Called by the framework after the view's document has been modified; this function is called by CDocument::UpdateAllViews and allows the view to update its display to reflect those modifications.
It is also called by the default implementation of OnInitialUpdate. The default implementation invalidates the entire client area, marking it for painting when the next WM_PAINT message is received. Override this function if you want to update only those regions that map to the modified portions of the document. To do this you must pass information about the modifications using the hint parameters.
To use lHint, define special hint values, typically a bitmask or an enumerated type, and have the document pass one of these values. To use pHint, derive a hint class from CObject and have the document pass a pointer to a hint object; when overriding OnUpdate, use the CObject::IsKindOf member function to determine the run-time type of the hint object.
Typically you should not perform any drawing directly from OnUpdate. Instead, determine the rectangle describing, in device coordinates, the area that requires updating; pass this rectangle to CWnd::InvalidateRect. This causes painting to occur the next time a WM_PAINT message is received.
If lHint is 0 and pHint is NULL, the document has sent a generic update notification. If a view receives a generic update notification, or if it cannot decode the hints, it should invalidate its entire client area.