callback function processes all messages sent to a Control Panel application
by a controlling application. The function expects to be sent messages in a
specific order. The controlling application expects the messages to be processed
in a specific way.
function receives the CPL_INIT
message when the controlling application first loads the Control Panel
application. The function should carry out any initialization, such as allocating
memory, and return nonzero. If CPlApplet
cannot complete the initialization, it must return zero, directing the
controlling application to terminate communication and release the DLL.
function receives the CPL_GETCOUNT
message only if the CPL_INIT message succeeded. The function must then return
the number of dialog boxes supported by the Control Panel application.
function receives one CPL_INQUIRE
message and one CPL_NEWINQUIRE
message for each dialog box supported by the Control Panel application. The
function fills in a CPLINFO
structure with information about your application, such as its name, icon,
and a descriptive string. Most applications should process the CPL_INQUIRE
message and ignore the CPL_NEWINQUIRE message. The CPL_INQUIRE message provides
information in a form that the controlling application can cache, resulting in much
better performance. The CPL_NEWINQUIRE message is useful only if you need to
change your application's icon or display strings based on the state of the
function receives a CPL_DBLCLK
message as a notification that the user has chosen the icon representing the
dialog box. The function may receive this message any number of times. The
message includes the dialog box identifier and the lData
value. The function should display the corresponding dialog box and process
subsequent user input.
Before the controlling application terminates, CPlApplet
receives the CPL_STOP
message once for each dialog box supported by the Control Panel application.
The message includes the identifier for the dialog box and the lData
value. The function should free any memory that it allocated for the
specified dialog box.
After the last CPL_STOP message, CPlApplet
receives a CPL_EXIT
message. The function should free all remaining allocated memory and
unregister any private window classes that it may have registered. Immediately after
the function returns from this message, the controlling application releases the
Control Panel application by calling the FreeLibrary
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