Webmaster''''s Guide to the Wireless Internet part 12 potx

82 Chapter 3 • A New Markup: WML Figure 3.3 shows that the SDK has picked up that the closing element for <card> did not match the closing element for , which causes an error. DTDs are essentially a set of rules that say what elements can exist and in what form they can appear.Thus, it determines whether the element contains content or not, what attributes it has, and where it can appear in the document. In the following example, taken from the WML DTD, you can see how the head element should appear (we added line numbers for reference): 1 <!ELEMENT head ( access | meta )+> 2 <!ATTLIST head 3 %coreattrs; 4 > Getting to know how to interpret DTDs can be worthwhile because they often provide a definitive guide to what can and cannot be used in the language they define.Although appearing complex at first, the DTD is written according to a convention known as Backus Naur Form (BNF). BNF is a notation that describes the syntax a language must use. Knowing how BNF works will allow you to “read” the specification with ease. Our preceding example reads as follows: ■ In Line 1, !ELEMENT specifies the name of the element (<head>) and what other elements it may contain. For the <head> element, this can be <access> or <meta>. www.syngress.com Figure 3.3 The Error in Detail 159_wg_wi_03 10/22/01 4:33 PM Page 82 A New Markup: WML • Chapter 3 83 ■ In line 2, !ATTLIST specifies which attributes the <head> element can contain. In this case, the <head> element can contain only those defined as core attributes elsewhere in the DTD; the attribute %coreattrs; is a reference to another part of the DTD where the core attributes are specified in a similar fashion to the head. The various WML elements are covered in the later section “WML Elements.” NOTE You can find out more about DTDs and XML at www.xml.com. Using WML Syntax As mentioned earlier,WML is XML-based, which means that every WML file is an XML file and must contain a reference to the DTD.Therefore, the following must be included at the beginning of every WML file: <?xml version="1.0"?> <!DOCTYPE wml PUBLIC "//WAPFORUM//DTD_WML_1.1//EN" http://www.wapforum.org/DTD/wml_1.1.xml> This code tells the device interpreting the file that this file is an XML document (in this case, conforming to XML version 1.1) and that the DTD is located at www.wapforum.org/DTD/wml_1.1.xml. WARNING Unlike HTML, the XML declaration must be on the first line. Even a single space before the XML declaration will cause an error—the browser cannot interpret white space until it knows the file is an XML file and to which DTD it should be referring. In HTML, white space at the beginning of a file is highly prevalent because pages are frequently constructed dynamically from a database, which results in files with expanses of white space where the server-side code was stripped out (this can of course be avoided by sensibly constructing the server-side code). www.syngress.com 159_wg_wi_03 10/22/01 4:33 PM Page 83 84 Chapter 3 • A New Markup: WML Following Syntax Rules WML can consist of a mixture of entities, elements, attributes, comments, and variables. Many of the syntax rules in WML are directly inherited from XML: ■ All WML documents must contain the root element WML that contains any other WML elements. ■ All elements must be lowercase (with the exception of keywords such as DOCTYPE and ENTITY, which are in uppercase). ■ All element attribute values must be quoted. ■ All elements must have corresponding endings. ■ Elements which are empty elements (that is, have no content) must use /> to signify closure of the element. ■ All white space is treated as significant. ■ Elements must not overlap, though they may be nested (as with HTML). ■ Element names are case-sensitive: name and Name are not the same. Replacing Entities Either numeric or named, entities are specific characters within the document character set that require escaping in WML or that may not be supported for entry within editors (due to their using a different character set, for example).All WML entities have the following form: &entityreference; Thus, the commonly used ampersand (&) becomes: & In WML, certain characters are used to denote special situations, such as the start and end of an element (< and >). Simply placing the character (<) in the content would thus generate an error. Entities are used to reference content by placing the entity on the content as a placeholder.Thus, to show a code snippet, relevant entities would be replaced as follows (Figure 3.4 shows the result when viewed on a device or emulator): The element www.syngress.com 159_wg_wi_03 10/22/01 4:33 PM Page 84 A New Markup: WML • Chapter 3 85 As you can see in Figure 3.4, the < and > entities as output on a device show how you can insert content into WML that would not otherwise be possible. Closing Elements Elements are similar to HTML elements (or tags) in that they specify the markup and structure of a WML file. However, due