Errors when calling functions and events dynamically – PB Docs 126

Functions

The rules for functions are similar to those for events, except
functions must exist: if a function is not found, an error always
occurs. Although events can exist without a script, if a function
is defined it has to have code. Consider the following statements:

1. This statement calls a function without looking
   for a return value:

   PowerBuilder

   object.DYNAMIC funcname( )

   1	object.DYNAMIC funcname( )

2. This statement looks for an integer return
   value:

   PowerBuilder

   int li_int

   1	int li_int

   PowerBuilder

   li_int = object.DYNAMIC funcname( )

   1	li_int = object.DYNAMIC funcname( )

3. This statement looks for an Any return
   value:

   PowerBuilder

   any la_any

   1	any la_any

   PowerBuilder

   la_any = object.DYNAMIC funcname( )

   1	la_any = object.DYNAMIC funcname( )

The following table uses these statements as examples.

Events

Consider these statements:

1. This
   statement calls an event without looking for a return value:

   PowerBuilder

   object.EVENT DYNAMIC eventname( )

   1	object.EVENT DYNAMIC eventname( )

2. This example looks for an integer return
   value:

   PowerBuilder

   int li_int

   1	int li_int

   PowerBuilder

   li_int = object.EVENT DYNAMIC eventname( )

   1	li_int = object.EVENT DYNAMIC eventname( )

3. This example looks for an Any return
   value:

   PowerBuilder

   any la_any

   1	any la_any

   PowerBuilder

   la_any = object.EVENT DYNAMIC eventname( )

   1	la_any = object.EVENT DYNAMIC eventname( )

The following table uses these statements as examples.

When an error occurs

You can surround a dynamic function call in a try-catch block
to prevent the application from terminating when an execution error
occurs. Although you can also handle the error in the SystemError
event, you should not allow the application to continue once the
SystemError event is invoked—the SystemError event should
only clean up and halt the application.

For information on using try-catch blocks, see the chapter
on exception handling in Application Techniques.

If the arguments do not match

Function arguments are part of the function’s definition.
Therefore, if the arguments do not match (a compatible match, not
an exact match), it is essentially a different function. The result
is the same as if the function did not exist.

If you call an event dynamically and the arguments do not
match, the call fails and control returns to the calling script.
There is no error.

Error-proofing your code

Calling functions and events dynamically opens up your application
to potential errors. The surest way to avoid these errors is to
always make static calls to functions and events. When that is not
possible, your design and testing can ensure that there is always
an appropriate function or event with the correct return datatype.

One type of error you can check for and avoid is data conversion
errors.

The preceding tables illustrated that a function or event
can return a null value either as an Any variable
or as a variable of the expected datatype when a function or event
definition exists but is not implemented.

If you always assign return values to Any variables
for dynamic calls, you can test for null (which
indicates failure) before using the value in code.

This example illustrates the technique of checking for null before
using the return value.