Change operators (SET, CHANGED, ...)

Change operators determine whether some types of changes have occurred for a certain property in the current session. All these operators are derived from the previous value operator (PREV), however, it is recommended to use them to improve readability and performance. The following table shows the supported types of changes and their description:

Operator	Value	Description
SET	f AND NOT PREV(f)	Value is set
DROPPED	NOT f AND PREV(f)	Value is reset
CHANGED	(f OR PREV(f)) AND NOT f==PREV(f)	Value is changed
SETCHANGED	f AND NOT f==PREV(f) or CHANGED(f) AND NOT DROPPED(f)	Value is changed to non-NULL
DROPCHANGED	CHANGED(f) AND NOT SET(f)	Value is either reset or changed from one non-NULL to another non-NULL
SETDROPPED	SET(f) OR DROPPED(f)	Value is either reset or changed from NULL to non-NULL

warning

These operators are computed differently inside the event handler: in this case, they return changes from the point of the previous occurrence of this event, or rather, from the point at which all its handlers are completed.

Language

To declare a property using change operators, the following syntax constructs are used.

Examples

quantity = DATA NUMERIC[14,2] (OrderDetail);
price = DATA NUMERIC[14,2] (OrderDetail);
sum(OrderDetail d) <- quantity(d) * price(d) WHEN CHANGED(quantity(d)) OR CHANGED(price(d));

createdUser = DATA CustomUser (Order);
createdUser (Order o) <- currentUser() WHEN SET(o IS Order);

numerator = DATA Numerator (Order);
number = DATA STRING[28] (Order);
series = DATA BPSTRING[2] (Order);
WHEN SETCHANGED(numerator(Order o)) AND
     NOT CHANGED(number(o)) AND
     NOT CHANGED(series(o))
     DO {
        number(o) <- curStringValue(numerator(o));
        series(o) <- series(numerator(o));
        incrementValueSession(numerator(o));
     }
;