XML Step by Step- P5 pptx

Chapter 3 Creating Well-Formed XML Documents 55 3 Well-Formed Documents Here’s an example of content in an element that consists of both character data and a nested element: C h a r acte r data Nested e l e m e n t Co n te n t o f TITLE e l e m e n t When adding character data to an element, you can insert any characters as part of the character data except the left angle bracket (<), the ampersand (&), or the string ]]>. note The XML parser scans an element’s character data looking for XML markup. You therefore cannot insert a left angle bracket (<), an ampersand (&), or the string ]]> as a part of the character data because the parser would interpret each of these characters or strings as markup or the start of markup. If you want to insert < or & as an integral part of the character data, you can use a CDATA section (discussed later in the list). You can also insert <, &, or any other character—including one not on your keyboard—by using a character reference, and you can insert certain characters by using predefined general entity references (such as < or & for inserting < or &). General entity and character references are discussed next. ■ General entity references or character references. Here’s an element containing one of each: A character reference A general entity reference Entity and character references are covered in Chapter 6. ■ CDATA sections. A CDATA section is a block of text in which you can freely insert any characters except the string ]]>. Here’s an example of a CDATA section in an element: 56 XML Step by Step A CDATA section CDATA sections are covered in Chapter 4. ■ Processing instructions. A processing instruction provides information to the XML application. Processing instructions are covered in Chapter 4. ■ Comments. A comment is an annotation to your XML document that people can read but that the XML processor ignores and (optionally) passes on to the application. Comments are covered in Chapter 4. Here’s an element containing both a processing instruction and a comment: A processing instruction A comment White Space in Elements White space consists of one or more space, tab, carriage-return, or line feed characters. (These characters are represented, respectively, by the decimal values 32, 9, 13, and 10, or by the equivalent hexadecimal values 20, 09, 0D, and 0A.) Sometimes you insert white space into an element because you want it to be an actual part of the element’s character data. For example, the leading white space in the last of the VERSE elements shown here is an integral part of the content of the poem: <VERSE>For the rare and radiant maiden<VERSE> <VERSE>whom the angels name Lenore </VERSE> <VERSE> Nameless here for evermore.</VERSE> Chapter 3 Creating Well-Formed XML Documents 57 3 Well-Formed Documents Other times, you insert white space into an element merely to make the XML source easy to read and understand (often a good idea). For instance, in the following source, the line breaks inserted after the <BOOK>, </TITLE>, and </AUTHOR> tags, and the space characters before the <TITLE> and <AUTHOR> tags, make the structure of the elements easier to see and aren’t intended to be part of the BOOK element’s actual character content: <BOOK> <TITLE>The Adventures of Huckleberry Finn</TITLE> <AUTHOR>Mark Twain</AUTHOR> </BOOK> According to the XML specification, however, the XML processor should not try to guess the purpose of various blocks of white space, but rather it must always preserve all white space characters and pass them on to the application. (The one exception is that in all text it passes to the application, the processor must convert a carriage-return and line feed character pair, or a carriage-return without a following line feed, to a single line feed character.) XML provides a reserved attribute, named xml:space, that you can include in any element to tell the application how you would like it to handle white space contained in that element. (Attributes are discussed later in this chapter.) The xml: indicates that this attribute belongs to the xml namespace. Because this namespace is predefined, you don’t have to declare it. (See “Using Namespaces” on page 69.) Keep in mind that this attribute has no effect on the XML processor, which always passes on all white space in elements to the application, and the application can use this information in any way, even ignoring it if appropriate. The two standard values you can assign to this attribute are default, which signals the application that it should use its default way of handling white space, and preserve, which informs the application that it should preserve all white space. The xml:space attribute specification applies to the element in which it occurs and to any nested elements, unless it is overridden by an xml:space attribute specification in a nested element. For example, the xml:space attribute in the following STANZA element tells the application that it should preserve all white space in the STANZA and nested VERSE elements: <STANZA xml:space=”preserve”> <VERSE>For the rare and radiant maiden<VERSE> <VERSE>whom the angels name Lenore </VERSE> <VERSE> Nameless here for evermore.