InvalidateRgn

InvalidateRgn is a function in Microsoft Windows programming environments, commonly used with the Windows API. Its primary purpose is to mark a region of a window's client area as invalid. When a region is marked as invalid, it means that the content within that area is no longer considered up-to-date or accurate and needs to be redrawn.

The function takes a window handle and a region handle as parameters. The window handle identifies the specific window whose area needs to be invalidated, and the region handle specifies the precise shape and extent of the area to be invalidated. If the region handle is NULL, the entire client area of the window is invalidated.

Invalidating a region does not immediately cause the window to redraw. Instead, it adds the specified region to the window's update region. The Windows operating system will then process these update regions and generate a WM_PAINT message for the window when it's appropriate, typically when the application has no other pending messages to process. The window procedure for the window then handles this WM_PAINT message, which includes calls to the BeginPaint and EndPaint functions, allowing the application to redraw the invalidated portion of its client area. This mechanism is fundamental for efficient redrawing and ensures that only necessary parts of the window are repainted, improving performance.