Answer Posted / saurabh shakya

n the preceding two examples, a CClientDC object was used to draw in the client area. This approach has the shortcoming that when the window is resized, the graphics are not repainted; in the preceding examples, they must be restored by again choosing the menu item. Using a CPaintDC object and putting the graphics calls into the OnPaint() function changes this behavior, so that the graphics are redrawn each time a WM_PAINT message is sent. This is done in all subsequent examples.

Windows OS sets the paint flag whenever a window gains one or more pixels. Windows OS sends a WM_PAINT message to the window whenever the client area needs to be repainted. When there are no higher-priority messages to be processed, the WM_PAINT message is processed and the window is repainted. The WM_PAINT message will be sent whenever any of the following events occur:

• The window has one or more invalid regions. (This is caused by one or more previously hidden areas of the window being brought back into view. Your window is brought back into view when a window that was above your window is closed or moved. Windows OS keeps track of all the invalid regions since the last repainting occurred and will repaint the invalid rectangle that encloses all invalid regions.)
• The user resizes the window.
• Part of the client area is scrolled.
• The application invalidates the client area, using one of the function calls: CWnd::Invalidate(), CWnd::InvalidateRect(), or CWnd::InvalidateRgn().

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