add extends attribute to fields

nexusformat · Jan 16, 2025 · f3a1ff7 · f3a1ff7
1 parent fb7142d
commit f3a1ff7
Showing 1 changed file with 41 additions and 0 deletions.
diff --git a/nxdl.xsd b/nxdl.xsd
@@ -961,6 +961,47 @@ https://stackoverflow.com/a/48980995/1046449 -->
 						</xs:restriction>
 					</xs:simpleType>
 				</xs:attribute>
+				<xs:attribute name="extends" use="optional">
+					<xs:annotation>
+						<xs:documentation>
+							The ``extends`` attribute allows to specify which field
+							in the inheritance hierarchy this field is derived from.
+
+							This attribute is to be used in cases where there is an ambiguity
+							in the inheritance. An example are the ``AXISNAME`` and ``DATA``
+							fields in ``NXdata``, which are both of type ``NX_NUMBER``.
+							Thus, when specializing ``NXdata`` in an application definition,
+							``extends`` attribute allows to specify whether a field in the
+							specialized ``NXdata`` is an ``AXISNAME`` and ``DATA``.
+
+							For example, consider the following NXDL snippet:
+
+							<definition xmlns="http://definition.nexusformat.org/nxdl/3.1" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" category="application" name="NXiv_temp" extends="NXsensor_scan" type="group" xsi:schemaLocation="http://definition.nexusformat.org/nxdl/3.1 ../nxdl.xsd">
+								<doc>
+									Application definition for temperature-dependent IV curve measurements.
+								</doc>
+								<group type="NXentry">
+									<group type="NXdata">
+										<doc>
+											This NXdata contains multiple fields. Temperature and voltage are supposed to specialize `AXISNAME`, whereas
+											`current` is specializing `DATA`.
+										</doc>
+										<field name="temperature" type="NX_NUMBER" extends="/NXdata/AXISNAME"/>
+										<field name="voltage" type="NX_NUMBER" extends="/NXdata/AXISNAME"/>
+										<field name="current" type="NX_NUMBER" extends="/NXdata/DATA"/>
+									</group>
+								</group>
+							</definition>
+
+							Here, the ``extends`` attribute is used to specify that the ``temperature`` and ``voltage`` fields are specializations
+							of the ``AXISNAME`` field, whereas the ``current`` field is a specialization of the ``DATA`` field.
+
+							Note that the ``extends`` keyword shall only be used in cases where there is an ambiguity in the inheritance, such as
+							the example above. The ``extends`` attribute must not be used when such ambiguity does not exist. Especially, it is
+							not alllowed to specify the ``extends`` attribute for fields that are not part of the inheritance hierarchy.
+						</xs:documentation>
+					</xs:annotation>
+				</xs:attribute>
 				<xs:attributeGroup ref="nx:nameTypeAttributeGroup"/>
 			</xs:extension>
 		</xs:complexContent>