# Support for Mixin Classes

Due to the fact that several implementation platforms including XML Schema support only single inheritance (type derivation from a single base type, element substitutablity in XML Schema is restricted to a single element, too), the use of multiple inheritance is currently not supported by GML 3.2 Annex E.

However, for conceptual modelling, the ability to define abstract types which capture a set of properties that are associated with a concept is sometimes very convenient.

The following additional rules for such abstract types are therefore supported by ShapeChange:

- If a class is a specialization of another class, then this class shall have one of the stereotypes <<featureType>>, <<dataType>>, no stereotype, <<type>> or <<interface>>.

- The class shall have zero or one supertype with the same stereotype and zero or more abstract supertypes of the stereotype <<type>> or <<interface>>.

- I.e., disregarding classes with stereotype <<type>> or <<interface>>, a generalization relationship shall be specified only between two classes that are either:

  - both feature types (stereotype <<featureType>>),

  - both object types (no stereotype), or

  - both data types (stereotype <<dataType>>).

- For every class <<type>> or <<interface>> all direct or indirect subtypes shall be either

  - all feature or object types (stereotypes <<featureType>>, no stereotype, <<type>> or <<interface>>),

  - all data types (stereotypes <<dataType>>, <<type>> or <<interface>>).

- All generalization relationships between classes shall have no stereotype. The discriminator property of the UML generalization shall be blank.

The abstract mixin class (example in the GSIP: GeometryInfo, example in INSPIRE: HydroObject) are marked as mixin classes by a tagged value 'gmlMixin' set to 'true'.

In order for ShapeChange to recognize and process mixin classes, include the encoding rule rule-xsd-cls-mixin-classes in the XML Schema conversion rules. Another rule that - depending on the use case - can be included is rule-xsd-cls-mixin-classes-as-group.

Note | If rule-xsd-cls-mixin-classes is part of the conversion rules, then abstract <<type>> classes are treated as mixin classes, even if the 'gmlMixin' tagged value is not set.

By default, mixin classes are not to object elements and their properties are not processed. A property type with references to all subtypes with object elements is created. The properties are included in all non-mixin subtypes.

For example, the property type for the HydroObject mixin class is:

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      <complexType name="HydroObjectPropertyType">
     <choice minOccurs="0">
      <element ref="hy-p:DrainageBasin"/>
      <element ref="hy-n:HydroNode"/>
      ...
      <element ref="hy-n:WatercourseLinkSequence"/>
      <element ref="hy-n:WatercourseSeparatedCrossing"/>
     </choice>
     <attributeGroup ref="gml:AssociationAttributeGroup"/>
     <attributeGroup ref="gml:OwnershipAttributeGroup"/>
    </complexType>

And the content model of hy-p:DrainageBasin contains the HydroObject properties:

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      <complexType name="DrainageBasinType">
     <complexContent>
      <extension base="gml:AbstractFeatureType">
       <sequence>
        ...
        <element maxOccurs="unbounded" minOccurs="0" name="hydroId" nillable="true">
         <complexType>
          <sequence>
           <element ref="hy:HydroIdentifier"/>
          </sequence>
          <attribute name="nilReason" type="gml:NilReasonType"/>
         </complexType>
        </element>
        <element maxOccurs="unbounded" minOccurs="0" name="relatedHydroObject" type="gml:ReferenceType"/>
         <annotation>
          <appinfo>
           <targetElement xmlns="http://www.opengis.net/gml/3.2">hy:HydroObject</targetElement>
          </appinfo>
         </annotation>
        </element>
        ...
       </sequence>
      </extension>
     </complexContent>
    </complexType>

If the conversion rule "rule-xsd-cls-mixin-classes-as-group" is active, the mixin class is encoded as a group with all properties (attributes and navigable association ends) encoded as usual. This group will be referenced from subtypes.

The following example is for the mixin class GeometryInfo and the instantiable subtype PointGeometryInfo:

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      <group name="GeometryInfoGroup">
     <annotation>
      <documentation>Geometry Information: An abstract modeling entity serving as a superclass that collects shared properties (attributes and associations) of modeling entities that specify geometric representation information about a feature. [desc] For example, the horizontal and/or vertical metadata, notes, and/or restriction(s) and/or security control(s) applicable to dissemination of data regarding the geometric representation of the feature. [constraint] There exists an associated: Event Entity or Feature Entity</documentation>
      <appinfo>
       <sc:taggedValue tag="primaryCode">GeometryInfo</sc:taggedValue>
       <sc:taggedValue tag="secondaryCode">ZI029</sc:taggedValue>
       <sc:taggedValue tag="oclExpressions">inv: self.eventEntity-&gt;notEmpty() or self.featureEntity-&gt;notEmpty()</sc:taggedValue>
      </appinfo>
     </annotation>
     <sequence>
      <element maxOccurs="unbounded" minOccurs="0″ name="eventEntity" type="gml:ReferenceType">
       <annotation>
        <documentation>Geometry of Event Entity: An event for which this geometry representation applies.</documentation>
        <appinfo>
         <targetElement xmlns="http://www.opengis.net/gml/3.2″>gsip:EventEntity</targetElement>
         <reversePropertyName xmlns="http://www.opengis.net/gml/3.2″>gsip:geometry</reversePropertyName>
         <sc:taggedValue tag="primaryCode">eventEntity</sc:taggedValue>
        </appinfo>
       </annotation>
      </element>
      <element maxOccurs="unbounded" minOccurs="0″ name="featureEntity" type="gml:ReferenceType">
       <annotation>
        <documentation>Geometry of Feature Entity: A feature entity for which this geometry representation applies.</documentation>
        <appinfo>
         <targetElement xmlns="http://www.opengis.net/gml/3.2″>gsip:FeatureEntity</targetElement>
         <reversePropertyName xmlns="http://www.opengis.net/gml/3.2″>gsip:geometry</reversePropertyName>
         <sc:taggedValue tag="primaryCode">featureEntity</sc:taggedValue>
        </appinfo>
       </annotation>
      </element>
      <element name="horizontalCoordMetadata" type="gml:ReferenceType">
       <annotation>
        <documentation>Horizontal Coordinate Metadata: The horizontal coordinate metadata of this geometry.</documentation>
        <appinfo>
         <targetElement xmlns="http://www.opengis.net/gml/3.2″>gsip:HorizCoordMetadata</targetElement>
         <reversePropertyName xmlns="http://www.opengis.net/gml/3.2″>gsip:geometryInfo</reversePropertyName>
         <sc:taggedValue tag="primaryCode">horizontalCoordMetadata</sc:taggedValue>
        </appinfo>
       </annotation>
      </element>
      <!– … –>
     </sequence>
    </group>

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      <element name="PointGeometryInfo" substitutionGroup="gml:Point" type="gsip:PointGeometryInfoType"/>
    <complexType name="PointGeometryInfoType">
     <annotation>
      <documentation>Point Geometry Information: A modeling entity collecting geometric representation information about a feature that is modeled as a spatial point. [desc] A spatial point is a 0-dimensional geometric primitive, representing a position.</documentation>
      <appinfo>
       <sc:taggedValue tag="primaryCode">PointGeometryInfo</sc:taggedValue>
       <sc:taggedValue tag="secondaryCode">ZI007</sc:taggedValue>
      </appinfo>
     </annotation>
     <complexContent>
      <extension base="gml:PointType">
       <sequence>
        <group ref="gsip:GeometryInfoGroup"/>
        <!– … –>
       </sequence>
      </extension>
     </complexContent>
    </complexType>