</VERSE> </STANZA> continued 58 XML Step by Step If the application abides by this example xml:space specification, it will preserve the leading spaces in the last VERSE element as well as the line breaks before and after each VERSE element. When you get to Chapters 5 and 7 on creating valid documents, keep in mind that in a valid document the xml:space attribute must be declared just like any other attribute. (This will make sense when you read those chapters.) In a document type definition (DTD), you must declare the attribute as an enumerated type, as shown in the following example attribute-list declaration: <!ATTLIST STANZA xml:space (default|preserve) ‘preserve’> Remember that when you use the methods for displaying and working with XML discussed in this book, Internet Explorer provides the application, or at least the front end of the application. So you also need to know what the XML application component of Internet Explorer does with the white space that it receives from Internet Explorer’s XML processor. This will tell you whether the white space will be displayed in the browser, or whether it will be available to the Web pages you write to display XML. The way Internet Explorer handles white space depends on which method you use for displaying and working with XML documents: ■ CSS. If you display an XML document using a cascading style sheet (CSS), as explained in Chapters 8 and 9, Internet Explorer handles white space just as it does in an HTML page (regardless of any xml:space settings included in the document). That is, it replaces sequences of white space characters within an element’s text with a single space character, and it discards leading or trailing white space. To format the text the way you want it, you can use CSS properties. ■ Data Binding. If you use data binding to display an XML document, as explained in Chapter 10, Internet Explorer automatically preserves all the white space within an XML element to which an HTML element is bound, regardless of any xml:space settings included in the document. An exception is an HTML element with the DATAFORMATAS=”HTML” attribute specification, as explained in Chapter 10. ■ XML DOM or XSLT Style Sheets. If you use an XML Document Object Model (DOM) script to display an XML document following the instructions in Chapter 11, Internet Explorer preserves most white space within an element. If, however, you use an XSLT style sheet as directed in Chapter 12, Internet Explorer handles white space as it does in HTML (described in the first list item). With both display methods, the exact handling of white space depends upon how you load and access the XML document, and follows fairly complex rules. For details, search for “white space” in the topic titles of the Microsoft XML SDK 4.0 help file. continued Chapter 3 Creating Well-Formed XML Documents 59 3 Well-Formed Documents Empty Elements You can also enter an empty element—that is, one without content—into your document. You can create an empty element by placing the end-tag immediately after the start-tag, as in this example: <HR></HR> Or, you can save typing by using an empty-element tag, as shown here: <HR/> These two notations have the same meaning. Because an empty element has no content, you might question its usefulness. Here are two possible uses: ■ You can use an empty element to tell the XML application to perform an action or display an object. Examples from HTML are the BR empty element, which tells the browser to insert a line break, and the HR empty element, which tells it to add a horizontal divid- ing line. In other words, the mere presence of an element with a par- ticular name—without any content—can provide important information to the application. ■ An empty element can store information through attributes, which you’ll learn about later in this chapter. An example from HTML is the IMG (image) empty element, which contains attributes that tell the processor where to find the graphics file and how to display it. tip As you’ll learn in Chapter 8, a cascading style sheet can use an empty element to display an image. In Chapter 10, you’ll learn how to use data binding to access the attributes belonging to an empty or non-empty element. And in Chapters 11 and 12, you’ll learn how to use HTML scripts (Chapter 11) and XSLT style sheets (Chapter 12) to access elements (empty or non-empty) and their attributes and then perform appropriate actions. Create Different Types of Elements 1 Open a new, empty text file in your text editor, and type in the XML document shown in Listing 3-2. (You’ll find a copy of this listing on the companion CD under the filename Inventory03.xml.) If you want, you can use the 60 XML Step by Step Inventory.xml document you created in Chapter 2 (given in Listing 2-1 and included on the companion CD) as a starting point. 2 Use your text editor’s Save command to save the document on your hard disk, assigning the filename Inventory03.xml. Inventory03.xml <?xml version=”1.0"?> <! File Name: Inventory03.xml > <?xml-stylesheet type=”text/css” href=”Inventory02.css”?