About Screen Savers
The Desktop application in Windows Control Panel lets users select from a list
of screen savers, specify how much time should elapse before the screen saver
is started, configure screen savers, and preview screen savers. Screen savers
are loaded automatically when Windows starts or when a user activates the screen
saver feature by using Control Panel.
Once a screen saver is chosen, Windows monitors keystrokes and mouse
movements, starting the screen saver after a period of inactivity. However, Windows does
not start the screen saver if any of the following conditions exist:
- The active application is not a Windows-based application.
- A computer-based training (CBT) window is present.
- The active application receives the WM_SYSCOMMAND message with wParam set to the SC_SCREENSAVE value, but does not pass the message to the DefWindowProc function.
Screen savers contain specific exported functions, resource definitions, and
variable declarations. The static-link library SCRNSAVE.LIB contains the
main function and other startup code required for a screen saver. When a screen
saver starts, the startup code in SCRNSAVE.LIB creates a full-screen window. The
window class for this window is declared as follows.
WNDCLASS cls;
cls.hCursor = NULL;
cls.hIcon = LoadIcon(hInst, MAKEINTATOM(ID_APP));
cls.lpszMenuName = NULL;
cls.lpszClassName = "WindowsScreenSaverClass";
cls.hbrBackground = GetStockObject(BLACK_BRUSH);
cls.hInstance = hInst;
cls.style = CS_VREDRAW | CS_HREDRAW |
CS_SAVEBITS | CS_DBLCLKS;
cls.lpfnWndProc = (WNDPROC) ScreenSaverProc;
cls.cbWndExtra = 0;
cls.cbClsExtra = 0;
To create a screen saver, most developers create a source-code module
containing three required functions and link them with SCRNSAVE.LIB. A screen saver
module is responsible only for configuring itself and for providing visual
effects.
One of the three required functions in a screen saver module is
ScreenSaverProc. This function processes specific messages and passes any unprocessed
messages back to SCRNSAVE.LIB. Following are some of the typical messages processed by
ScreenSaverProc.
Message
| Meaning
|
WM_CREATE
| Retrieve any initialization data from the file REGEDIT.INI. Set a window timer
for the screen saver window. Perform any other required initialization.
|
WM_ERASEBKGND
| Erase the screen saver window and prepare for subsequent drawing operations.
|
WM_TIMER
| Perform drawing operations.
|
WM_DESTROY
| Destroy the timer(s) created when the application processed the WM_CREATE message. Perform any additional required cleanup.
|
ScreenSaverProc passes unprocessed messages to SCRNSAVE.LIB by calling the
DefScreenSaverProc function. The following table describes how this function processes various
messages.
The second required function in a screen saver module is
ScreenSaverConfigureDialog. This function displays a dialog box that enables the user to configure the
screen saver. (Thus, an application must provide a corresponding dialog box
template.) Windows displays the configuration dialog box when the user chooses the
Setup button in the Screen Saver box in the Desktop application in Control
Panel. The data entered by the user with the configuration dialog box is stored in
the REGEDIT.INI file.
The third required function in a screen saver module is
RegisterDialogClasses. This function must be called by all screen saver applications. However,
applications that do not require special windows or custom controls in the
configuration dialog box can simply return TRUE. Applications requiring custom controls
or special windows should use this function to register the corresponding
window classes.
In addition to creating a module that supports the three functions just
described, a screen saver should supply an icon. This icon is visible only when the
screen saver is run as a standalone application. (To be run by Control Panel, a
screen saver must have the .SCR filename extension; to be run as a standalone
application, it must have the .EXE filename extension.) The icon must be
identified in the screen saver's resource file by the constant ID_APP, which is
defined in the header file SCRNSAVE.H.
One final requirement is a screen saver description string. The resource file
for a screen saver must contain a descriptive string that Control Panel
displays as the screen saver name. The description string must be the first string in
the resource file's string table (identified with the ordinal value 1).
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