I have been working with GDI for a time now and I am trying to up my understanding of it to be able to create a wrapper class for it.
Why cant I hold on to a window device context until the app ends? Does it get invalidated upon any of the events? I heard someone say after WM_SIZE the context gets invalidated, is this true? I never experienced this even though I always cache the window context. It makes it much easier to createCompatibleDC()s and createCompatibleBitmaps()s in the constructor. I also cache these created DC and use them to avoid flickering, for local bounds drawing, caching the draw operation incase window is minimized then maximized.
Why would an application ever fetch more than 1 window context at a time? As the documemtation complains about running out the window context limit.
Remember, in the context you are talking about 'invalidate' means to require some portion of the window to be redrawn, it does not mean that a handle retrieved from GetDC will be rendered unusable.
A window with CS_HREDRAW or CS_VREDRAW will require at least partial redrawing on any resize in the indicated direction as well as any sizing operation that increases the window size.
Because of the limitations on the number of GDI objects the normal pattern is to handle WM_PAINT messages and use the handle returned by BeginPaint (which will only remain usable so long as EndPaint is not called).
And as Some programmer dude said in a comment, a second DC can be used to perform flicker-free drawing, the time consuming drawing is done to an off-screen DC and then simply copied onto the screen DC. And even if that is done entirely within a WM_PAINT handler the off-screen DC will normally be released once the drawing operation completes so as to not hog GDI resources.
I have a custom Windows control subclassed from CButton (no idea why that was selected--this is 17-year-old code; no semblance of button functionality is present).
Its DrawItem( LPDRAWITEMSTRUCT pdis ) method is called by CButton::OnChildNotify in response to WM_DRAWITEM. It renders its scene with the DC CDC::FromHandle( pdis->hDC ).
The mouse event method OnMouseMove() calculates the new cursor position and calls RedrawWindow( NULL, NULL, RDW_INVALIDATE ). A cursor that follows the mouse duly appears at the new mouse position. It works fine, but it's slow. In fact, only the previous and new cursor cells need be redrawn (if that) yet the graphic updates start to lag as the entire scene is rendered many times.
I thought in my OnMouseMove() method, instead of repainting the entire scene, could just paint the cells in question. It already has the exact X and Y coordinates of the cells and pointers to their data. I thought CPaintDC(this) would provide a DC that allowed this, but it doesn't paint. (Doesn't crash either, which is a rare joy.)
My hazy recollection is that the "optimal" way to do this would be to invalidate just the areas of the two cells, and the DrawItem() method would eventually be told these areas were invalidated, and rather than totally repainting it could just work out from the coordinates which cells they were (not an easy operation btw) and repaint them, and that would streamline not only this cursor problem but also ensure only a few cells be painted were the partially-obscured control partially revealed. But time pressure doesn't allow and the use cases don't seem to call for this to be optimized.
So the question is: is there some nice way for OnMouseMove() to re-render a single control immediately, and if so with what DC? (For instance can I cache the DC that I've received in DrawItem() via FromHandle()?
Right now the only idea I have is to have an object member pointing to a single cell to be redrawn, to call RedrawWindow() with this RDW_UPDATENOW flag, and have DrawItem(), if that flag be set, do just that one item. That would result in DrawItem() getting a DC that presumably would work in the way it always has. Seems like a real hack though, is there a better way?
In a Windows application, it is customary to perform all rendering in response to a WM_PAINT (or WM_NCPAINT) message. Code that needs to trigger a repaint marks part or all of window's client area as dirty, by calling InvalidateRect (and friends). The system is optimized for this approach, coalescing multiple requests into a single update region, and subsequently issuing a WM_PAINT message, when there is no more important work to do (like handling input).
This works reliably, and is usually easier to implement than spreading the rendering across several places. It is, however, perfectly legal to deviate from this, and perform rendering anywhere in your code. While WM_PAINT messages can still arrive at any time, it is desirable to have the out-of-band rendering produce identical visual results as the WM_PAINT handler would, to prevent visual artifacts.
All rendering goes through an abstraction called a device context (DC). When handling a WM_PAINT message in an MFC application, a suitable DC can be obtained by constructing a CPaintDC instance. When rendering anywhere else you cannot use a CPaintDC, but need to use a CClientDC instead (or a CWindowDC, to render the non-client area as well). In general, rendering code need not know, which type of DC it is rendering to, and can usually be reused without change.
Background
We are running our application in XenDesktop mode and our window shows some real time information. But if some other application is also launched in that XenDekstop and that application is overlapping our window then we want to stop rendering. And once its moved out of our window then we want to start rendering again. Unfortunately, right now these kind of notifications are not supported by Citrix.
