Creating draggable windows - SDI and dialogs

• Download Dialog project - 19 Kb
• Download SDI  project - 25 Kb

Introduction

The standard practice for moving a window is to drag it's title bar. This is handled for us by the operating system itself. But there are some applications that allow us to move the entire window by dragging anywhere within it's body. Sometimes it is pretty annoying when that happens, but there might be occasions where this is required. For dialog based applications there is a despicable trick which we can use to achieve this. The secret is to handle WM_NCHITTEST and to trick the OS into thinking that the mouse click or movement was made in the title bar. I explain the technique in more detail later down the article. But in SDI applications, there is a slight issue. Because all the mouse clicks and moves are handled by the view class! And if you attempt to use the same technique you used in the case of the dialog based application, the results will be wretchedly peculiar. Of course this just means we'll have to write that much more code. I show how this is done later down the article. I haven't tried out this technique on MDI applications, but my guess is that with a little bit of adjusting it should work fine on MDI applications as well.

Of course there is always more than one way to skin a cat, and it's not much different when it comes to programming. The same goes for this article too. Roman Nurik, has explained a much easier way to accomplish the same as I have. I have included this much easier method in terms of number of lines and effort at the end of the article. I could have put them on the top of the article too, but I wanted the flow of the article to be from good to better. In fact after Roman's method, I have also provided a solution offered by Albert Ling, which in my opinion is  the most innovative of all the methods discussed here.

Draggable dialogs

The WM_NCHITTEST message will sent to the dialog when the mouse is moved through it or a mouse click is made on it. Of course since we are using MFC, we have the OnNcHitTest function which handles  the WM_NCHITTEST message. The function returns one of several enumerated values, each of which indicates where the mouse action took place. Now one of these enumerated values is HTCAPTION which indicates that the mouse action took place on the title bar. So what we do is to verify if the mouse is currently within the client area of the window, and if it is within the client area of the window, we check if the mouse is down through a flag that is set and unset from the OnLButtonDown and OnLButtonUp handlers. If all our checks are passed, we return HTCAPTION, thus fooling the OS into thinking that the action is taking place on the title bar. It is very important to verify that the mouse action is within the dialog's client area, otherwise any buttons we have on the title bar, like the close and maximize buttons will be rendered useless.

UINT CDragDialogDlg::OnNcHitTest(CPoint point)
{
    CRect r;
    GetClientRect(&r);
    ClientToScreen(&r);

    //Chk to see if the mouse is within 
    //the dialog client area
    if(r.PtInRect(point))
    {       
        if(m_mousedown)
        {       
            return HTCAPTION;
        }
    }

    return CDialog::OnNcHitTest(point);

}

m_mousedown is a bool member variable which is set and unset from the OnLButtonDown and OnLButtonUp handlers. We also set m_mousedown  to true in the OnInitDialog handler because otherwise the first drag attempt will fail, as m_mousedown will still be false when the OnNcHitTest handler is called.

void CDragDialogDlg::OnLButtonDown(UINT nFlags, 
                                   CPoint point)
{   
    m_mousedown = true;     

    CDialog::OnLButtonDown(nFlags, point);
}

void CDragDialogDlg::OnLButtonUp(UINT nFlags, 
                                 CPoint point)
{
    m_mousedown = false;

    CDialog::OnLButtonUp(nFlags, point);
}

Draggable SDI windows

As I have mentioned earlier we cannot use the technique we used for dialog based applications here. The whole issue here is that the CView derived class is a wrapper for the view window and not for the main window of the application. The main window of an SDI application is wrapped by the CMainFrame class which is derived from CFrameWnd by the App wizard. All mouse clicks and movements within the view are handled by the view class and not by the frame window class. Well, this time we use another solution to settle our issue. We do the most obvious thing to do in the situation, which is that we move the window using code.

void CDragSDIView::OnLButtonDown(UINT nFlags, 
                                 CPoint point)
{
    m_mousedown = true; 
    ClientToScreen(&point);
    m_lastpoint = point;

    CView::OnLButtonDown(nFlags, point);
}

Just as in the previous case, we override OnLButtonDown. We set the m_mousedown flag to true. This time as you'll notice we also have a CPoint member variable called m_lastpoint in our CView derived class. We set m_lastpoint to the CPoint passed to us in OnLButtonDown.

void CDragSDIView::OnLButtonUp(UINT nFlags, 
                               CPoint point)
{   
    m_mousedown = false;

    CView::OnLButtonUp(nFlags, point);
}

OnLButtonUp is also overridden. But here we haven't done anything different from what we did in the case of the dialog based application. All our window moving work is done in the OnMouseMove function which we override as shown below.

