTIME_CLASS(CMultiPagePreviewView), pState))
        {
            // In derived classes, reverse special window handling here for
            // Preview failure case
            
            TRACE0("Error: DoPrintPreview failed.\n");
            AfxMessageBox(AFX_IDP_COMMAND_FAILURE);
            delete pState;      // preview failed to initialize, delete State now
            pState = NULL;
        }
    }
}

Well, once I had this and the standard code from the enhanced print preview code merged sufficiently to get this far, I ran some tests to see how well it worked.

And it failed very badly. :( In fact I could not immediately see why it was failing, so I placed judicious TRACE statements throughout the code. One of the symptoms of the failure was I was getting to see the standard MFC print preview dialog bar:

Now during the print preview initialisation, this standard dialog bar is being destroyed and replaced with our own, which enabled the mouse wheel messages, so why is it being shown? This was the first clue to my problem. It seems that even though I was trying to suppress the ID_FILE_PRINT_PREVIEW message in the standard MFC code, I was failing to, and two preview views were being created, mine and MFCs. This was the main source of the problem. So how do I fix it?

Message suppression in the library

In the MFC extension library that handles the plug-ins, each plug-in had a bool m_bSuppressThisMessage variable which when we needed to suppress the message was being set to true, so I checked the state of the flag in the ProcessCommandMessageMaps() function after my plug-in message had just been processed.

And it was false!

After some thought it occurred to me that when we create the preview view and we go about displaying it, other messages may be being processed by the library in the background, probably due to a message pump being done due to a SendMessage() call, and as we only have a single flag for all passes through the library, any additional messages that do get processed before we return from our plug-in function will reset the suppressed message flag. This is not what we want to happen.

Now that we have identified the flaw in the library we need to fix it.

Fixing the problem

Well the problem we have here is in the library, and not our plug-in, so I went back to the library source and decided that we needed a stack of suppression flags, one gets added for every message being processed, and is removed when that message gets completed. This led to some changes in the CPlugInMap class as follows:

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    // changes from a single variable into a vector
    std::vector&ltbool&gt    m_bSuppressThisMessage;

// pre/post calls push a new entry onto the end of the vector
void CPlugInMap::SetPreCall()
{
    m_bPreCall = true;
    m_bPostCall = false;
    m_bSuppressThisMessage.push_back(false);
}

void CPlugInMap::SetPostCall()
{
    m_bPreCall = false;
    m_bPostCall = true;
    m_bSuppressThisMessage.push_back(false);
}

// checking to see whether the message was suppressed pops an entry of the vector
bool CPlugInMap::IsSuppressed()
{
    bool bSuppressed = m_bSuppressThisMessage[m_bSuppressThisMessage.size() - 1];
    m_bSuppressThisMessage.pop_back();
    return bSuppressed;
}

// an additional query function can return the supressed state without
// removing the entry from the queue
bool CPlugInMap::GetSuppressedState() const
{
    bool bSuppressed = m_bSuppressThisMessage[m_bSuppressThisMessage.size() - 1];
    return bSuppressed;
}

Well this also made me think about a problem that was originally experienced by early users of the plug-in library, that is that when WM_NCDESTROY messages were processed, some objects called delete this on themselves, causing Post message handler to fail with a GPF when trying to access the deleted map objects. I did not want to re-introduce the same problem, so had to also change the ProcessCommandMessageMaps() and ProcessWindowMessageMaps() functions to check the CPIState object related to the plug-in object.

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// one of the updated functions
BOOL CPlugInApp::ProcessWindowMessageMaps(
    CPIState& state,    // new parameter added to the function
    bool pre, 
    bool* pbSuppress, 
    CPlugInMap** pMaps, 
    int count, 
    UINT message, 
    WPARAM wParam, 
    LPARAM lParam, 
    LRESULT* pResult)
{
    BOOL    ret = FALSE;
    BOOL    local_ret = FALSE;

    *pbSuppress = false;
    
    for (int i = 0; i &lt count; ++i)
    {
        if (pre)
        {
            pMaps[i]-&gtSetPreCall();
        }
        else
        {
            pMaps[i]-&gtSetPostCall();
        }
        local_ret = CallWindowMessageMap(pMaps[i], 
             message, wParam, lParam, pResult);
        if (local_ret)
        {
            ret = TRUE;   // show that it was handled somewhere
        }
        // works due to the order of evaluation
        if (!state.IsDestroyed() && pMaps[i]-&gtIsSuppressed())
        {
            *pbSuppress = true;
        }
    }
    return ret;
}

Once all this had been done, I rebuilt the project and the problem had been fixed.

This new version of the plug-in library is now V1.3 - It seems every time I create a new plug-in, I discover another flaw in it. Hopefully there will be less and less of these in the future.