Question
How can we detect when a part or the whole of the application window has been overlapped by other windows, and also detect when that's no longer the case?
I found the WindowFromPoint family of functions when Googling, however, that is not practical for my purpose because I'd need to keep polling all co-ordinates that my window covers.
Bonus points: For a start, it's enough if I can just detect when such overlapping occurs. However, if I can detect exactly which area(s) of my window is/are covered that would be great.
There is no such API function. And usually the it isn't needed. WM_PAINT cares for itself.
If you get a WM_PAINT message you receive a region and a update rectangle of the area that needs a repaint. But it is a rectangle only, no complex region. Also there is a clipping region too.
But it should be possible to calculate the region by yourself. If we are talking about a top level window.
Create a rectangular region that is consists of your window rect
Walk all top level windows from back to front
Ignore all windows until you find your top level window
For each visible top level window create a rectangular region and XOR it with your current one.
Should be easy with GetWindow GW_HWNDNEXT
The resulting region is what you are searching for.
Again: There is no such function or message that determine, that is fired or can be executed to find such overlapping. There is no need for such an information. The system cares for itself with the appropriate WM_PAINT message. If an area is covered. There is no need for an action. If an area is uncovered WM_PAINT gets fired.
I think you should be able to get this kind of information when processing the WM_PAINT message, since normally the clipping region would be set accordingly. Calls to the RectVisible() function should tell you, for any part of your window, whether it "should be painted" (and so, whether it was just uncovered).
Despite this is not a solution to the OP's problem, I want to remark that once an overlapping window reveals part of your window (and also if you drag more area of your window back to screen), you will get a WM_ERASEBKGND message before the WM_PAINT.
Is it necessary to invalidate the update region before immediate painting (in response to a user action) if I use Direct2D? Or is calling RenderTarget::BeginDraw() enough? I need to quickly repaint some part of the window outside of the WM_PAINT message. If I don't invalidate the update region, sometimes the whole client area of the window becomes black, no drawings are visible. But if I do invalidate the update region, the system sends a WM_PAINT message before I validate the update region back, which causes unnecessary drawing operations to be performed. How should I implement the immediate drawing operations outside the WM_PAINT message handler if I use Direct2D?
Invalidating and validating are ways to get a WM_PAINT message and to handle one. If you're painting outside of a WM_PAINT handler, you shouldn't be invalidating or validating as part of that drawing.
That said, it's very uncommon to paint outside of the WM_PAINT handler. It can be very hard to get it right. My advice would be to get everything working via a traditional WM_PAINT handler first, and then decide whether it's really necessary to do some painting outside that handler.
Typically when using Direct2D in a game-like application you perform no drawing in WM_PAINT and draw using Direct2D many times per second. Another common technique, for cases where you draw something once and do not create a new drawing many times per second is storing the drawing in a back buffer of some sort; that way you can 'blit' it to the screen in response to a WM_PAINT message.
Of course, this could be different for your case, depending on the type of program you are creating.
When the user grabs a corner of a resizable window, and then moves it, windows first moves the contents of the window around, then issues a WM_SIZE to the window being resized.
Thus, in a dialog where I want to control the movement of various child controls, and I want to eliminate flickering, the user first sees what windows OS thinks the window will look like (because, AFAICT, the OS uses a bitblt approach to moving things around inside the window before sending the WM_SIZE) - and only then does my dialog get to handle moving its child controls around, or resize them, etc., after which it must force things to repaint, which now causes flicker (at the very least).
My main question is: Is there a way to force windows NOT to do this stupid bitblt thing? Its definitely going to be wrong in the case of a window with controls that move as the window is resized, or that resize themselves as their parent is resized. Either way, having the OS do a pre-paint just screws the works.
I thought for a time that it might be related to CS_HREDRAW and CSVREDRAW class flags. However, the reality is that I don't want the OS to ask me to erase the window - I just want to do the repainting myself without the OS first changing the contents of my window (i.e. I want the display to be what it was before the user started resizing - without any bitblit'ing from the OS). And I don't want the OS to tell every control that it needs to be redrawn either (unless it happened to be one that was in fact obscured or revealed by the resize.
What I really want:
To move & resize child controls before anything gets updated onscreen.
Draw all of the moved or resized child controls completely so that they appear without artifacts at their new size & location.
Draw the spaces inbetween the child controls without impacting the child controls themselves.
NOTE: Steps 2 and 3 could be reversed.
The above three things appear to happen correctly when I use DeferSetWindowPos() in combination with the dialog resource marked as WS_CLIPCHILDREN.
