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The most common child  window are controls such as push buttons,radio buttons,
cheak boxes,scroll bars,edit boxes and list boxes that appears in dialog boxes. child 
window are never themselves active window.
a child window can have the input focus only if it is a descent of the active window. 
child window controls indicates  that teey have the input focus generally by displaying 
a flashing caretor a dotted line.
sometimes no window has the input focus. this is the case if all our programs have 
been minimized. window,but these messages are in different form keyboard message
 sent to active window that not minimized.
a window procedure can determine when its window has the input focus by trapping WM_SETFOCUS 
and WM_KILLFOCUS message.WM_SETFOCUS indicates that the window is receiving the input 
focus & WM_KILLFOCUS signals that the window is loging the input focus.
keystrokes and character
the message that an application receive from window about keybord events distinguish 
between keystrokes & character. this is in accordance with the two ways we can view the
 keuboard. we can think of 
keyboard as a collection of keys pressing & releasing key is keystroke. 