===== The <descriptorSwitch> Element =====
==== Definition and Usage ====

The [[descriptorswitch|<descriptorSwitch>]] element contains a set of alternative
descriptors to be associated with an object.  The choice is made during
presentation time.

Test rules used to choose the [[descriptor|<descriptor>]] element are defined by
[[rule|<rule>]] and [[compositerule|<compositeRule>]] elements; [[bindrule|<bindRule>]] child
elements associate these rules to child [[descriptor|<descriptor>]] elements of the
[[descriptorswitch|<descriptorSwitch>]] element.

The rules are evaluated in the order they are defined.  The first rule
evaluated as true determines the chosen [[descriptor|<descriptor>]] element.  If all
rules are evaluated as false, a default [[descriptor|<descriptor>]] element, defined
by the [[defaultdescriptor|<defaultDescriptor>]] element,is chosen; if the
[[defaultdescriptor|<defaultDescriptor>]] element is not defined no descriptor is selected
and the NCL player behaves as if the element does not exist.

==== Syntax ====
^ Element ^ Attributes ^ Content ^ Parent ^
| [[descriptorswitch|<descriptorSwitch>]] | __//id//__ | ([[defaultdescriptor|<defaultDescriptor>]]?, ([[bindrule|<bindRule>]] <nowiki>|</nowiki> [[descriptor|<descriptor>]])*) | [[descriptorbase|<descriptorBase>]] |
==== Attributes ====

^ Name ^ Value ^ Description ^
| //id// | It may receive any string value that begins with a letter or an underscore and that only contains letters, digits, <".">, and <"_">. | The //id// attribute uniquely identifies the element within a document. |