Inside Macintosh: Overview /

Chapter 9 - Processes

Handling Null Events

Recall that the Event Manager sends your application a null event when there are no other events to report. The WaitNextEvent function reports a null event by returning a function result of FALSE and by setting the what field of the event record to nullEvt.

When your application receives a null event, it can perform idle processing. Your application should do only minimal processing in response to a null event, so that other processes can use the CPU and so that the foreground process (or your application, when it is in the foreground) can respond promptly to the user. For example, if your application is in the foreground when it receives a null event, you can make the insertion point blink in the active window (if your application supports text entry).

If your application receives a null event in the background, it can perform tasks or do other processing while in the background. However, your application should not perform any tasks that would slow down the responsiveness of the foreground process. Your application also should not interact with the user if it is in the background.

Note

Remember that your application receives null events while it is in the background only if you've set the canBackground flag in your application's 'SIZE' resource. If you don't want your application to receive null events when it is in the background, you should set the cannotBackground flag.

The Venn Diagrammer application uses null events in a somewhat interesting way. Whenever the application receives a null event, it calls the application-defined procedure DoIdle, which checks to see whether the user wants it to automatically adjust the Venn diagram and whether the diagram might need adjusting. If both of these are true, then DoIdle calls the application-defined procedure DoVennIdle to perform the automatic adjustment. The DoIdle procedure is defined in Listing 9-3.

Listing 9-3 Handling null events

PROCEDURE DoIdle (myEvent: EventRecord);
   VAR
      myWindow:   WindowPtr;
      myHandle:   MyDocRecHnd;
BEGIN
   myWindow := FrontWindow;
   IF IsAppWindow(myWindow) THEN
      IF gAutoAdjust THEN
         BEGIN
            myHandle := MyDocRecHnd(GetWRefCon(myWindow));
            IF myHandle^^.needsAdjusting THEN
               DoVennIdle(myWindow);
         END;
END;

The document record contains the field needsAdjusting, which is set to TRUE each time the user clicks anywhere within the Venn diagram circles. If the user's preference is for automatic diagram adjustment, then DoIdle calls the application-defined procedure DoVennIdle to adjust the diagram. Figure 9-2 shows the state of a diagram needing adjustment, and Figure 9-3 shows the same diagram after DoVennIdle has adjusted
the diagram.

Note

The DoVennIdle procedure is not defined in this book. In addition to determining whether and how to adjust the diagram, DoVennIdle resets the needsAdjusting field of the document record to FALSE.

Figure 9-2 A Venn diagram before automatic adjusting

Figure 9-3 A Venn diagram after automatic adjusting