WM_NCCALCSIZE

WM_NCCALCSIZE

The WM_NCCALCSIZE message is sent when the size and position of a window's client area must be calculated. By processing this message, an application can control the content of the window's client area when the size or position of the window changes.

WM_NCCALCSIZE

fCalcValidRects = (BOOL) wParam; // valid area flag

lpncsp = (LPNCCALCSIZE_PARAMS) lParam; // points to size calculation data

lpncsp = (LPRECT) lParam; // points to new window coordinates

Parameters

fCalcValidRects

Value of wParam. A Boolean value that, if TRUE, specifies that the application should indicate which part of the client area contains valid information. The operating system copies the valid information to the specified area within the new client area. Also, if this parameter is TRUE, lParam points to an NCCALCSIZE_PARAMS structure.

If this parameter is FALSE, the application does not need to indicate the valid part of the client area. Also, if this parameter is FALSE, lParam points to a RECT structure.

lpncsp

Value of lParam.

If wParam is TRUE, lParam points to an NCCALCSIZE_PARAMS structure that contains information an application can use to calculate the new size and position of the client rectangle.

If wParam is FALSE, lParam points to a RECT structure that contains the new coordinates of the window that has been moved or resized. This is equivalent to rgrc[0] of an NCCALCSIZE_PARAMS structure.

Return Values

If the fCalcValidRects parameter is FALSE, the application should return zero.

If fCalcValidRects is TRUE, the application can return zero or a valid combination of the following values:

Value		Meaning
WVR_ALIGNTOP, WVR_ALIGNLEFT, WVR_ALIGNBOTTOM, WVR_ALIGNRIGHT
	These values, used in combination, specify that the client area of the window is to be preserved and aligned appropriately relative to the new position of the window. For example, to align the client area to the lower-left corner, return the WVR_ALIGNLEFT and WVR_ALIGNTOP values.
WVR_HREDRAW, WVR_VREDRAW
	These values, used in combination with any other values, cause the window to be completely redrawn if the client rectangle changes size horizontally or vertically. These values are similar to the CS_HREDRAW and CS_VREDRAW class styles.
WVR_REDRAW
	This value causes the entire window to be redrawn. It is a combination of WVR_HREDRAW and WVR_VREDRAW values.
WVR_VALIDRECTS
	This value indicates that, upon return from WM_NCCALCSIZE, the rectangles specified by the rgrc[1] and rgrc[2] members of the NCCALCSIZE_PARAMS structure contain valid source and destination area rectangles, respectively. Windows combines these rectangles to calculate the area of the window to be preserved. Windows copies any part of the window image that is within the source rectangle and clips the image to the destination rectangle. Both rectangles are in parent-relative or screen-relative coordinates.
		This return value allows an application to implement more elaborate client-area preservation strategies, such as centering or preserving a subset of the client area.

If fCalcValidRects is TRUE and an application returns zero, the old client area is preserved and is aligned with the upper-left corner of the new client area.

Default Action

The window may be redrawn, depending on whether the CS_HREDRAW or CS_VREDRAW class style is specified. This is the default, backward-compatible processing of this message by the DefWindowProc function (in addition to the usual client rectangle calculation described in the preceding table).