Dr. Dobb's Journal - May 2008 - (Page 35)

d05much_p3db 3/19/08 8:21 AM Page 35 The persist property is optional in the tag. However, the advantage of setting persist to true is that once the image is downloaded, it is stored on the device; thus, on startup of the application next time around, the image is available. HTTP, XML, and XSLT Given the nature of mobile devices and their ability to communication over the air, many Mojax applications access remote resources. The tag is for content types accessible via HTTP. Listing Four (available online) presents the httprequest for the sample application. The url property is set to match the format for calling the Zillow property details API. The async property is set to true, enabling the request to be completed in the background. I apply an XSLT filter (Listing Five; available online) to the XML response to tailor the data specifically for the application. A side-effect of using the filter is reducing network traffic to only the information needed, thus improving application performance. The bulk of the interesting code regarding the HTTP request is inside the tag onLoaded, which is called once the request has completed. The response from the HTTP request is used to create two XPath expressions for accessing the returned XML. For completeness, it’s important to note that Mojax currently supports XML, JSON, images, and Mojax script as the supported data types through an HTTP request. elements. Attributes are defined, one for each softkey, left and right. Two tags are defined, one for each label to be associated with the left and right keys. Once the prototype is defined, you can use the object as you would any other Mojax tag. For example, the code below shows a tag definition that declares a tag used to associate the text “Exit” with the left softkey, and the text “Search” with the right. Referring back to Example 5, notice that inside the tag I referenced the tags using the IDs assigned to the textbox tags; for example, this.lefttext.value. When you define a tag, an event is generated for each attribute defined in the tag. For example, left="Exit" generates a call to the attribute tag within the prototype definition with the name left, which sets the value of the first textbox to Exit and sets its visible property to true. The same logic applies for the right="Search" definition, setting the value and visible properties of the object. The reason for the visible property in the prototype definition is to allow use of a tag, which has just one key definition. In other words, you may want to enable just one softkey for a screen: Creating Objects Using Prototypes Prototypes are a powerful concept for encapsulating common code into reusable classes. Within Mojax, the tag is the equivalent of the prototype object in ECMAScript. One common use of a prototype within Mojax apps is for managing softkeys. Notice in Example 5 how the prototype extends the Box object, which is a logical choice as this object is designed specifically as a container for other visual By definition, the textboxes in the prototype are not visible; thus, just one label (Back) is shown on the device display. In Listing Two, you see the reference to a tag. Figure 1 shows the device output when using this tag, which displays the property information returned for the Zillow API. In Listing Four (available online) the variable propertyXML was assigned the value of the XPath expression: May 2008 l www.ddj.com l Dr. Dobb’s Journal 35 http://www.birdstep.com/database http://www.ddj.com

Table of Contents Feed for the Digital Edition of Dr. Dobb's Journal - May 2008

Dr. Dobb's Journal - May 2008 Contents Friday Night Fish Fry Alia Vox Developer Diaries Software Development Goes to the Movies Cat: A Functional Stack-Based Little Language Mojax: Mobile Ajax Framework Kernel-Mode Databases Getting Better Search Results Effective Concurrency The Agile Edge

Dr. Dobb's Journal - May 2008

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