Palm Programming: The Developer's Guide

Other Times Your Application Is Called

The Palm OS makes a distinction between communicating with the active application and communicating with a possibly inactive application. In this first case, the active application is busy executing an event loop and can be communicated with by posting events to the event queue. As shown in Hello World, this was how our application got closed; the appStopEvent was posted to the event queue. When the active application gets that event, it quits.

Because there are other times that your application gets called by the Palm OS, there needs to be a way to communicate with it in those instances. First, let’s look at a list of the circumstances under which the system might want to talk to your application:

• When the user does a Find, the system must ask each installed application to look for any records that match the Find request.

• When beamed data is received, the system must ask the appropriate application (the one that is registered to receive the data) to handle the incoming item.

• When a synchronization occurs, each application is notified after its data has been synced.

• After a reset, each application is notified that a reset has occurred.

• If the system time or date changes, each application is notified.

• If the country changes, each application is notified.

In all these cases, a communication must take place to an inactive or closed application. The question is how the system does this. The answer is launch codes; all these communications are handled ...