> <INVENTORY> <! Inventory of selected 19th Century American Literature > <BOOK> <COVER_IMAGE Source=”Huck.gif” /> <TITLE>The Adventures of Huckleberry Finn</TITLE> <AUTHOR>Mark Twain</AUTHOR> <BINDING>mass market paperback</BINDING> <PAGES>298</PAGES> <PRICE>$5.49</PRICE> </BOOK> <BOOK> <COVER_IMAGE Source=”Leaves.gif” /> <TITLE>Leaves of Grass</TITLE> <AUTHOR>Walt Whitman</AUTHOR> <BINDING>hardcover</BINDING> <PAGES>462</PAGES> <PRICE>$7.75</PRICE> </BOOK> <BOOK> <COVER_IMAGE Source=”Faun.gif” /> <TITLE>The Marble Faun</TITLE> <AUTHOR>Nathaniel Hawthorne</AUTHOR> <BINDING>trade paperback</BINDING> <PAGES>473</PAGES> <PRICE>$10.95</PRICE> </BOOK> <BOOK> <COVER_IMAGE Source=”Moby.gif” /> <TITLE>Moby-Dick <SUBTITLE>Or, The Whale</SUBTITLE></TITLE> Chapter 3 Creating Well-Formed XML Documents 61 3 Well-Formed Documents <AUTHOR>Herman Melville</AUTHOR> <BINDING>hardcover</BINDING> <PAGES>724</PAGES> <PRICE>$9.95</PRICE> </BOOK> </INVENTORY> Listing 3-2. note The document you typed uses the cascading style sheet (CSS) named Inventory02.css that you created in a previous exercise. (It’s given in Listing 2-4 and is on the companion CD.) Make sure that this style sheet file is in the same folder as Inventory03.xml. 3 In Windows Explorer or in a folder window, double-click the name of the file that you saved, Inventory03.xml: Internet Explorer will now display the document as shown here: 62 XML Step by Step The document you entered contains the following types of elements: ■ An element with a comment as part of its content (INVENTORY). Notice that the browser doesn’t display the comment text. ■ An empty element named COVER_IMAGE at the beginning of each BOOK element. The purpose of this element is to tell the XML application to display the specified image of the book’s cover. (The Source attribute contains the name of the image file.) To be able to actually show the image, however, you would need to display the XML document using one of the methods discussed in Chapters 10 through 12, rather than using a simple CSS as in this example. ■ An element (the TITLE element for Moby-Dick) that contains both character data and a child element (SUBTITLE). Notice that the browser displays both the character data and the child element on a single line, using the same format. (The CSS format assigned to the TITLE element is inherited by the SUBTITLE element.) Adding Attributes to Elements In the start-tag of an element, or in an empty-element tag, you can include one or more attribute specifications. An attribute specification is a name-value pair that is associated with the element. For example, the following PRICE element includes an attribute named Type, which is assigned the value retail: For other books, this attribute might, for example, be set to wholesale. The following BOOK element includes two attributes, Category and Display: <BOOK Category=”fiction” Display=”emphasize”> <TITLE>The Marble Faun</TITLE> <AUTHOR>Nathaniel Hawthorne</AUTHOR> <BINDING>trade paperback</BINDING> <PAGES>473</PAGES> <PRICE>$10.95</PRICE> </BOOK> Chapter 3 Creating Well-Formed XML Documents 63 3 Well-Formed Documents The following empty element includes an attribute named Source, which indicates the name of the file containing the image to be displayed: <COVER_IMAGE Source=”Faun.gif” /> Adding an attribute provides an alternative way to include information in an element. Attributes offer several advantages. For example, if you write a valid document using a document type definition (DTD), you can constrain the types of data that can be assigned to an attribute and you can specify a default value that an attribute will be assigned if you omit the specification. (You’ll learn these techniques in Chapter 5.) In contrast, in a DTD you can’t specify a data type or a default value for the character data content of an element. note If you write a valid document using an XML schema, as described in Chapter 7, you can constrain the data type for either an attribute’s value or an element’s character data. Typically, you place the bulk of the element’s data that you intend to display within the element’s content. And you use attributes to store various properties of the element, not necessarily intended to be displayed, such as a category or a display instruction. The XML specification, however, makes no rigid distinctions about the types of information that should be stored within attributes or content, and you can use them any way you want to organize your XML documents. note When you display an XML document using a CSS (the method covered in Chapters 8 and 9), the browser does not display attributes or their values. Dis- playing an XML document using data binding (Chapter 10), a script in an HTML page (Chapter 11), or an XSLT style sheet (Chapter 12), however, allows you to access attributes and their values and to display the values or perform other appropriate actions. Rules for Creating Attributes As you can see, an attribute specification consists of an attribute name followed by an equal sign (=) followed by an attribute value. You can choose any attribute name you want, provided that you follow these rules: Chapter 3 Creating Well-Formed XML Documents 65 3 Well-Formed Documents Rules for Legal Attribute Values The value you assign to an attribute is a series of characters delimited with quotes, known as a quoted string or literal. You can assign any literal value to an attribute, provided that you observe these rules: ■ The string can be delimited using either single quotes (') or double quotes ("). ■ The string cannot contain the same quote character used to delimit it. ■ The string can contain character references or references to general internal entities. (I’ll explain character and entity references in Chapter 6.) ■ The string cannot include the ampersand (&) character, except to begin a character or entity reference. ■ The string cannot include the left angle bracket (<) character. You’ve already seen examples of legal attribute specifications. The following attribute specifications are illegal: <EMPLOYEE Status=””downsized””> <! Can’t use delimiting quote within string. > <ALBUM Type=”<CD>”> <! Can’t use < within string. > <WEATHER Forecast=”Cold & Windy”> <! Can’t use & except to start a reference. > If you want to include double quotes (") within the attribute value, you can use single quotes (') to delimit the string, as in this example: <EMPLOYEE Status=’”downsized”’> <! Legal attribute value. > Likewise, to include a single quote within the value, delimit it using double quotes: <CANDIDATE name=”W.T. ‘Bill’ Bagley”> <! Legal attribute value. > tip You can get around the character restrictions and enter any character into an attribute value (including a character not on your keyboard) by using a character reference or—if available—a predefined general entity reference. I’ll explain character and predefined general entity references in Chapter 6. [...]...68 XML Step by Step Internet Explorer will now display the document as shown here: The document you typed is based on Inventory .xml, which you created in a previous exercise In addition to having fewer elements than Inventory .xml, the new document has two modifications that illustrate the use of attributes: I In each... Nathaniel Hawthorne $10.95 and that you keep track of your CDs in another XML document: < ?xml version=”1.0"?> Violin Concerto in D Well-Formed Documents Using Namespaces 70 XML Step by Step Beethoven $14.95 Violin Concertos Numbers 1, 2, and 3 Mozart... solution would probably involve rewriting parts of either the XML document or the application The XML namespace mechanism provides an easier way It allows you to easily differentiate two or more elements, or two or more attributes, that have the same name by assigning each to a separate namespace Listing 3-4 shows a combined document created by merging the two documents shown above (You’ll find a copy... Hawthorne $10.95 Listing 3-4 Well-Formed Documents 3 .xml > Chapter 3 Creating Well-Formed XML Documents 73 I URLs, which use traditional addressing schemes such as http (for example, http://www.mjyOnline.com), ftp (for... that they belong to the namespace A valid namespace prefix must begin with a letter or underscore (_), followed by zero or more letters, digits, periods (.), hyphens (-), or underscores Beginning a prefix with the letters xml (in any case combination) is reserved for prefixes defined by XML- related specifications You can use the prefix anywhere within the element in which the namespace has been defined... item (book:ITEM) from a CD item (cd:ITEM), a book title (book:TITLE) from a CD title (cd:TITLE), or a book price (book:PRICE) from a CD price (cd:PRICE) Chapter 3 Creating Well-Formed XML Documents 71 Collection .xml < ?xml version=”1.0"?> The Adventures of Huckleberry Finn Mark Twain $5.49 ... You’ll see more in Chapter 5 Combining XML data from several sources can result in conflicts in the names of elements and attributes Suppose, for example, that you keep track of your books in one XML document: < ?xml version=”1.0"?> The Adventures of Huckleberry Finn Mark Twain $5.49 Leaves... store but not necessarily display Chapter 3 Creating Well-Formed XML Documents 69 One way to hide such information—and indicate its lesser importance—is to assign it to an attribute rather than placing it in the content of an element 3 These are only a few of the many possible uses for attributes You’ll see more in Chapter 5 Combining XML data from several sources can result in conflicts in the names... each to a separate namespace Listing 3-4 shows a combined document created by merging the two documents shown above (You’ll find a copy of this listing on the companion CD under the filename Collection .xml. ) In the combined document, each of the elements for a book (ITEM, TITLE, AUTHOR, and PRICE) is assigned to the book namespace and each of the elements for a CD (ITEM, TITLE, COMPOSER, and PRICE) is... urn:w3-org-ns:HTML For complete information on URN syntax, see http://www.ietf.org/rfc/rfc2141.txt For much more information on URIs, see the following page on the W3C Web site: http://www.w3.org/Addressing/ The xmlns in the namespace declaration is a predefined prefix (you don’t have to declare it) that is used specifically for defining namespaces And book is the namespace prefix, which is a shorthand notation . Nameless here for evermore.</VERSE> </STANZA> continued 58 XML Step by Step If the application abides by this example xml: space specification, it will preserve the leading spaces in. in the XML document shown in Listing 3-2. (You’ll find a copy of this listing on the companion CD under the filename Inventory03 .xml. ) If you want, you can use the 60 XML Step by Step Inventory .xml. of your CDs in another XML document: < ?xml version=”1.0"?> <COLLECTION> <ITEM> <TITLE>Violin Concerto in D</TITLE> 70 XML Step by Step <COMPOSER>Beethoven</COMPOSER>