to the use of XML,WML elements do not seek to provide formatting information.This is left to the end device to work out. Elements can be found in one of two forms: <element>Content</element> or: <element/> Although the first example, with its opening and ending tag, will be familiar to anyone who has used HTML, notice the backslash on the second example. WML is far less forgiving than HTML, and if an element is not a container for content, you must use the backslash as shown to explicitly close it. If the element does not contain any content but has a corresponding closing element, you must still include the closing element, for example: <element> </element> www.syngress.com Figure 3.4 WML Entities 159_wg_wi_03 10/22/01 4:33 PM Page 85 86 Chapter 3 • A New Markup: WML Characterizing the Element with Attributes Although an element can provide information, such as identifying enclosed content as a paragraph (as in the element), further information about the element’s characteristics is provided by adding attributes to the element, as necessary. Thus, if you want to right-align a paragraph, the attribute would read as follows: Right-aligned text All attributes must be enclosed in quotation marks (either single or double)— this is strictly required by WML. Failure to enclose all attributes in quotes generate an error, and the content will fail to load in the device. NOTE No attribute may be named more than once in any element that contains it. You can find more information about which attributes are applicable to which elements in the later “WML Elements” section. Case Sensitivity In addition to enclosing all element attributes in quotes,WML—unlike HTML—is case-sensitive.All elements and attributes must be in lowercase. Handling White Space White space within text is handled by WML in the same manner as XML default white space handling.White space before or after elements is ignored completely, and in fact, sequences of white space (such as tabs and spaces) will be collapsed into a single space, the treatment of which will depend on the locale.Thus, you cannot space words within text by using multiple spaces or tabs.The handling of white space within elements and attributes is dependent on the rules defined in the XML specification referenced within the XML declaration at the beginning of the WML deck. Collapsing Carriage Returns The numerous examples in this chapter make use of carriage returns and tabs to format the content, making it easier to follow.You can continue this practice www.syngress.com 159_wg_wi_03 10/22/01 4:33 PM Page 86 A New Markup: WML • Chapter 3 87 through to actual WML files to make code easier to read when editing. In WML, the user-agent (or the WAP Gateway) will take carriage returns, tabs, new lines, and multiple spaces, and collapse them to a single space as part of the process of minimizing the file size. Commenting WML comments are much like their HTML counterparts, though they must not extend across more than one line.Thus, the following is valid: <! A comment > This, however, is invalid: <! A Comment and another > Using Variables A variable is a temporary parameter, commonly stored on the user-agent into which a value can be inserted for future access during the session.A variable within WML has a very exact syntax. Where white space can be expected to signify the end of a variable, the following is acceptable: $identifier If this is not the case, the variable must be enclosed in parentheses: $(identifier) Variables are covered in more detail in Chapter 4. Formatting Text Now for the bad news:Whereas HTML contains much to help you format your content, such as font tags and Cascading Style Sheets, the WML specification leaves that pretty much up to the receiving device.This behavior was intentional, given the huge range of possible wireless devices that are bound to be created, and means that if you rely on some kind of formatting to get across a key part of your application functionality, you could be heading for problems.This is com- pounded by the haphazard implementation of what little formatting control is allowable in WML by the currently available devices. www.syngress.com 159_wg_wi_03 10/22/01 4:33 PM Page 87 88 Chapter 3 • A New Markup: WML The formatting that is available addresses the bare minimum necessary and includes the following: ■ Elements for bold and strong emphasis ( or ) ■ Elements for italic and emphasized text ( or ) ■ Elements for underlined text () ■ Elements to manipulate font size ( and <big>) Displaying Fonts You have very little influence over the fonts used in a device.They will almost certainly be nonproportional in nature, which rules out fun ASCII text of the type sometimes used to create basic pictures because these require proportional fonts.Take a look at the differences in the interpretation of markup containing formatting instructions in the comparison between Nokia and Phone.com devices in Figures 3.5 and 3.6. Both emulators are displaying the same WML content, but