XML Getting Started Tutorial-Attribute Declaration Code Example

A valid XML document must declare the attributes of the element. Use the ATTLIST statement to complete. An ATTLIST can declare multiple attributes for an element type.
#REQUIRED>
The above example declares that the image element must have a src attribute, and the value of this attribute is character data. You can use the ATTLIST statement to declare multiple attributes for an element, such as:
width CDATA #REQUIRED height CDATA #REQUIRED alt CDATA #IMPLIED > ;
The above statement points out that the src, width, and height attributes are required, and the alt attribute is optional. 1. Attribute type

CDATA type attribute value can contain any text string . DTDThe attribute cannot be specified as an integer or a date. Schema can provide more powerful data types.

The NMTOKEN type attribute value is an XML name token. XML name tokens are similar to XML names, but XML name tokens allow all characters to be used as the starting character of a name, while the first letter of an XML name must be a letter, an ideographic character, and an underscore. So 10, .bashrc is a legal XML name tag, but not a legal XML name. Every XML name is an XML name tag, but not all XML name tags are XML names. If the attribute contains integers such as 1990, 2005, its type should be specified as NMTOKEN. For example:

<!ELEMENT person birthday NMTOKEN #REQUIRED>

The NMTOKENS type attribute contains one or more XML name tokens separated by whitespace. For example:

<person dates="02-01-2005 03-01-2005 05-01-2005">person</person>

The corresponding declaration should be:

<!ATTLIST person dates NMTOKENS #REQUIRED>

On the other hand, this declaration cannot be used for the form 01/02/2005 because the forward slash is not a legal name. character.

Enumeration declaration, no keywords are used for enumerations. List all values ​​directly, separated by vertical bars. For example:

<!ATTLIST date month(January | February | March | April | May | June | July | August | September | October | November | December) #REQUIRED>

In response to the above statement, the month attribute of the date element can select one of the twelve months.

Attributes of type ID must contain an XML name that is unique within the document. The ID attribute assigns a unique identifier to the element.

<!ATTLIST name card_id ID #REQUIRED>

Since numbers are not legal XML names, the ID number cannot start with a number. The solution is to add an underscore or letter in front.

The IDREF type attribute points to the ID type attribute of an element in the document. Therefore, it must be an XML name whose purpose is to establish a many-to-many relationship between elements when a simple containment relationship does not suffice. For example:

<project project_id="p1"> <goal>deploy linux</goal> 
<team_member person_card_id="c123"> </project> 
<person card_id="c123"> <name>linuxsir</name> 
<assignment project_project_id="p1"> </person>

The project_id attribute of the project element and the card_id attribute of the person element should be of ID type. The person_card_id attribute of the team_member element and the project_project_id attribute of the assignment element are of type IDREF. The corresponding declaration is as follows:

<!ATTLIST person card_id ID #REQUIRED> 
<!ATTLIST project project_id ID #REQUIRED> 
<!ATTLIST team_member person_card_id IDREF #REQUIRED> 
<!ATTLIST assignment project_project_id IDREF #REQUIRED>

IDREFS type attribute contains a list of XML names. The names are separated by whitespace, and each name is the ID of an element in the document. This element is used when an element needs to reference multiple other elements. For example:

<!ATTLIST person card_id ID #REQUIRED assignment IDREFS #REQUIRED> 
<!ATTLIST project project_id ID #REQUIRED team IDREFS #REQUIRED>

The corresponding document can be written as:

<project project_id="p1" team="c123"> 
<gold>deploy linux</gold> </project> 
<person card_id="c123" assignment="p1"> 
<name>Linuxsir</name> </person>

Attributes of ENTITY type are included in the names of unparsed entities declared in other positions of the DTD. For example, the movie element may have an entity attribute that identifies the mpeg or rm file to be played when activated:

<!ATTLIST movie src ENTITY #REQUIRED>

If the DTD declares an unparsed entity named play, this movie element can be used to embed in the XML document

VideoFile:

<movie src="play" />

The ENTITIES type attribute contains multiple unparsed entity names declared elsewhere in the DTD, separated by blanks.

<!ATTLIST slide_show slides ENTITIES #REQUIRED>

If the DTD declares the unparsed entities slide1, slide2, slide3,..., you can use the slide_show element to embed the slide in the XML document.

<slide_show slides="slide1 slide2 slide3" />

NOTATION type attribute contains the name of a notation declared in the document's DTD. This attribute type is rarely used. In theory, you can use this attribute to associate certain special elements with types. The following example declares four notations for different image types, and then stipulates that each image element must choose a type attribute from them.

<!NOTATION gif SYSTEM "image/gif"> 
<!NOTATION tiff SYSTEM "image/tiff"> 
<!NOTATION jpeg SYSTEM "image/jpeg"> 
<!NOTATION png SYSTEM "image/png"> 
<!ATTLIST image type NOTATION (gif | tiff | jpeg | png) #REQUIRED>

The value of the type attribute of each image element can be one of the four values ​​​​of gif, tiff, jpeg and png. This property has a slight advantage over enumeration types, since the notation's actual MIME media type is theoretically available. Since the slash is not a legal character in XML names, the enumeration type cannot specify image/png or image/jpeg as allowed values.

2.属性缺省值 
每个ATTLIST声明除了要提供一种数据类型外，还要声明属性的缺省行为。 

#IMPLIED，属性可选。 

#REQUIRED，属性必须有。 

#FIXED，属性是常量，不能更改。 

<!ATTLIST person name CDATA #FIXED "linuxsir"

Literal，作为一个引用字符串的实际缺省值。 

<!ATTLIST person name NMTOKEN "linuxsir"

如果没有显示指明person元素的name属性，则该值为linuxsir。

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