If a runtime error occurs, an error structure that describes the error is created. An error can occur when an expression that uses dot notation to refer to data and properties of a DataWindow object is invalid under some runtime conditions. The DataWindow error Event is triggered, with the information in the error structure as arguments.
The error can be handled in this Error event by use of a special reference argument that allows the error to be ignored. If the error does not occur in the context described above, or if the error in that context is not dealt with, then the error structure information is used to populate the global error variable, and the SystemError event on the Application object is triggered.
In the SystemError event, unexpected error conditions can be dealt with in a limited way. In general, it is not a good idea to continue running the application after the SystemError event is triggered. However, error-handling code can and should be added to this event. Typically you could use the SystemError event to save data before the application terminates and to perform last-minute cleanup (such as closing files or database connections).
If you write code in the Error event, then that code is executed first in the event of a thrown exception.
If the exception is not thrown in any of the described contexts, or the object’s Error event does not handle the exception, or you do not code the Error event, then the exception is handled by any active exception handlers (CATCH clauses) that are applicable to that type of exception. Information from the exception class is copied to the global error variable and the SystemError event on the Application object is fired only if there are no exception handlers to handle the exception.
In PocketBuilder applications, you can handle errors by using try-catch blocks or by coding the Error event. You should always have a SystemError event coded in your Application object to handle any uncaught exceptions. The SystemError event essentially becomes a global exception handler for a PocketBuilder application.
