Adding flexibility and facilitating object reuse – PB Docs 125

Adding flexibility and facilitating object reuse

You can use exception handling to add flexibility to your
PowerBuilder applications, and to help in the separation of business
rules from presentation logic. For example, business rules can be
stored in a non-visual object (nvo) that has:

  • An
    instance variable to hold a reference to the presentation object:

  • A function that registers the presentation object

    The registration function could use the following syntax:

  • Code to call a dynamic function implemented by the
    presentation object, with minimal assumptions about how the data
    is displayed

    The dynamic function call should be enclosed in a try-catch
    block, such as:

    This try-catch block catches all system and user-defined errors
    from the presentation object and throws them back up the calling
    chain (to the object that called the nvo). In the above example,
    the thrown object in the CATCH statement is an
    object of type Throwable, but you could also instantiate and throw
    a user exception object:

Code for data processing could be added to the presentation
object, to the business rules nvo, or to processing objects called
by the nvo. The exact design depends on your business objectives,
but this code should also be surrounded by try-catch blocks. The
actions to take and the error messages to report (in case of code
processing failure) should be as specific as possible in the try–catch
blocks that surround the processing code.

There are significant advantages to this type of approach,
since the business nvo can be reused more easily, and it can be
accessed by objects that display the same business data in many
different ways. The addition of exception handling makes this approach
much more robust, giving the application user a chance to recover
from an error condition.

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