XSD 声明不能有子元素或其他内容的元素的语法

Question

Would this be the proper method of declaring an XML element Foo that cannot have a child element or other content?

<xs:element name="Foo" type="xs:string" fixed="" nillable="true" />

The valid examples of this element in an XML document would be:

<Foo></Foo>

and

<Foo/>

Anything else would be invalid, eg:

<Foo>stuff</Foo>

It would be nice, though not essential, that the following be valid

<Foo>
</Foo>

Answer 1

I combined a few options into a single schema and attempted to use it to validate some test elements.

Sources: http://www.w3schools.com/schema/schema_complex_empty.asp

http://www.herongyang.com/XML-Schema/complexType-Empty-Element-Declaration.html

The schema:

<?xml version="1.0" encoding="ISO-8859-1" ?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
  <xs:element name="root">
    <xs:complexType>
      <xs:choice maxOccurs="unbounded">
        <xs:element name="foo">
          <xs:complexType></xs:complexType>
        </xs:element>
        <xs:element name="y1">
          <xs:simpleType>
            <xs:restriction base="xs:string">
              <xs:length value="0" />
            </xs:restriction>
          </xs:simpleType>
        </xs:element>
        <xs:element name="y2">
          <xs:complexType>
            <xs:complexContent>
              <xs:restriction base="xs:anyType" />
            </xs:complexContent>
          </xs:complexType>
        </xs:element>
      </xs:choice>
    </xs:complexType>
  </xs:element>
</xs:schema>

Test cases:

<?xml version="1.0" encoding="UTF-8"?>
<root xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
xsi:noNamespaceSchemaLocation="NewXMLSchema.xsd">
 <!--  These six are all valid -->
  <foo />
  <y1 />
  <y2 />
  <foo></foo>
  <y1></y1>
  <y2></y2>
  <!-- These three are all invalid! -->
  <foo>
  </foo>
  <y1>
  </y1>
  <y2>
  </y2>
  <!--  These are invalid too. -->
  <foo>invalid</foo>
  <y1>invalid</y1>
  <y2>invalid</y2>
</root>

It looks like any of the three element declarations would do what you want, except for the extra blank line feature.

I wouldn't include type="xs:string", fixed="", or nillable="true" because these are all semantically different from an empty tag. An empty tag wouldn't really have a type (like string) because it's empty. It also wouldn't have a fixed value of the null string, because that's different from being empty. A nillable element is semantically different from an empty tag because it is a semantically equivalent to the tag being absent (Source: XSLT 2.0 and XSLT 2.0 Programmer's Reference by Michael Kay, p. 182.). An empty tag like <br/> does not mean that, in this case, the line break is absent. It means that the line break is present but has no content or attriubtes.

Answer 2

There are two approaches to defining a type that enforces empty content:

• one is to use a simple type of xs:string which is constrained using an enumeration or pattern facet (or using fixed) to be zero-length;
• the other is to use a complex typ e defined as (say) an empty sequence of elements.

For validation purposes, there's not much to choose between them; but if you're interested in doing more advanced things with your schema, like defining type hierarchies and substitution groups, or mapping the schema to a Java object model, it could make a difference which you choose.

Answer 3

<xs:element name="Foo">
  <xs:complexType>
    <xs:attribute name="Bar" type="xs:positiveInteger"/>
  </xs:complexType>
</xs:element>

You could leave out the attribute tag to leave it totally empty...