Message Processing - Win32 Programmer's Reference

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Message Processing

The CPlApplet 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.

The CPlApplet 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.

The CPlApplet 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.

The CPlApplet 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 or NEWCPLINFO 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 computer.

The CPlApplet 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 function.

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