c++ - InvalidateRect in Windows API, Charles Petzold checker4.c program -


full program here:

http://examples.oreilly.com/9781572319950/cd_contents/chap07/checker4/checker4.c

the code in childwndproc confuses me:

 case wm_lbuttondown :       setwindowlong (hwnd, 0, 1 ^ getwindowlong (hwnd, 0)) ;       setfocus (hwnd) ;       invalidaterect (hwnd, null, false) ;       return 0 ;             // focus messages, invalidate window repaint   case wm_setfocus:       idfocus = getwindowlong (hwnd, gwl_id) ;             // fall through   case wm_killfocus:       invalidaterect (hwnd, null, true) ;       return 0 ;

why in childwndproc in case of message wm_lbuttondown:

the last statement before return 0 invalidaterect, since right before program sends wm_setfocus message setfocus function, falls through wm_killfocus, has invalidaterect, last argument true, instead of false.

in understanding program should work without invalidaterect in wm_lbuttondown since calls setfocus, makes invalidate window, when comment out invalidaterect wm_lbuttondown, program doesn't repaint window after mouse clicks or button presses.

why , why in wm_lbuttondown invalidaterect last argument false , in wm_killfocus true?

i running ms visual studio c++ 2010 express in windows xp 32bit.

calling setfocus() doesn't unconditionally send wm_setfocus message. if window has focus before call, setfocus() not anything.

put way, wm_setfocus message sent if focus changed.

as true/false parameter, that's berase, whether erase whole window before re-drawing it, or whether paint on what's there. is, in book, performance hack. if know can away not erasing window (because updating small part of it), can pass true here.

i don't think in 2015 there's value in optimization. if need high performance display updates, you'd use opengl or directx anyway.


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