Making the preview view plug-in enabled

I also wanted the preview view to make use of any other plug-ins that may enhance its appearance. One such is the owner-drawn menu plug-in. So I added versions of the OnCmdMsg and OnWndMsg functions along with the additional support varaibles:

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// header file
    virtual BOOL OnCmdMsg(UINT nID, int nCode, void* pExtra, 
        AFX_CMDHANDLERINFO* pHandlerInfo);
    virtual BOOL OnWndMsg(UINT message, WPARAM wParam, 
         LPARAM lParam, LRESULT* pResult);
private:
    CPlugInMap**    m_pMaps;
    int                m_MapCount;
    bool            m_bSuppressThisMessage;
    CPIState        m_state;

// C++ file
CMultiPagePreviewView::CMultiPagePreviewView()
{
    // replace the PAGE_INFO array with our one to make sure its large enough
    m_pPageInfo = m_pageInfoArray2;
    m_Across = 2;           // default number of pages across the screen
    m_Down = 1;            // default number of pages down the screen
    m_nPages = 2;
    
    // NOTE : If you change m_nPages outside of this 
    // code you will also need to modify
    // the m_Across and m_Down member vars to get the pages to show correctly
    m_pMaps = NULL;
    m_MapCount = 0;

    CPlugInApp *pApp = static_cast&ltCPlugInApp*&gt(AfxGetApp());
    // get our pointers to any plug in maps
    m_pMaps = pApp-&gtGetMessageMaps(this, m_MapCount);
}

CMultiPagePreviewView::~CMultiPagePreviewView()
{
    // release message map pointers
    for (int i = 0; i < m_MapCount; ++i)
    {
        delete m_pMaps[i];
        m_pMaps[i] = NULL;
    }
    delete []m_pMaps;
    m_pMaps = NULL;
    m_state.SetDestroyed();
}

BOOL CMultiPagePreviewView::OnCmdMsg(
        UINT nID, 
        int nCode, 
        void* pExtra, 
        AFX_CMDHANDLERINFO* pHandlerInfo) 
{
    // check to see whether any of the plug-in DLL's
    // want to intercept this message and process it before the application class
    CPlugInApp *pApp = static_cast&ltCPlugInApp*&gt(AfxGetApp());
    CPIState    stateCopy(m_state);

    ASSERT(pApp);
    BOOL ret_pre = FALSE;
    BOOL ret_app = FALSE;
    BOOL ret_post = FALSE;
    m_bSuppressThisMessage = false;
    
    // allow DLL's to process this message before the exe
    ret_pre = pApp-&gtProcessCommandMessageMaps(
            stateCopy, 
            true, 
            &m_bSuppressThisMessage, 
            m_pMaps, 
            m_MapCount, 
            nID, 
            nCode, 
            pExtra, 
            pHandlerInfo);
    if (!m_bSuppressThisMessage)
        {
        // allow the exe to process the message is it hasn't been suppressed by a DLL
        ret_app = CPreviewView::OnCmdMsg(nID, nCode, pExtra, pHandlerInfo);
        }
    // allow DLL to process the message after the exe
    // note that DLLs get to do this whether the message has been supressed or not
    if (!stateCopy.IsDestroyed())
    {
        ret_post = pApp-&gtProcessCommandMessageMaps(
                stateCopy, 
                false, 
                &m_bSuppressThisMessage, 
                m_pMaps, 
                m_MapCount, 
                nID, 
                nCode, 
                pExtra, 
                pHandlerInfo);
    }

    if (ret_pre || ret_app || ret_post)
        return  TRUE;
    return FALSE;
}

BOOL CMultiPagePreviewView::OnWndMsg(
        UINT message, 
        WPARAM wParam, 
        LPARAM lParam, 
        LRESULT* pResult)
{
    LRESULT    lResult;
    // special case for commands
    if (message == WM_COMMAND)
        {
        if (OnCommand(wParam, lParam))
            {
            lResult = 1;
            if (pResult != NULL)
                *pResult = lResult;
            return TRUE;
            }
        return FALSE;
        }

    // special case for notifies
    if (message == WM_NOTIFY)
        {
        NMHDR* pNMHDR = (NMHDR*)lParam;
        if (pNMHDR-&gthwndFrom != NULL && OnNotify(wParam, lParam, &lResult))
            {
            if (pResult != NULL)
                *pResult = lResult;
            return TRUE;
            }
        return FALSE;
        }

