MFC SDI GetActiveView() always returns NULL

Question

I need help with switching between CFormViews in my MFC SDI C++ project. I have been digging for a long time, and can't figure out why my code isn't working. Through searching the internet (and this site included) I came across several tutorials for switching forms by adding two functions to MainFrm.cpp (a CMainFrame object that inherits from CFrameWnd). One of them is passed an id of the form I want to switch to, then gets a pointer to the active view, and runs some other code from there. However, GetActiveView() always returns a NULL pointer value. I know there's an active view because I'm clicking a button from an active form. My code is below. This is just the function I'm referring to. It resides in MainFrm.cpp (the default window file created when you start a new MFC project).

So far I've tried the code from the Microsoft Knowledge Base that talks about How to Get Current CDocument or CView from Anywhere, I tried to get the active frame first, then called GetActiveView from CFrameWnd, and I tried the code below. All to no avail. I clearly do not know enough about MFC to figure anything out. If you need more information from me, please ask. I probably didn't mention everything I should have. I chose to do MFC for a school project, and can't proceed to create a UML or writing any other code until I know that I can get these forms to work.

void CMainFrame::SelectView(UINT ViewID)
{
    // If the view the user selected is already displaying, do nothing
    if (ViewID == m_CurrentView)
        return;

    // Get a pointer to the current view
    CView* pCurrentView = GetActiveView();

    // We are about to change the view, so we need a pointer to the runtime class
    CRuntimeClass* pNewView = NULL; // Added = NULL because it wouldn't allow program to be run without initialization of pNewView

    // We will process a form
    // First, let's change the identifier of the current view to our integer
    ::SetWindowLong(pCurrentView->m_hWnd, GWL_ID, m_CurrentView);

    // Now we will identify what form the user selected
    switch (ViewID)
    {
    case IDD_CHOOSE_ITEM:
        pNewView = RUNTIME_CLASS(CChooseItemView);
        break;

    case IDD_ITEM_INFORMATION:
        pNewView = RUNTIME_CLASS(CItemInformationView);
        break;
    }

    // We will deal with the frame
    CCreateContext crtContext;

    // We have a new view now. So we initialize the context
    crtContext.m_pNewViewClass = pNewView;
    // No need to change the document. We keep the current document
    crtContext.m_pCurrentDoc = GetActiveDocument();

    CView* pNewViewer = STATIC_DOWNCAST(CView, CreateView(&crtContext));

    // Now we can create a new view and get rid of the previous one
    if (pNewViewer != NULL)
    {
        pNewViewer->ShowWindow(SW_SHOW);
        pNewViewer->OnInitialUpdate();
        SetActiveView(pNewViewer);
        RecalcLayout();
        m_CurrentView = ViewID;
        pCurrentView->DestroyWindow();
    }
}

Answer 1

To get the a non active View but associated CView from CDocument, you can implement this schema in Doc

// ----- GetCChooseItemView() -- -Search the first associated CView in  INACTIVE Views too ! ------ 
CView* CMyDoc::GetCChooseItemView(void)
{
  CRuntimeClass* prt = RUNTIME_CLASS(CChooseItemView);
  CView* pView = NULL;

  // Continue search in inactive View by T(o)m

  POSITION pos = GetFirstViewPosition();
  while (pos != NULL)
  {
    pView = GetNextView(pos);
    if (pView->GetRuntimeClass() == prt)
    {
        if (pView->IsKindOf(RUNTIME_CLASS(CChooseItemView)))
            break;
    }
    pView = NULL;       // not valid vie
  }

  return static_cast<CChooseItemView*>(pView);
}

then add in your SelectView Code

void CMainFrame::SelectView(UINT ViewID)
{
  : (code as before)
  :      
  // Get a pointer to the current view
  CView* pCurrentView = GetActiveView();

  // Get a pointer to the current view
  CView* pCurrentView = GetActiveView();
  if (pCurrentView == NULL
  { 
    CMyDoc* pDoc = static_cast<CMyDoc*>(GetActiveDocument());
    if (pDoc)
    {
      pCurrentView = pDoc->GetChhoseItemView(); 
      if (pCurrentView == NULL)
         mpCurrentView = pDoc->GetCItemInformationView()  // let as exercise for the OP

      if (pCurrentView == NULL
      {
          DebugBreak();     // Errror No View found..
      }
    } 

  :  (code as befeore)
  :   
}

Answer 2

The following code is working for me:

virtual CView* SwitchToView(CView* pNewView);

and in cpp:

CView* CMyDoc::SwitchToView(CView* pNewView)
{
 CMDIFrameWndEx* pMainWnd = (CMDIFrameWndEx*)AfxGetMainWnd();
 // Get the active MDI child window
 CMDIChildWndEx* pChild = (CMDIChildWndEx*)pMainWnd->MDIGetActive();
 // Get the active view attached to the active MDI child window.
 CView* pOldActiveView = pChild->GetActiveView();
 // Exchange control ID of old view
 // note: if you have more than two view you have to remember which view you switched to
 // so you can set it's old control ID correctly
 if(pNewView == m_pMyView)
  pOldActiveView->SetDlgCtrlID(CTRLID_MYVIEW2);
 if(pNewView == m_pMyView2)
  pOldActiveView->SetDlgCtrlID(CTRLID_MYVIEW);
 // Exchange control ID of new new
 // note: the control ID of the active view must always be AFX_IDW_PANE_FIRST
 pNewView->SetDlgCtrlID(AFX_IDW_PANE_FIRST);
 // Set flag so that document will not be deleted when view is dettached.
 BOOL bAutoDelete = m_bAutoDelete;
 m_bAutoDelete = FALSE;
 // Dettach existing view
 RemoveView(pOldActiveView);
 // restore flag
 m_bAutoDelete = bAutoDelete;
 // Show the newly active view and hide the inactive view.
 pNewView->ShowWindow(SW_SHOW);
 pOldActiveView->ShowWindow(SW_HIDE);
 // Attach new view
 AddView(pNewView);
 pChild->RecalcLayout();
 pNewView->UpdateWindow();
 pChild->SetActiveView(pNewView);

 return pOldActiveView;
}

I hope it help you.