void CDragSDIView::OnMouseMove(UINT nFlags, 
                               CPoint point)
{   
    CRect r;
    GetClientRect(&r);
    ClientToScreen(&r);
    ClientToScreen(&point);

    if(r.PtInRect(point))
    {       
        if(m_mousedown)
        {   
            AfxGetMainWnd()->GetWindowRect(&r); 

            AfxGetMainWnd()->MoveWindow(
                r.left - (m_lastpoint.x - point.x),
                r.top - (m_lastpoint.y - point.y),
                r.Width(),r.Height());
            m_lastpoint = point;                
        }
    }

    CView::OnMouseMove(nFlags, point);
}

Well, we first check to see if the point is within the client area of the view window.  If it is, then we check the m_mousedownflag is true. If m_mousedownflag is true, we figure out the current window coordinates by calling GetWindowRect on the main frame window. Now we call MoveWindow on the main frame window and pass it the new values which we calculate using the CPoint passed to us by OnMouseMove, the m_lastpoint member variable and the CRect obtained by calling GetWindowRect on the main frame window. And finally we set m_lastpoint to the new CPoint.

A much easier way - Roman Nurik

As I have mentioned in the introduction, there are multiple ways to skin cats, though why anyone would ever want to skin cats beat me. Cat skins are not exactly useful in my opinion. Alright, let's get to Roman's method for making draggable windows. First override WM_LBUTTONDOWN  and then send a WM_NCLBUTTONDOWN message to the main window of your application. For dialog apps, this will be the dialog window itself and for SDI apps, this will be your CMainFrame window. The WM_NCLBUTTONDOWN message is sent to a window when a left mouse click is made on the non client area of the window. The wParam specifies the hit-test enumeration value. We pass HTCAPTION and the lParam specifies the cursor position, which we pass as a 0 so that it's sure to be in the title bar. Thus we end up with one of the following implementations.

ReleaseCapture(); //This is not compulsory
POINT pt; 
GetCursorPos(&pt);
POINTS pts = {pt.x, pt.y}; 
::SendMessage(m_hWnd,WM_NCLBUTTONDOWN,HTCAPTION,(LPARAM)&pts);

ReleaseCapture(); //This is not compulsory
::SendMessage(m_hWnd,WM_NCLBUTTONDOWN,HTCAPTION,0);

m_hWnd is the window handle of the main application window. With MFC, you usually have a CWnd* and not an HWND. In such cases you can do an MFC CWnd version of the  SendMessage call.

pWnd->SendMesage(WM_NCLBUTTONDOWN,HTCAPTION,0);

Well, that was sure easier than the previously discussed techniques, wasn't it and thanks goes to Roman Nurik for this really cool tip. But then I guess each method would have it's pros and cons which may make themselves visible at random.

Another way - Albert Ling

Well, we come to that matter of cats and skins again. Here is yet another solution suggested by Albert Ling, that seems to me to be the best of all the methods we have investigated. He overrides OnNcHitTest and then calls the base class implementation. He checks the value returned by the base class implementation and if it is HTCLIENT, he returns HTCAPTION. This one is for dialog based applications.

UINT CYourDlg::OnNcHitTest(CPoint point)
{
    UINT hit = CDialog::OnNcHitTest(point);
    if ( hit == HTCLIENT ) 
    {
        return HTCAPTION;
    }
    else
        return hit;
}

If you thought Albert Ling's solution for dialog based apps was cool, you are yet to see his solution for SDI apps. It was simply amazing! This is what he did. He overrides OnNcHitTest in the CView derived class and calls the base class implementation first. If the base class call returns HTCLIENT, he returns HTTRANSPARENT. Now, HTTRANSPARENT indicates that the mouse action was on a window that's covered by another window, and thus the message gets sent to the underlying window in the thread, which in our case would be the main frame window. Thus we override OnNcHitTest in CMainFrame and call the base class method. If the base class method returns HTCLIENT, then we return HTCAPTION.

UINT CYourView::OnNcHitTest(CPoint point)
{
    UINT hit = CView::OnNcHitTest(point);
    if (hit == HTCLIENT )
        return HTTRANSPARENT;
    else
    {
        return hit;
    }
}

UINT CMainFrame::OnNcHitTest(CPoint point)
{
    UINT hit = CFrameWnd::OnNcHitTest(point);
    if ( hit == HTCLIENT ) 
    {
        return HTCAPTION;
    }
    else
        return hit;

}

Conclusion

Well, when I wrote this article I was under the thoroughly mistaken impression that my method was the only way to go about doing it. That's when Roman came and proved me wrong by offering his solution which was quite simpler to implement. Just when I was trying to be complacent about all this by talking about the cat-skins in the old adage, out comes Albert Ling with yet another amazing solution. Now I live in constant fear of being bombarded with other solutions and feel like a haunted man. Heheh. No, actually I don't. Was just kidding. If any of you have other solutions, feel free to suggest it here via the forum, or email me directly so that we can make this a single point source for all methods used for creating draggable windows.