    // check to see whether any of the plug-in DLL's
    // want to intercept this message and process it before the application class
    CPlugInApp *pApp = static_cast&ltCPlugInApp*&gt(AfxGetApp());
    ASSERT(pApp);
    CPIState    stateCopy(m_state);
    BOOL ret_pre = FALSE;
    BOOL ret_app = FALSE;
    BOOL ret_post = FALSE;
    m_bSuppressThisMessage = false;
    // allow DLL's to process this message before the exe
    ret_pre = pApp-&gtProcessWindowMessageMaps(
            stateCopy, 
            true, 
            &m_bSuppressThisMessage, 
            m_pMaps, 
            m_MapCount, 
            message, 
            wParam, 
            lParam, 
            pResult);
    if (!m_bSuppressThisMessage)
        {
        // allow the exe to process the message is it hasn't been suppressed by a DLL
        ret_app = CPreviewView::OnWndMsg(message, wParam, lParam, pResult);
        }
    if (!stateCopy.IsDestroyed())
    {
        // allow DLL to process the message after the exe
        // note that DLLs get to do this whether the message has been supressed or not
        ret_pre = pApp-&gtProcessWindowMessageMaps(
                stateCopy, 
                false, 
                &m_bSuppressThisMessage, 
                m_pMaps, 
                m_MapCount, 
                message, 
                wParam, 
                lParam, 
                pResult);
    }

    if (ret_pre || ret_app || ret_post)
        return  TRUE;
    return FALSE;
}

This made the preview view plug-in enabled, so it makes use of the owner-drawn menu plug-in for the popup menu used to select how many pages the user wishes to preview at a time. A toolbar resource, which is loaded by the owner-drawn menu plug-in was added with the correct images.

So how did it look?

At this point I had all the code that changes the print orientation and printer disabled, so what we had was this:

I Now needed to enable the printer selection and the paper orientation features.

In the original aritcle, I had a class which enumerates all the printers installed on the local system. Along with this class, the user had to add some functions to their CWinApp derived class to allow the page orientation and the printer to be changed. But we cannot do this in our plug-in. We have to find a different method of doing this because we cannot modify the CWinApp or CPlugInApp derived classes.

If you look in the original article, you will see that the paper orientation change is done like this:

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// code section
class CMultiPagePrintPreviewApp : public CWinApp
{
    CEnumPrinters        m_Printers;

    void        SetPrintOrientation(int mode);
}

void CMultiPagePrintPreviewApp::SetPrintOrientation(int mode)
{
    m_Printers.SetPrintOrientation(m_hDevMode, mode);
}

bool CEnumPrinters::SetPrintOrientation(HANDLE &hDevMode, int mode)
{
    if (hDevMode == INVALID_HANDLE_VALUE)
        return false;

    switch (mode)
        {
        case DMORIENT_PORTRAIT :
                {
                // portrait mode
                LPDEVMODE pDevMode = (LPDEVMODE)::GlobalLock(hDevMode);
                // set orientation to portrait
                pDevMode->dmOrientation = DMORIENT_PORTRAIT;
                ::GlobalUnlock(hDevMode);
                }
                break;
        case DMORIENT_LANDSCAPE :
                {
                // landscape mode
                LPDEVMODE pDevMode = (LPDEVMODE)::GlobalLock(hDevMode);
                // set orientation to landscape
                pDevMode->dmOrientation = DMORIENT_LANDSCAPE;
                ::GlobalUnlock(hDevMode);
                }
                break;
        default :    
                ASSERT(FALSE);        // invalid parameter
                return false;
        }
    return true;
}

In this version we had to call the relevant function of the CWinApp derived class as we needed access to the CWinApp::m_hDevMode global handle, which manages the currently selected printer. We need to find a different way of doing this.

After looking through the MSDN, I discovered that calling the function CWinApp::GetPrinterDeviceDefaults() returns the required handles in a PRINTDLG structure. So we can extract the function from the CEnumPrinters class and rewrite it as follows:

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bool CMultiPagePreviewView::SetPrintOrientation(int mode) const
{
    PRINTDLG    pd;

    pd.lStructSize = (DWORD)sizeof(PRINTDLG);
    BOOL bRet = AfxGetApp()->GetPrinterDeviceDefaults(&pd);

    if (bRet)
    {
        switch (mode)
        {
        case DMORIENT_PORTRAIT :
            {
            // portrait mode
            LPDEVMODE pDevMode = (LPDEVMODE)::GlobalLock(pd.hDevMode);
            // set orientation to portrait
            pDevMode-&gtdmOrientation = DMORIENT_PORTRAIT;
            ::GlobalUnlock(pd.hDevMode);
            }
            break;
        case DMORIENT_LANDSCAPE :
            {
            // landscape mode
            LPDEVMODE pDevMode = (LPDEVMODE)::GlobalLock(pd.hDevMode);
            // set orientation to landscape
            pDevMode-&gtdmOrientation = DMORIENT_LANDSCAPE;
            ::GlobalUnlock(pd.hDevMode);
            }
            break;
        default :    
            ASSERT(FALSE);        // invalid parameter
            return false;
        }
        return true;
    }
    return false;
}

So we can hook up the paper orientation, but how do we switch printers?

Well again we need to avoid using functions added to the CWinApp derived class so we need to make use of existing MFC code. The most obvious of which was CWinApp::SelectPrinter(), but after playing with this for a while, I could still only get GPFs!

So, following his advice, I removed the printer combo box, and added a print setup button and mapped the relevant code. And hey presto, it works without any problems.