Lecture 2: Windows Basics

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Client Area of Window Everything except title
bar, window edge, scrollbars.
Client Area
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Invalid Rectangle

• Smallest rectangle that covers all invalid areas
  called invalid rectangle. Causes WM_PAINT message
  to be generated.
• Continue to be generated until invalid rectangle
  becomes validated.
• BeginPaint(hwnd, ps)
• EndPaint(hwnd, ps)
• Treat as pair. Validates invalid rectangle.

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Invalid Rectangle
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Invalid Region
Invalid Region
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Invalid Rectangle Smallest rectangle the covers
all invalid regions.
(Does not include title bar and SB)
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WM_PAINT Message

• Windows does not save contents of window.
• Generates WM_PAINT message when any part of the
  window becomes invalid.
• When previously hidden area becomes visible.
• When user resizes window (and class style set
  with CS_HREDRAW CS_VREDRAW).
• Application calls
• InvalidateRect, InvalidateWindow, ScrollWindow,
  ScrollDC.

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Device Context

• Hardware device and device driver.
• Application gets a handle to a device context.
• Must obtain hdc before writing text or graphics
  to window.
• Device context determines
• Area of window accessible to application.
• Default graphical attributes
• Text size and color.
• Text font.

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Obtaining Handle to DC

• hdc BeginPaint(hwnd, ps)
• Actions
• 1. Validates any invalid areas.
• 2. Erases window background (Most of the
  time).
• 3. Fills in fields of ps.
• 4. CD restricts outputting to invalid
  rectangle.

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typedef struct tagPAINTSTRUCT HDC hdc
BOOL fErase //Background needs erasing?
RECT rcPaint // Coords of invalid rectangle
. // Other fields. PAINTSTRUCT
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Invalid Rectangle Clipping Region for hdc
obtained by BeginPaint/EndPaint.
(Does not include title bar and SB)
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A Different Device Context

• hdc GetDC(hwnd)
• ReleaseDC(hwnd, hdc)
• Allows application to write anywhere within the
  client area. That is, clipping region is the
  client area of window.
• Does not validate any invalid areas.

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HDC returned by GetDC/ReleaseDC has a clipping
region equal to the entire client area of the
window.
Client Area
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A Couple of Bad Ideas
switch(message) case WM_PAINT hdc
GetDC(hwnd) ReleaseDC(hwnd, hdc)
return 0
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switch(message) case WM_PAINT return 0

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One approach that will work
switch(message) case WM_PAINT hdc
GetDC(hwnd) . ReleaseDC(hwnd,
hdc) ValidateRect(hwnd, NULL)
return 0
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InvalidateRect (hwnd, NULL, TRUE)
hwnd Handle to window. NULL Invalidate entire
client area of the window. Alternatively
InvalidateRect(hwnd, rect, TRUE) TRUE/FALSE
Erase/Do not erase the background of the window.
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• TextOut (hdc, x, y, buf, length)