XML Schema: Extend xsd:choice so that union (not sequence) of choices is created

问题

I have the following XML Schema

<xsd:complexType name="SimpleThing">
    <xsd:choice maxOccurs="unbounded">
        <xsd:group ref="simpleContent" />
    </xsd:choice>
</xsd:complexType>

<xsd:complexType name="ExtendedThing">
    <xsd:complexContent>
        <xsd:extension base="SimpleThing">
            <xsd:choice maxOccurs="unbounded">
                <xsd:element name="qux" />
            </xsd:choice>
        </xsd:extension>
    </xsd:complexContent>
</xsd:complexType>

<xsd:group name="simpleContent">
    <xsd:choice>
        <xsd:element name="foo" />
        <xsd:element name="bar" />
    </xsd:choice>
</xsd:group>

<xsd:group name="extendedContent">
    <xsd:choice>
        <xsd:group ref="simpleContent" />
        <xsd:element name="qux" />
    </xsd:choice>
</xsd:group>

<xsd:element name="root" type="ExtendedThing" />

I would like the type ExtendedThing to contain foo, bar and qux in arbitrary order. But with this schema, qux must come after foo and bar because a sequence, not a union of the choices is created from the choice of the supertype and the choice of the subtype.

The alternative, replacing the type definition for ExtendedThing with

<xsd:complexType name="ExtendedThing">
    <xsd:complexContent>
        <xsd:extension base="SimpleThing">
            <xsd:group ref="extendedContent" />
        </xsd:extension>
    </xsd:complexContent>
</xsd:complexType>

doesn't work either, because the Unique Particle Attribution constraint is violated.

Is there any solution to this problem or is it impossible to create a type in XML Schema that is an extension of another type such that the content of the new type is a set of elements (in any order) that is a superset of the content of the supertype?

回答1:

This is not possible, for practical reasons. The content model of a super-type must always come in entirety before the content model of the sub-type. It helps to keep the implementation of validators fast.

来源：https://stackoverflow.com/questions/4216313/xml-schema-extend-xsdchoice-so-that-union-not-sequence-of-choices-is-created

标签

xsd

choice