The framework calls this member function for the child window being deactivated and the child window being activated.
An MDI child window is activated independently of the MDI frame window. When the frame becomes active, the child window that was last activated with a OnMDIActivate call receives an WM_NCACTIVATE message to draw an active window frame and caption bar, but it does not receive another OnMDIActivate call.
This member function is called by the framework to allow your application to handle a Windows message. The parameters passed to your function reflect the parameters received by the framework when the message was received. If you call the base-class implementation of this function, that implementation will use the parameters originally passed with the message and not the parameters you supply to the function.