How to declare an xs:element with two different sets of attributes? - xsd

Question

My new XML private language includes elements <figure>, representing illustrations (image + caption).
Whenever illustrations refer to some image in the local database I just want to type
<figure id="9809" width="full" />
to identify image number 9809 and its associated caption.
On the other side, if images come from outside I will need a slightly different syntax:
<figure href="https://some-url-here" width="full">Some ad hoc catpion</figure>
So far I have declared an element that combine both behaviours, like this:
<!-- Figures -->
<xs:simpleType name="FigureWidthEnum">
<xs:restriction base="xs:token">
<xs:enumeration value="center" />
<xs:enumeration value="half" />
<xs:enumeration value="full" />
</xs:restriction>
</xs:simpleType>
<xs:element name="figure">
<xs:complexType mixed="true">
<xs:complexContent>
<xs:extension base="Inline">
<xs:attribute name="href" type="URI" />
<xs:attribute name="id" type="xs:nonNegativeInteger" />
<xs:attribute name="width" type="FigureWidthEnum" default="full" />
</xs:extension>
</xs:complexContent>
</xs:complexType>
</xs:element>
It works fine but a fresh editor can mess with the 3 attributes and type impossible things I don't want to pass the Schema Validator that easy. For example:
<figure id="9809" width="full" href="https://some-unexpected-url">Some unexpected caption that should not be here</figure>
I want to have two completely separate sintaxes for <figure>, as if I could declare these two elements with the same name:
<xs:element name="figure">
<xs:complexType mixed="true">
<xs:complexContent>
<xs:extension base="Inline">
<xs:attribute name="href" type="URI" />
<xs:attribute name="width" type="FigureWidthEnum" default="full" />
</xs:extension>
</xs:complexContent>
</xs:complexType>
</xs:element>
<xs:element name="figure">
<xs:complexType>
<xs:attribute name="id" type="xs:nonNegativeInteger" />
<xs:attribute name="width" type="FigureWidthEnum" default="full" />
</xs:complexType>
</xs:element>
In fact it is not possible.
Can it be done somehow?

Answer 1

Yes, it is possible with XSD 1.1, which provides <xs:alternative> element intended specifically for such requirements. Here's a complete XML schema I've designed to validate exactly as you need:
<?xml version="1.0" encoding="utf-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
xmlns:vc="http://www.w3.org/2007/XMLSchema-versioning"
vc:minVersion="1.1">
<!-- Figures -->
<xs:simpleType name="FigureWidthEnum">
<xs:restriction base="xs:token">
<xs:enumeration value="center" />
<xs:enumeration value="half" />
<xs:enumeration value="full" />
</xs:restriction>
</xs:simpleType>
<!-- stub definition -->
<xs:complexType name="Inline"/>
<!-- 'figure' element is declared once, but its type has two alternatives -->
<xs:element name="figure">
<!-- The first alternative is selected, when the condition specified
in 'test' attribute is true. The condition must be a boolean XPath
expression. The '#id' returns the value of 'id' attribute. However,
when converted to boolean, it indicates whether 'id' attribute is
specified at all. The anonymous complexType inside <xs:alternative>
provides the element type valid for this case.
-->
<xs:alternative test="#id">
<xs:complexType>
<xs:attribute name="id" type="xs:nonNegativeInteger" />
<xs:attribute name="width" type="FigureWidthEnum" default="full" />
</xs:complexType>
</xs:alternative>
<!-- The second alternative has no 'test' attribute. That means, it must
be selected by default, when all other alternatives (with a specified
test condition) do not pass. Here, the anonymous complexType inside
<xs:alternative> defines the element type in case of reference:
when 'href' is present.
-->
<xs:alternative>
<xs:complexType mixed="true">
<xs:complexContent>
<xs:extension base="Inline">
<xs:attribute name="href" type="xs:anyURI" />
<xs:attribute name="width" type="FigureWidthEnum" default="full" />
</xs:extension>
</xs:complexContent>
</xs:complexType>
</xs:alternative>
</xs:element>
<!-- this element is defined just to test the whole schema in XML below -->
<xs:element name="figures">
<xs:complexType>
<xs:sequence>
<xs:element ref="figure" minOccurs="0" maxOccurs="unbounded"/>
</xs:sequence>
</xs:complexType>
</xs:element>
</xs:schema>
A complete XML file this schema validates:
<?xml version="1.0" encoding="UTF-8"?>
<figures>
<!-- passes validation -->
<figure id="9809" width="full" />
<!-- passes validation -->
<figure width="full" href="https://some-unexpected-url">Some ad hoc caption</figure>
<!-- does not pass the validation -->
<figure id="9809" width="full" href="https://some-unexpected-url">
Some unexpected caption that should not be here
</figure>
</figures>
The validation was done with Apache Xerces 2.11.0 (XSD 1.1 aware).
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