• Choose a category
• All
• Classic-Fiction

By Root 4107 0

for any errors we encounter during the loading process. We can change the message or add style parameters to it. The last global variable that is going to affect the outcome of the script is the array, arrayRSSFeeds. In this array, we add the URLs of the RSS feeds that we want to access. In this case, we access four separate RSS

feeds from JavaRanch.com’s radio blogs.

The next set of global variables that we declare, in listing 13.6, are used to communicate between our separate functions. These global variables hold the state of the RSS feeder. Their values change depending on the action that is being performed.

Listing 13.6 Global variables that maintain the state

var currentMessage = 0;

var layerFront = 2;

var timerSwitch;

var bPaused = false;

var arrayMessage = new Array();

var intLoadFile = 0;

var bLoadedOnce = false;

Licensed to jonathan zheng

520

CHAPTER 13

Building stand-alone applications with Ajax

The first global variable that we instantiate in listing 13.6 is currentMessage. The variable currentMessage keeps track of the message that is being viewed. It can be considered a counter that is reset when it reaches the maximum number of records. The next global variable is layerFront, which holds the state of our layers. When we designed our RSS reader layout, we had two layers on top of each other. This variable is keeping track of the state of those layers. The variable timerSwitch holds the timer object that determines when the next frame is going to be loaded. If the user pauses the feeder, we cancel this timer and change the state of our next variable, bPaused. The boolean value that bPaused holds allows us to determine the state of the timer: true if it is paused and false if it is running.

The global variable arrayMessage holds the formatted messages that we retrieved from the RSS items. The array is multidimensional and holds all the information we want to show. As stated earlier, the item elements in the RSS feed hold more information than we may need; therefore, we only grab the few items that interest us and store them in arrayMessage.

The last variable, intLoadFile, leads us into our next section of code. The variable is a counter, which holds the current file count that is being loaded from our array, arrayRSSFeeds, during the preloading process.

Now that all the global variables have been declared, we can see a global picture of where this project is heading. We preload the RSS feeds from an array of URLs. We use a counter to track the status of the preload process. During the preloading, we are only selecting the desired information from each XML file. After the preload process has finished, the messages are displayed with a fading transition to create a slideshow, which we can pause and manipulate. With the global variables that we declared, we are able to control the functionality of the script. This has led us to the starting point of the RSS preloader function. 13.3.2 Ajax preloading functionality

One of the problems developers face with Ajax is how to preload multiple files without sending too many requests to the external websites and having them step all over each other. One solution is to use queuing, and that is what our Ajax ContentLoader does. Making the repeated requests

The ContentLoader allows the queuing mechanism to fire the requests in an orderly fashion. In listing 13.7, we take our array (which was populated when the Licensed to jonathan zheng

Loading the RSS feeds

521

page was loaded in listing 13.5) that contains the URLs to our feeds and prepare them for our ContentLoader.

Listing 13.7 Preload JavaScript function

window.onload = function(){

var loader= new Array()

for(i=0;iloader[loader.length] = new

net.ContentLoader(arrayRSSFeeds[i],

BuildXMLResults,BuildError);

}

}

The code in listing 13.7 is fired with the onload event handler. As the page is loaded, we prepare an array variable, loader, to hold all the requests to the server. We loop through our array arrayRSSFeeds to obtain