How to repaint a child window only once every frame using WinAPI? - c++

Question

I'm using Direct3D and WinAPI to create windows and render 3D objects in the client area. I use the standard Windows message loop to invalidate the rectangle of the render window, and in the message handler of the render window, I perform a render call in Direct3D when processing the WM_PAINT message:
BOOL bRet;
HWND mainWnd; // Main window
HWND renderWnd; // Child of mainWnd, takes up a portion of the client area
// to be used as a Direct3D render target
MSG msg = {};
while ((bRet = GetMessage( &msg, NULL, 0, 0 )) != 0)
{
if(bRet != -1)
{
TranslateMessage(&msg);
DispatchMessage(&msg);
InvalidateRect(renderWnd, nullptr, false);
}
}
// ...
// Window procedure used by renderWnd
LRESULT renderWndProc(HWND hWnd, UINT uMsg, WPARAM wParam, LPARAM lParam)
{
switch (uMsg)
{
// ...
case WM_PAINT:
{
// Perform Direct3D rendering
// ...
}
break;
// ...
}
return DefWindowProc(hWnd, uMsg, wParam, lParam);
}
This setup seems to work correctly as long as my app only has one window, as InvalidateRect is called each frame and ensures that the client area needs to be redrawn, which in turn will result in a Direct3D draw call. It doesn't feel very elegant though, especially once I try to create multiple windows, as I'd have to invalidate their rectangles in that same piece of code, even if the features that the windows serve otherwise have nothing to do with each other (some of the windows might not even exist at any one time).
That said, my question is this: is it possible to have a window invalidate part of its client area exactly once each frame (assuming it's currently not minimized, etc.)? Maybe through the use of the message queue? Referring back to the above code segment: I'd want some way for mainWnd (perhaps in its own window procedure) to call InvalidateRect on renderWnd exactly once each frame.
Thanks in advance!
EDIT: small error in the code sample

Answer 1

Note that the 'standard' way to handle this is something more like this:
// Main message loop
MSG msg = {};
while (WM_QUIT != msg.message)
{
if (PeekMessage(&msg, nullptr, 0, 0, PM_REMOVE))
{
TranslateMessage(&msg);
DispatchMessage(&msg);
}
else
{
// Update your simulation/animation/etc. based on elapsed time
// -or-
// multiple fixed time-steps.
//
// Then render one frame.
}
}
…
LRESULT CALLBACK WndProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam)
{
PAINTSTRUCT ps;
HDC hdc;
switch (message)
{
case WM_PAINT:
hdc = BeginPaint(hWnd, &ps);
EndPaint(hWnd, &ps);
break;
This model is a 'flat-out' rendering model where you will render as fast as the system can do it, limited by Present details like refresh rate and automatic throttling if you have 3 or more frames ready.
You should take a look at GitHub for some more details like implementing WM_ENTERSIZEMOVE / WM_EXITSIZEMOVE.
The only time you'd implement WM_PAINT / InvalidateRect to render is if you are doing an editor or some 'mixed' UI and Direct3D application.

Answer 2

So with the help of the comments on the original post, I figured out that the best option was to use the Timers provided by WinAPI to schedule a repaint of the render window at the required framerate. Going to mark this as solved.