I'd get an additional small benefit if I could do the above to a memory DC, and then only do a single bitblt at the end of the WM_SIZE handler.
I have played with this for a while now, and I cannot escape two things:
I still am unable to suppress Windows from doing a 'predictive bitblt'. Answer: See below for a solution that overrides WM_NCCALCSIZE to disable this behavior.
I cannot see how one can build a dialog where its child controls draw to a double buffer. Answer: See John's answer (marked as answer) below for how to ask Windows OS to double buffer your dialog (note: this disallows any GetDC() in-between paint operations, according to the docs).
My Final Solution (Thank you everyone who contributed, esp. John K.):
After much sweat and tears, I have found that the following technique works flawlessly, both in Aero and in XP or with Aero disabled. Flicking is non-existent(1).
Hook the dialog proc.
Override WM_NCCALCSIZE to force Windows to validate the entire client area, and not bitblt anything.
Override WM_SIZE to do all of your moves & resizes using BeginDeferWindowPos/DeferWindowPos/EndDeferWindowPos for all visible windows.
Ensure that the dialog window has the WS_CLIPCHILDREN style.
Do NOT use CS_HREDRAW|CS_VREDRAW (dialogs don't, so generally not an issue).
The layout code is up to you - its easy enough to find examples on CodeGuru or CodeProject of layout managers, or to roll your own.
Here are some code excerpts that should get you most of the way:
LRESULT ResizeManager::WinProc(HWND hwnd, UINT msg, WPARAM wparam, LPARAM lparam)
{
switch (msg)
{
case WM_ENTERSIZEMOVE:
m_bResizeOrMove = true;
break;
case WM_NCCALCSIZE:
// The WM_NCCALCSIZE idea was given to me by John Knoeller:
// see: http://stackoverflow.com/questions/2165759/how-do-i-force-windows-not-to-redraw-anything-in-my-dialog-when-the-user-is-resiz
//
// The default implementation is to simply return zero (0).
//
// The MSDN docs indicate that this causes Windows to automatically move all of the child controls to follow the client's origin
// and experience shows that it bitblts the window's contents before we get a WM_SIZE.
// Hence, our child controls have been moved, everything has been painted at its new position, then we get a WM_SIZE.
//
// Instead, we calculate the correct client rect for our new size or position, and simply tell windows to preserve this (don't repaint it)
// and then we execute a new layout of our child controls during the WM_SIZE handler, using DeferWindowPos to ensure that everything
// is moved, sized, and drawn in one go, minimizing any potential flicker (it has to be drawn once, over the top at its new layout, at a minimum).
//
// It is important to note that we must move all controls. We short-circuit the normal Windows logic that moves our child controls for us.
//
// Other notes:
// Simply zeroing out the source and destination client rectangles (rgrc[1] and rgrc[2]) simply causes Windows
// to invalidate the entire client area, exacerbating the flicker problem.
//
// If we return anything but zero (0), we absolutely must have set up rgrc[0] to be the correct client rect for the new size / location
// otherwise Windows sees our client rect as being equal to our proposed window rect, and from that point forward we're missing our non-client frame
// only override this if we're handling a resize or move (I am currently unaware of how to distinguish between them)
// though it may be adequate to test for wparam != 0, as we are
if (bool bCalcValidRects = wparam && m_bResizeOrMove)
{
NCCALCSIZE_PARAMS * nccs_params = (NCCALCSIZE_PARAMS *)lparam;
// ask the base implementation to compute the client coordinates from the window coordinates (destination rect)
m_ResizeHook.BaseProc(hwnd, msg, FALSE, (LPARAM)&nccs_params->rgrc[0]);
// make the source & target the same (don't bitblt anything)
// NOTE: we need the target to be the entire new client rectangle, because we want windows to perceive it as being valid (not in need of painting)
nccs_params->rgrc[1] = nccs_params->rgrc[2];
// we need to ensure that we tell windows to preserve the client area we specified
// if I read the docs correctly, then no bitblt should occur (at the very least, its a benign bitblt since it is from/to the same place)
return WVR_ALIGNLEFT|WVR_ALIGNTOP;
}
break;
case WM_SIZE:
ASSERT(m_bResizeOrMove);
Resize(hwnd, LOWORD(lparam), HIWORD(lparam));
break;
case WM_EXITSIZEMOVE:
m_bResizeOrMove = false;
break;
}
return m_ResizeHook.BaseProc(hwnd, msg, wparam, lparam);
}
The resizing is really done by the Resize() member, like so:
// execute the resizing of all controls
void ResizeManager::Resize(HWND hwnd, long cx, long cy)
{
// defer the moves & resizes for all visible controls
HDWP hdwp = BeginDeferWindowPos(m_resizables.size());
ASSERT(hdwp);
// reposition everything without doing any drawing!
