Back in Chapter 3, we added an Ajax.Request object to News to sync the web feeds to the device, but didn’t describe the object or the return handling in any detail. Ajax.Request manages the complete Ajax lifecycle, with support for callbacks at various points to allow you to insert processing within the lifecycle where you need to. In Chapter 3, we used updateFeedRequest to initiate the Ajax request:

// updateFeedRequest - function called to setup and make a feed request
updateFeedRequest: function(currentFeed) {
    Mojo.Log.info("URL Request: ", currentFeed.url);

    var request = new Ajax.Request(currentFeed.url, {
        method: "get",
        evalJSON: "false",
        onSuccess: this.updateFeedSuccess.bind(this),
        onFailure: this.updateFeedFailure.bind(this)
    });
},

An Ajax.Request is created using the new operator, the only way that a request can be generated, and initiates an XMLHttpRequest as soon as it is created. In this case, a get request is initiated on the URL defined in currentFeed.url, and two callbacks are defined for success or failure of the request. You can also make post requests and define other callbacks, which map to the request lifecycle shown in Table 6-3.

To clarify:

Ajax requests are asynchronous by default, one of the virtues of Ajax. While Ajax.Request supports an override, the synchronous XMLHttpRequest has been disabled in webOS. Since the UI and applications run as part of a common process, this is necessary to preserve UI responsiveness. It may be supported in a later release when there is concurrency support.

There are more Ajax.Request options available. Ajax.Request is covered thoroughly in most Prototype references, including the Prototype 1.6 API reference available at http://www.prototypejs.org/api.

An Ajax.Response object is passed as the first argument to any callback. In our sample, we have used the HTTP status codes under the status property and the responseText and responseXML properties. Table 6-4 provides a summary of the full list of properties.

We haven’t discussed JSON specifically, but it is an increasingly important tool for developers. Prototype includes some powerful JSON tools in Ajax.Request, which supports automatic conversion of JSON data and headers. If you’re handling structured data, you should look at JSON, particularly for data interchange.

Prototype includes some additional functions for consolidating listeners for Ajax callbacks, called responders, and for updating DOM elements directly from an Ajax request.

Ajax.Responders.register({
   onCreate: function()  {
       spinnerModel.value = true;
       this.controller.modelChanged(spinnerModel, this);
   }.bind(this),

   onSuccess: function(response)  {
       spinnerModel.value = false;
       this.controller.modelChanged(spinnerModel, this);
       this.process.Update(response);
   }.bind(this),

   onFailure: function(response) {
       this.spinnerModel.value = false;
       this.controller.modelChanged(spinnerModel, this);
       var status = response.status;
       Mojo.Log.info("Invalid URL - Status returned: ", status);
       Mojo.Controller.errorDialog("Invalid feed - http failure: "+status);
   }.bind(this)
});