Remembering a Window Position Across Application Invocations

Thursday Jun 19th 2003 by Kiran Sanjeeva
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Many applications need to remember their position on the Desktop across invocations. Learn a way to implement this functionality through a simple C++ class that can be used in MFC or non-MFC applications.

Environment: VC6

I wrote this for a very simple requirement. I wanted the main window of the application to appear at the same position across invocations. Well, what better a place than the Registry to store these positions?

The class for this purpose, called WindowPosition, is a simple class with a handful of public methods.

class WindowPosition
{
public:
  WindowPosition(HKEY hKey,TCHAR* szSubKey);
  virtual ~WindowPosition();

private:
  DWORD m_dwPosArray[5];
  HKEY  m_hKey;
  TCHAR m_szSubKey[MAX_PATH];

  BOOL ValidatePositions(DWORD& dwxPos,DWORD&dwyPos,DWORD&
                                dwWidth,DWORD& dwHeight,DWORD&
                                dwSum);

public:
  BOOL GetWindowPositions(DWORD& dwxPos,DWORD& dwyPos,DWORD&
                                 dwWidth,DWORD& dwHeight);
  void OnWindowPosChanged(WINDOWPOS* lpwndpos);
  BOOL SaveWindowPositions();
};

Here's an explanation of the classes used in the preceding code:

  • GetWindowPositions. Used to get the window positions stored in the Registry.
  • OnWindowPosChanged. Used to set the window positions into the WindowPosition class' member variables.
  • SaveWindowPositions. Used to save the WindowPosition class's data to the Registry.

When constructing the object, one would have to specify the root key and the subkey to store.

Usage

A typical usage of this class can be something like this, and this is how I have implemented it in the sample project that is included with this article.

  1. In the main window class, one would add a member variable of class WindowPosition, say m_wndPos:

    #include "WindowPosition.h"
    
    class CWndPosDlg : public CDialog
    {
    ...............
    // Implementation
    protected:
        HICON m_hIcon;
      WindowPosition  m_wndPos;
        // Generated message map functions
    ...............
    };
    
  2. One would then intercept the WM_WINDOWPOSCHANGED message for the main window and in the handler and simply pass the WINDOWPOS* data to m_wndPos by calling the m_wndPos.OnWindowPosChanged(...) method.

    BEGIN_MESSAGE_MAP(CWndPosDlg, CDialog)
    //{{AFX_MSG_MAP(CWndPosDlg)
    .......
      ON_WM_WINDOWPOSCHANGED()
    .......
    //}}AFX_MSG_MAP
    END_MESSAGE_MAP()
    
    void CWndPosDlg::OnWindowPosChanged( WINDOWPOS* lpwndpos )
    {
      m_wndPos.OnWindowPosChanged(lpwndpos);
      m_wndPos.SaveWindowPositions();
    }
    
  3. One could then save the same to the Registry by calling m_wndPos.SaveWindowPositions() right at that moment (as is done above), or defer it to some other time. For example, one could save the window positions on application exit only.

  4. The next time the application is invoked, we would want the saved window positions to be reflected. You could do this by reading m_wndPos.GetWindowPositions in OnInitDialog (or the equivalent) of the main window and calling SetWindowPos with the retrieved data.

    BOOL CWndPosDlg::OnInitDialog()
    {
      CDialog::OnInitDialog();
    
      // Set the icon for this dialog. The framework does this
      // automatically when the application's main window is not a
      // dialog
      SetIcon(m_hIcon, TRUE);     // Set big icon
      SetIcon(m_hIcon, FALSE);    // Set small icon
    
      // TODO: Add extra initialization here
      DWORD dwXPos,dwYPos,dwWidth,dwHeight;
    
      if(TRUE == m_wndPos.GetWindowPositions(dwXPos,dwYPos,
                                             dwWidth,dwHeight))
      {
        SetWindowPos(NULL,dwXPos,dwYPos,dwWidth,dwHeight,
                     SWP_NOOWNERZORDER | SWP_NOZORDER|SWP_NOSIZE);
      }
      else
        CenterWindow();
          return TRUE;    // return TRUE unless you set the focus
                          // to a control
    }
    

Caveats

Yes, there are a few caveats with this. Nothing so serious, but yes, I would like to fix it and clean it up once I find an elegant way to deal with the issues. If one changes the display resolution, things can get messy!! The main window may even disappear from the visible area of the screen. This is pretty serious and can be annoying.

I would be glad to receive any ideas or suggestions in this matter, or anything for making this a handy class.

Downloads

Download sample - 5 Kb
Download source - 12 Kb
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