The WM_PAINT Message
Typically, an application draws in a window in response to a WM_PAINT
message. Windows sends this message to a window procedure when changes to the
window have altered the content of the client area. Windows sends the message
only if there are no other messages in the application message queue.
Upon receiving a WM_PAINT message, an application can call BeginPaint
to retrieve the display DC for the client area and use it in calls to GDI
functions to carry out whatever drawing operations are necessary to update the
client area. After completing the drawing operations, the application calls the EndPaint
function to release the display DC.
returns the display DC, Windows prepares the DC for the given window. It
first sets the clipping region for the DC to be equal to the intersection of the
portion of the window that needs updating and the portion that is visible to the
user. Only those portions of the window that have changed are redrawn. Attempts
to draw outside this region are clipped and do not appear on the screen.
Windows can also send WM_NCPAINT
messages to the window procedure before BeginPaint
returns. These messages direct the application to draw the nonclient area and
window background. The nonclient area
is the part of a window that is outside of the client area. The area includes
features such as the title bar, window menu (also known as the System menu),
and scroll bars. Most applications rely on the default window function, DefWindowProc
, to draw this area and therefore pass the WM_NCPAINT message to this
function. The window background
is the color or pattern a window is filled with before other drawing
operations begin. The background covers any images previously in the window or on the
screen under the window. If a window belongs to a window class having a class
background brush, the DefWindowProc
function draws the window background automatically.
fills a PAINTSTRUCT
structure with information such as the dimensions of the portion of the window
to be updated and a flag indicating whether the window background has been
drawn. The application can use this information to optimize drawing. For example,
it can use the dimensions of the update region, specified by the rcPaint
member, to limit drawing to only those portions of the window that need
updating. If an application has very simple output, it can ignore the update region
and draw in the entire window, relying on Windows to discard (clip) any
unneeded output. Because the system clips drawing that extends outside the clipping
region, only drawing that is in the update region is visible.
sets the update region of a window to NULL. This clears the region,
preventing it from generating subsequent WM_PAINT
messages. If an application processes a WM_PAINT message but does not call BeginPaint
or otherwise clear the update region, the application continues to receive
WM_PAINT messages as long as the region is not empty. In all cases, an
application must clear the update region before returning from the WM_PAINT message.
After the application finishes drawing, it should call EndPaint
. For most windows, EndPaint
releases the display DC, making it available to other windows. EndPaint
also shows the caret, if it was previously hidden by BeginPaint
hides the caret to prevent drawing operations from corrupting it.
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