relation.md
In rswipl: Embed 'SWI'-'Prolog'

class relation {#class-relation}

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

->create Called when the relation is established
->forwards Called if the constraint <-from side changes.
->backwards Called if the constraint <-to side changes.

@see 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 ->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 ->create has not been redefined, this method is also called when the constraint object is established.