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The Pre-Display/Verify Event Point

"As far as I can see the reason that the pre-display / verify event point exists is because the access/verify in Speed II was there. Is this true? Also, I noticed that previous Speed II programmers put their error-checks in the pre-display/verify, while they should be in the Verify."

The answer to your question is both yes and no. The PRE-DISPLAY/VERIFY event point does exist because it existed in SPEED II. However, it also exists because it is quite often the appropriate place for commands which need to be executed by APPX INPUT processes.

There are three event points which complement each other in order to provide the flexibility actually needed when designing a well behaved, complex INPUT process. We will ignore the OPTION INTERCEPT event point (and others) for the purposes of this discussion. They are:

PRE-DISPLAY
PRE-DISPLAY/VERIFY
VERIFY

The VERIFY event point is the easiest to understand. It is executed after the operator enters data and presses return. It is not executed prior to displaying the screen. It is only executed after data entry. The primary purpose of this event point is to validate data entered and to allow for displaying of ERROR and WARNING messages. This event point did not exist in SPEED II.

The PRE-DISPLAY event point is a little more difficult to understand. It is executed only once when a record is initially retrieved in CHANGE or INQUIRE mode - just before the screen is displayed. In ADD mode it is executed once for each ENTRY LEVEL which occurs - just before the screen is displayed.

The primary purpose of the PRE-DISPLAY event point is to allow default values to be set, screen attributes such as NO INPUT, etc.

The PRE-DISPLAY/VERIFY event point is the most difficult to understand because it is more or less a combination of the other two event points although there are some differences. It is executed before the screen is displayed after retrieving a record (much like PRE-DISPLAY).

However, it is also executed after data entry - after the VERIFY event point - just before the screen is redisplayed. It can be executed multiple times for a record being processed, not just once like PRE-DISPLAY. This is intended to facilitate displaying of related information such as data lookups, results of calculations, etc.

This event point should not be used to validate data unless the ERROR and WARNING messages are intended to be displayed when the screen is initially displayed since they will be executed at that point as well.

ERROR and WARNING messages have effect only in ADD, CHANGE, and DELETE modes. They do not display messages in INQUIRE mode (varies somewhat by APPX release, some versions display error/warning messages & let the user continue, some don't display it at all).

If an application converted from SPEED II has ERROR and WARNING messages which are being executed at undesirable times, then those commands should be moved to the VERIFY event point.

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Topic revision: r2 - 2016-02-18 - JeanNeron
 
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