Simulating CWinApp::OnIdle in C

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MFC allows you to override CWinApp::OnIdle function to perform idle-time processing.

This function is called whenever there’re no messages are waiting for processing in the message queue.

In this function, you can perform some secondary processing like updating the status bar, toolbar, etc.

The definition of this function is as follows:

virtual BOOL OnIdle(LONG lCount);

If you are interested you can override this function and do some processing. The following example paints random rectangle while the application is idle (that is no processing is carried on.) Thus, this code doesn’t make the application irresponsive.

	BOOL OnIdle(LONG lCount)

		CClientDC dc(m_pMainWnd);
		RECT rct;

			rand() % rct.right,
			rand() % rct.bottom,
			rand() % rct.right,
			rand() % rct.bottom);

		return TRUE;

This function receives only a single argument, lCount; it contains a value incremented each time OnIdle is called, and it is reset to 0 each time a new session is established. A new session is established each time your application finishes processing pending messages and no messages are left.

MFC continues to call CWinApp::OnIdle (incrementing lCount each time) as long as there’re no messages are waiting for processing. If the application received a new message, MFC ends the current session and stops calling OnIdle until it establishes a new session again. If you want, you can return TRUE to indicate that further processing is required and MFC should call OnIdle again (as long as we are in the current session,) or FALSE to indicate that processing have been finished and MFC should not call OnIdle again until a new session is established.

Notice that, you should call the base implementation of CWinApp::OnIdle because that the default implementation of OnIdle updates command UI objects like menu items and toolbars besides doing some internal structure cleanup.

Unfortunately, C doesn’t include OnIdle function. However, we could work out ours.

The following C example shows our new handmade message queue that simulates CWinApp::OnIdle:

	while (TRUE) {
		if (PeekMessage(&msg, 0, 0, 0, PM_REMOVE)) {

			// if a quit message
			if (WM_QUIT == msg.message)
				break; // exit the loop

			// process other messages
		else {	// no messages are waiting
			// do some idle processing

The key function is PeekMessage. This function checks the message queue and if a message was found it removes the message from the queue (if PM_REMOVE specified,) initializes the MSG object with message data, and returns TRUE to the caller. If no message was found, PeekMessage returns FALSE thus executing the code (secondary processing) in the else statement.

You can also create your fully-featured handmade OnIdle. Consider the following code:

BOOL OnIdle(LONG lCount);

int WINAPI WinMain(. . .)
	MSG msg;
	LONG lCount;

	. . .

	while (TRUE) {
		if (PeekMessage(&msg, 0, 0, 0, PM_REMOVE)) {
			// found a message, resetting
			lCount = 0;

			if (WM_QUIT == msg.message)

		else {
			if (lCount >= 0) {
				if (!OnIdle(lCount)) {
					// set a flag indicates
					// no further OnIdle
					// calls till a new
					// session
					lCount = -1;

				// increment the counter

	return msg.wParam;

BOOL OnIdle(LONG lCount)

	do some processing


	return FALSE;

Have a nice day!

Converting Colors to Gray Shades

هذه المقالة متوفرة أيضا باللغة العربية، اقرأها هنا.


Contents of this article:

  • Contents
  • Introduction
  • Colors in Grayscale
  • Detecting Print Preview Mode
  • Detecting Black-and-White Printers
  • Mixing all Together


This article discusses how you can display the page in print preview as grayscale if the printer is black-and-white. It discusses first how you can convert colors to grayscale. After that is discusses how to detect whether you are in print preview or not and whether the current printer is color or black-and-white printer. Let’s go€¦

Colors in Grayscale

If your application offers printing capability to the user, it should be aware of whether the user has a black-and-white or color printer while previewing the page. Of course, the user won’t be happy at all if he previewed his page in full-colors and printed it in black-and-white.

To solve this dilemma, you should render your print preview in grayscale if the user has a black-and-white printer and in full-colors if the user has a color printer.

The formula that converts a color to gray shade is very easy:

R/G/B = (red * 0.30) + (green * 0.59) + (blue * 0.11)

Set all the red, green, and blue to the sum of 30% of the red value, 59% of the green, and 11% of the blue.

For example, we can convert the color Magenta (0xFF, 0x00, 0xFF) to grayscale using the following steps:

-> 255 * 0.30 = 76
-> 0 * 0.59 = 0
-> 255 * 0.11 = 28
-> 76 + 0 + 28 = 104
-> R/G/B = 76

Thus, Magenta in grayscale equals to the RGB values 76, 76, 76.

The following function converts the color to grayscale:

	BYTE byColor;

	byColor =
		( GetRValue(cr) * 0.30 ) +
		( GetGValue(cr) * 0.59 ) +
		( GetBValue(cr) * 0.11 );

		RGB( byColor, byColor, byColor );

Detecting Print Preview Mode

If you have a CPrintInfo you can detect whether you are “print-previewing” or printing by checking the m_bPreview flag. If you don’t have a CPrintInfo (i.e. you are in the context of the OnDraw function) you can detect print preview mode by comparing CDC’s m_hDC and m_hAttribDC members.

MFC does some magic using m_hDC and m_hAttribDC. It uses m_hDC for output, while it uses m_hAttribDC for queries about DC attributes. How this helps?

If you are printing to the screen or to the printer, both m_hDC and m_hAttribDC will refer to the same HDC that’s used for drawing and retrieving attributes. On the other hand, while in print preview, MFC sets m_hDC to the window DC and sets m_hAttribDC to the HDC of the current printer. The results are unimaginable. If you are drawing, the calls are carried out to the screen. If you are querying about attributes (i.e. calling GetDeviceCaps,) the calls are carried out to the printer.

Therefore, you can detect print preview mode using a single line of code:

BOOL bPreview = (pDC->m_hDC != pDC->m_hAttribDC);

Or you can use the following code if you have a CPrintInfo:

BOOL bPreview = pInfo->m_bPreview;

Detecting Black-and-White Printers

Another point of interest is detecting whether the current printer is monochrome (black-and-white) or color printer.

This can be done through the GetDeviceCaps function with the NUMCOLORS item specified. It returns the number of colors if the device has a color depth of 8 bits per pixel or less. It’s not limited to printer DCs only. It can be used with display DCs too.

The following code detects if the device is monochrome:

BOOL bMono = (pDC->GetDeviceCaps(NUMCOLORS) == 2);

Mixing all Together

We have seen how to convert the color to grayscale, how to detect print preview mode, and how to detect a black-and white printer. Now, let’s mix them all together.

In OnDraw and OnPrint, we can solve the dilemma of black-and-white print-previewing by a simple change in the code. The following code segment sets the color based on the type of printer (it works fine too even if we are painting to the screen.)

BOOL bMono =
	(pDC->GetDeviceCaps (NUMCOLORS) == 2) &&
	(pDC->m_hDC != pDC->m_hAttribDC);

CBrush brush	(bMono ?
	GetGrayscale(RGB (255, 0, 255)) :
	RGB (255, 0, 255));

bMono is set to TRUE only if we are in print preview mode and the current printer is black-and-white printer.