for (ResizeAgentVector::const_iterator it = m_resizables.begin(), end = m_resizables.end(); it != end; ++it)
VERIFY(hdwp == it->Reposition(hdwp, cx, cy));
// now, do all of the moves & resizes at once
VERIFY(EndDeferWindowPos(hdwp));
}
And perhaps the final tricky bit can be seen in the ResizeAgent's Reposition() handler:
HDWP ResizeManager::ResizeAgent::Reposition(HDWP hdwp, long cx, long cy) const
{
// can't very well move things that no longer exist
if (!IsWindow(hwndControl))
return hdwp;
// calculate our new rect
const long left = IsFloatLeft() ? cx - offset.left : offset.left;
const long right = IsFloatRight() ? cx - offset.right : offset.right;
const long top = IsFloatTop() ? cy - offset.top : offset.top;
const long bottom = IsFloatBottom() ? cy - offset.bottom : offset.bottom;
// compute height & width
const long width = right - left;
const long height = bottom - top;
// we can defer it only if it is visible
if (IsWindowVisible(hwndControl))
return ::DeferWindowPos(hdwp, hwndControl, NULL, left, top, width, height, SWP_NOZORDER|SWP_NOACTIVATE);
// do it immediately for an invisible window
MoveWindow(hwndControl, left, top, width, height, FALSE);
// indicate that the defer operation should still be valid
return hdwp;
}
The 'tricky' being that we avoid trying to mess with any windows that have been destroyed, and we don't try to defer a SetWindowPos against a window that is not visible (as this is documented as "will fail".
I've tested the above in a real project that hides some controls, and makes use of fairly complex layouts with excellent success. There is zero flickering(1) even without Aero, even when you resize using the upper left corner of the dialog window (most resizable windows will show the most flickering and problems when you grab that handle - IE, FireFox, etc.).
If there is interest enough, I could be persuaded to edit my findings with a real example implementation for CodeProject.com or somewhere similar. Message me.
(1) Please note that it is impossible to avoid one draw over the top of whatever used to be there. For every part of the dialog that has not changed, the user can see nothing (no flicker whatsoever). But where things have changed, there is a change visible to the user - this is impossible to avoid, and is a 100% solution.
You can't prevent painting during resizing, but you can (with care) prevent repainting which is where flicker comes from. first, the bitblt.
There a two ways to stop the bitblt thing.
If you own the class of the top level window, then just register it with the CS_HREDRAW | CS_VREDRAW styles. This will cause a resize of your window to invalidate the entire client area, rather than trying to guess which bits are not going to change and bitblting.
If you don't own the class, but do have the ability to control message handling (true for most dialog boxes). The default processing of WM_NCCALCSIZE is where the class styles CS_HREDRAW and CS_VREDRAW are handled, The default behavior is to return WVR_HREDRAW | WVR_VREDRAW from processing WM_NCCALCSIZE when the class has CS_HREDRAW | CS_VREDRAW.
So if you can intercept WM_NCCALCSIZE, you can force the return of these values after calling DefWindowProc to do the other normal processing.
You can listen to WM_ENTERSIZEMOVE and WM_EXITSIZEMOVE to know when resizing of your window starts and stops, and use that to temporarily disable or modify the way your drawing and/or layout code works to minimize the flashing. What exactly you want to do to modify this code will depend on what your normal code normally does in WM_SIZE WM_PAINT and WM_ERASEBKGND.
When you paint the background of your dialog box, you need to not paint behind any of the child windows. making sure that the dialog has WS_CLIPCHILDREN solves this, so you have this handled already.
When you do move the child windows, Make sure that you use BeginDeferWindowPos / EndDefwindowPos so that all of the repainting happens at once. Otherwise you will get a bunch of flashing as each window redraws their nonclient area on each SetWindowPos call.
If I understood the question properly, it's exactly the question Raymond addressed today.
Here's a 2018 update, since I just ran through the very same gauntlet as you.
The "final solution" in your question, and the related answers, that mention tricks with WM_NCCALCSIZE and CS_HREDRAW|CS_VREDRAW are good for preventing Windows XP/Vista/7 from doing the BitBlt that molests your client area during resizing. It might even be useful to mention a similar trick: you can intercept WM_WINDOWPOSCHANGING (first passing it onto DefWindowProc) and set WINDOWPOS.flags |= SWP_NOCOPYBITS, which disables the BitBlt inside the internal call to SetWindowPos() that Windows makes during window resizing. This has the same eventual effect of skipping the BitBlt.
