XPCE Reference Manual: Section 1.134

1.134 class relation

A relation describes the reusable part of a constraint object. Class relation may be subclassed to define new relations. Subclasses normally redefine relation->create, relation->forward and/or relation->backwards.

The current system contains two types of relations, two of which are strictly between graphicals and one general. They are:

class spatial Spatial relation between two graphicals
class identity Identity of attributes.
New relations may be defined as user-defined subclasses of this class. A relation should define the methods:
relation->create Called when the relation is established
relation->forwards Called if the constraint<-from side changes.
relation->backwards Called if the constraint<-to side changes.

See also

class constraint

relation ->backwards: from=object, to=object: Called after to has changed. Its task is to update from accordingly.
relation ->create: from=object, to=object: Invokes relation->forwards to itself. This method is activated when the constraint object this relation is referred from is initialised.
relation ->forwards: from=object, to=object: Called after from has changed. Its task is to update to accordingly. If relation->create has not been redefined, this method is also called when the constraint object is established.