Simple event

Simple event - an event that occurs when the value of the specified property (which shall be called an event condition) changes to non-NULL. This event is a kind of extension of a basic event (adding an additional condition), which means that all the same parameters must be set as for an basic event, in particular:

• event type, which determines the point in time when this simple event will occur (including checking the condition).
• event handler - an action to be executed upon the occurrence of this simple event

Compared to a basic event, a simple event just wraps its handling in a loop operator whose condition is an event condition. However, simple events have a number of important additional features:

• If the condition does not contain the previous value operator (PREV), the platform itself wraps the specified condition in a change operator (SET). This significantly reduces the risk of creating incorrect handling (which will have consequences if the change session is empty)
• Such events are more understandable and readable, as they reflect the classic cause-effect relationship (when one thing occurs, another is done)
• For these events there is the possibility of "recalculation" - that is, executing handlers in a mode where all previous values are NULL (as if the database were empty). This is useful when an event is created for an existing database, and the old data must also follow the logic of this event.
• If the action in a simple event is a single change of a given property, this event is easily made calculated and vice versa.

Language

To create simple events, use the WHEN statement.

Examples

CLASS Stock;
name = DATA STRING[50] (Stock);

balance = DATA INTEGER (Sku, Stock);

// send an email when the balance is less than 0 as a result of applying session changes
WHEN balance(Sku s, Stock st) < 0 DO
      EMAIL SUBJECT 'The balance has become negative for the item ' + name(s) + ' in the warehouse ' + name(st);

CLASS OrderDetail;
order = DATA Order (OrderDetail) NONULL DELETE;
discount = DATA NUMERIC[6,2] (OrderDetail);

WHEN LOCAL CHANGED(customer(Order o)) AND name(customer(o)) == 'Best customer' DO
    discount(OrderDetail d) <- 50 WHERE order(d) == o;