And some people mentioned that your WM_NCCALCSIZE trick no longer works in Windows 10. I think that might be because the code you wrote returns WVR_ALIGNLEFT|WVR_ALIGNTOP when it should be returning WVR_VALIDRECTS in order for the two rectangles you constructed (nccs_params->rgrc[1] and nccs_params->rgrc[2]) to be used by Windows, at least according to the very skimpy dox in the MSDN pages for WM_NCCALCSIZE and NCCALCSIZE_PARAMS. It's possible that Windows 10 is more strict about that return value; I would try it out.
However, even if we assume that we can convince Windows 10 not to do BitBlt inside SetWindowPos(), it turns out there's a new problem...
Windows 10 (and possibly also Windows 8) adds another layer of client area molestation on top of the old legacy molestation from XP/Vista/7.
Under Windows 10, apps do not draw directly to the framebuffer, but instead draw into offscreen buffers that the Aero Window manager (DWM.exe) composites.
It turns out that DWM will sometimes decide to "help" you by drawing its own content over your client area (sort of like a BitBlt but even more perverse and even further out of your control).
So in order to be free of client area molestation, we still need to get WM_NCCALCSIZE under control but we also need to prevent DWM from messing with your pixels.
I was fighting with exactly the same problem and created a roundup Question/Answer which brings together 10 years of posts on this topic and offers some new insights (too long to paste the content here in this question). The BitBlt mentioned above is no longer the only problem, as of Windows Vista. Enjoy:
How to smooth ugly jitter/flicker/jumping when resizing windows, especially dragging left/top border (Win 7-10; bg, bitblt and DWM)?
For some controls, you can use WM_PRINT message to make the control draw into a DC. But that doesn't really solve your primary problem, which is that you want Windows to NOT draw anything during resize, but to let you do it all.
And the answer is that you just can't do what you want as long as you have child windows.
The way I ended up solving this eventually in my own code is to switch to using Windowless Controls. Since they have no window of their own, they always draw at the same time (and into the same DC) as their parent window. This allows me to use simple double buffering to completely remove flicker. I can even trivially suppress painting of the children when I need to just by not calling their draw routine inside the parent's draw routine.
This is the only way I know of to completely get rid of flicker and tearing during resize operations.
If you can find a place to plug it in, CWnd::LockWindowUpdates() will prevent any drawing from occuring until after you unlock the updates.
But keep in mind this is a hack, and a fairly ugly one at that. Your window will look terrible during resizes. If the problem you are having is flickering during resizes, then the best thing to do is diagnose the flickering, rather than hiding the flickering by blocking paints.
One thing to look for are redraw commands that get called too often during the resize. If you r window's controls are calling RedrawWindow() with the RDW_UPDATENOW flag specified, it is going to repaint then and there. But you can strip out that flag and specify RDW_INVALIDATE instead, which tells the control to invalidate the window without repainting. It will repaint at idle time, keeping the display fresh without spazzing out.
There are various approaches, but I found the only one that can be used generally is double buffering: draw to an offscreen buffer, then blit the entire buffer to screen.
That comes for free in Vista Aero and above, so your pain might be shortlived.
I am not aware of a general double-buffering implementation for windows and system controls under XP, However, here are some things to explore:
Keith Rule's CMemDC for double-buffering anything you draw yourself with GDI
WS_EX_COMPOSITED Window style (see the remarks section, and something here on stackoverflow)
there is only one way to effectively diagnose repainting problems - remote debugging.
Get a 2nd PC. Install MSVSMON on it. Add a post build step or utility project that copies your build products to the remote PC.
Now you should be able to place breakpoints in WM_PAINT handlers, WM_SIZE handlers and so on and actually trace through your dialog code as it performs the size and redraw. If you download symbols from the MS symbol servers you will be able to see full call stacks.
Some well placed breakpoints - in your WM_PAINT, WM_ERAGEBKGND handlers and you should have a good idea of why your window is being synchronously repainted early during the WM_SIZE cycle.
There are a LOT of windows in the system that consist of a parent window with layered child controls - explorer windows are massivly complicated with listviews, treeviews preview panels etc. Explorer does not have a flicker problem on resizing, so It is celarly possible to get flicker free resizing of parent windows :- what you need to do is catch the repaints, figure out what caused them, and, well, ensure that the cause is removed.
What appears to work:
Use the WS_CLIPCHILDREN on the parent dialog (can be set in WM_INITDIALOG)
During WM_SIZE, loop through the child controls moving and resizing them using DeferSetWindowPos().
This is very close to perfect, in my testing under Windows 7 with Aero.