Display Device Contexts
Windows provides three types of device contexts for video displays: class,
common and private. Class and private device contexts are used in applications
that perform numerous drawing operations such as computer-aided design (CAD)
applications, desktop-publishing applications, drawing and painting applications,
and so on. Common device contexts are used in applications that perform
infrequent drawing operations.
An application obtains a display device context by calling either the
BeginPaint or
GetDC function and identifying the window in which the corresponding output will
appear. (The type of device context Windows returns is dependent on how the
application registered its window class.) Typically, an application obtains a
display device context only when it must draw in the client area. When the
application is finished drawing, it must release the device context by calling the
EndPaint or
ReleaseDC function.
Class Device Contexts
Class device contexts are supported strictly for compatibility with previous
versions of Windows. When writing a Win32-based application, avoid using the
class device context. Use a private device context instead.
Common Device Contexts
Common device contexts are display device contexts maintained in a special
cache by the window-management component of the Win32 API. An application obtains
a handle identifying one of the available common device contexts by calling the
GetDC,
GetDCEx, or
BeginPaint function. Before returning this device context handle, Windows initializes a
common device context with default objects, attributes, and modes. Any drawing
operations performed by the application use these defaults unless one of the
GDI functions is called to select a new object, change the attributes of an
existing object, or select a new mode.
Because only a limited number of common device contexts exist in the window
manager's heap, an application should release these device contexts after it has
finished drawing. An application releases a common device context by calling
the
ReleaseDC or
EndPaint function. When the application releases a common device context, any changes
to the default data are lost.
Private Device Contexts
Private device contexts are display device contexts that, unlike common device
contexts, retain any changes to the default data
even after an application calls the
ReleaseDC or
EndPaint function. Private device contexts are not part of the window manager's cache
and therefore need not be released after use. The window manager automatically
removes a private device context after the last window of that class has been
destroyed.
An application creates a private device context by first specifying the
CS_OWNDC window-class style when it initializes the
style member of the
WNDCLASS structure and calls the
RegisterClass function. (For more information about window classes, see
Window Classes.) After creating a window with the CS_OWNDC style, an application can call
the
GetDC,
GetDCEx, or
BeginPaint function once to obtain a handle identifying a private device context. The
application can continue using this handle (and the associated device context)
until it deletes the window created with this class. Any changes to graphic
objects and their attributes, or graphic modes are retained by Windows until the
window is deleted.
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