the Nokia device fails to display the bold text as intended. In fact, the actual Nokia 7110 does not display formatting such as emphasized, bold, or underlining. www.syngress.com Figure 3.5 Use of on a Phone.com Browser 159_wg_wi_03 10/22/01 4:33 PM Page 88 A New Markup: WML • Chapter 3 89 Both emulators will interpret the WML, which is well-formed and valid, but the Nokia browser doesn’t have the capacity to render bold text, so the element is simply ignored, and the text is displayed in the normal default font. NOTE Figures 3.5 and 3.6 are for guidance only. Although sophisticated, emulators do not necessarily exactly reflect what your WML will look like on an actual live device. Always remember to check your WML on a real device to prevent any unwelcome surprises. You can learn more about emulators later in this chapter in the section called “WML Editors, WAP SDKs, and Emulators.” Reserved Characters WML uses a number of special characters to denote characters that may not be present within the current document encoding.The characters have the following syntax: & entity reference ; www.syngress.com Figure 3.6 Use of on a Nokia Browser 159_wg_wi_03 10/22/01 4:33 PM Page 89 90 Chapter 3 • A New Markup: WML The entity reference can be numerical, named, or hexadecimal, for example: < These will all cause the < character to be displayed on the device screen. It is vitally important to include the ampersand and semicolon because missing either of these will generate an error.You need to consider seven important entities within WML, as shown in Table 3.1. Table 3.1 Character Entity Reference Table Entity Name Entity Entity Description quot " Quotation mark amp &#38; Ampersand apos &#39 Apostrophe lt &#60; Less-than gt > Greater-than nbsp   Nonbreaking space shy Soft hyphen Displaying Tables Tables in WML are fairly simple in comparison to HTML tables, reflecting the limitations of the display generally available.You cannot nest tables, nor can you set explicit widths, although you must specify the number of columns within a row set. WARNING Only the more advanced devices, such as the Ericsson R320, are likely to be capable of displaying data marked up using tables. You must carefully inspect the target device to ensure that the device is capable of handling the display of tables and that the contents of the table will display as intended. www.syngress.com 159_wg_wi_03 10/22/01 4:33 PM Page 90 A New Markup: WML • Chapter 3 91 Meta Information Meta information is information designed for use by the browser or gateway and is not displayed to the user. Meta information is included within the head of the document, the precise range of which is generally left to the device manufacturer to specify. Controlling Caching Whenever a request is made for a WML page, the device will cache the received file, allowing it to be called up from the cache instead of having to download the whole file again.This can considerably speed up browsing .The current devices do not have large caches; when dealing with dynamic data that may have the same URL but different contents each time the URL is accessed, you may want to disable caching. You can control caching in a number of ways. Caching is a somewhat com- plicated issue to address because many devices cache differently than others and also differ in what instructions can be given to them to control caching.The following meta information is intended to cater to as many devices as possible: <meta http-equiv="Cache-Control" forua="true" content="no-cache, max-age=0,must-revalidate, proxy-revalidate, s-maxage=0"/> NOTE As you can see from the examples in this section, you can specify from a number of headers to cater for as many eventualities as possible. Whenever caching is a concern, make sure you do the appropriate back- ground research and testing to target the device on which you will be deploying your solution. Additionally, making use of the Expires header is an ideal way to control browser caching. It works very well on many different devices. If the document is set to expire in the future, the browser will always re-request the content, thus ensuring the document is never reloaded from the cache.You can include this in server-side script, such as ASP: <% Response.Expires = -1 www.syngress.com 159_wg_wi_03 10/22/01 4:33 PM Page 91 . %coreattrs; is a reference to another part of the DTD where the core attributes are specified in a similar fashion to the head. The various WML elements are covered in the later section “WML Elements.” NOTE You. designed for use by the browser or gateway and is not displayed to the user. Meta information is included within the head of the document, the precise range of which is generally left to the device manufacturer to. into WML that would not otherwise be possible. Closing Elements Elements are similar to HTML elements (or tags) in that they specify the markup and structure of a WML file. However, due to the

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