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Listing 13.22 The RSSItem Model class
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RSSItem = Class.create(); RSSItem.prototype = { initialize: function( rssFeed, title, link, description ) { this.rssFeed = rssFeed; this.title = title; this.link = link; this.description = description; } };
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Nothing much to get excited about here. The item encapsulates the title, link, and description attributes but also holds a reference to the RSSFeed object that it belongs to. Given these two Model classes, now we can envision that an item and one of its feeds could be constructed as shown here:
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var rssFeed = new RSSFeed( 'JavaRanch News', 'http://radio.javaranch.com/news/', 'Stories from around the ranch' ); var feed1 = new RSSItem( rssFeed, 'Win a copy of JBoss', 'http://radio.javaranch.com/news/05/07/20/9.html', 'Text of Article' ); rssFeed.addItem(feed1);
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So far, so good. The Model is a very straightforward encapsulation of the attributes of an RSS feed and its items. The two Model classes that encapsulate these two concepts are RSSFeed and RSSItem, respectively. Now let s consider the construction of the Model itself. We know that these objects will get instantiated as a result of the XML data being loaded into the client by an Ajax request. So let s define an API that our Ajax handler can call for converting the XML response into an instance of our RSSFeed Model class. Let s start by defining the contract of our Model creator as follows:
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var rssFeed = RSSFeed.parseXML( rssXML );
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This contract implies that we ll pass the XML response returned from our Ajax handler to the parse method of our RSSFeed type, and it will return to us an instance of an RSSFeed. Given that assumption, let s implement the parseXML() method as shown in listing 13.23.
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Listing 13.23 The RSS XML parsing
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RSSFeed.parseXML = function(xmlDoc) { var rssFeed = new RSSFeed( RSSFeed.getFirstValue(xmlDoc, 'title'), RSSFeed.getFirstValue(xmlDoc, 'link' ), RSSFeed.getFirstValue(xmlDoc, 'description')); var feedItems = xmlDoc.getElementsByTagName('item'); for ( var i = 0 ; i < feedItems.length ; i++ ) { rssFeed.addItem(new RSSItem(rssFeed, RSSFeed.getFirstValue(feedItems[i], 'title'), RSSFeed.getFirstValue(feedItems[i], 'link' ), RSSFeed.getFirstValue(feedItems[i], 'description')) } return rssFeed; }
This method does the textbook response XML parsing that we ve done many times already. It takes the values of the title, link, and description elements and uses them to create the RSSFeed. It then iterates over all of the item elements and does the same, creating an RSSItem instance for each. Within each iteration, the addItem() method is used to add the item to its parent RSS feed. Note that a helper method is used here to get the node value from the first child of an element with a given tag name. The helper method, getFirstValue, is shown in listing 13.24.
Listing 13.24 Parsing helper method
RSSFeed.getFirstValue = function(element, tagName) { var children = element.getElementsByTagName(tagName); if ( children == null || children.length == 0 ) return ""; if ( children[0].firstChild && children[0].firstChild.nodeValue ) return children[0].firstChild.nodeValue; return ""; }
This is everything we need from a Model perspective. Obviously, we could add attributes for all the optional parts of an RSS feed and populate them if they are present in the feed. We didn t do that in this case because the RSS reader doesn t use or need any of the optional attributes. But it s definitely an opportunity to
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provide extended metadata for future features. We could also define accessor methods for the attributes to provide a more formal contract for accessing them. For example, we could write a getTitle()/setTitle() method pair for accessing the title attribute. Since JavaScript doesn t support visibility semantics like other object-oriented languages (for example, the private/protected keywords in Java), we didn t bother. Now let s take a gander at our View.
13.7.2 RSS reader view
With our Model classes securely in place, we can now consider a View class. We could develop a View class for the RSSFeed, and another for the RSSItem, but because our RSSReader doesn t really view a feed independently of an item, we ll define a single View class called RSSItemView, which encapsulates the View for an RSSItem in the context of its parent RSSFeed. Since the View in this case is obviously HTML, our View class is really just responsible for the generation of HTML. Let s start by looking at the constructor in listing 13.25.
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