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In all of this, note again that we don’t need to explicitly reload these variables within the terminated
case because WinJS reloads sessionState automatically. If we managed our state more directly, such as
storing some variables in roaming settings within the checkpoint event, we would reload and apply
those values at this time.
Note Using ms-appdata:/// and getFileFromPathAsync works because the file exists in a location
that we can access programmatically by default. It also works for libraries for which we declare a
capability in the manifest. If, however, we obtained a StorageFile from the file picker, we’d need to
save that in the Windows.Storage.AccessCache to preserve access permissions across sessions.
Data from Services and WinJS.xhr
Though we’ve seen examples of using data from an app’s package (via URIs or Windows.ApplicationModel.Package.current.installedLocation)
as well as in appdata, it’s very likely that your app
will incorporate data from a web service and possibly send data to services as well. For this, the most
common method is to employ XmlHttpRequest. You can use this in its raw (async) form, if you like, or
you can save yourself a whole lot of trouble by using the WinJS.xhr function, which conveniently wraps
the whole business inside a promise.
Making the call is quite easy, as demonstrated in the SimpleXhr example for this chapter. Here we
use WinJS.xhr to retrieve the RSS feed from the Windows 8 developer blog:
WinJS.xhr({ url: "http://blogs.msdn.com/b/windowsappdev/rss.aspx" })
.done(processPosts, processError, showProgress);
That is, give WinJS.xhr a URI and it gives back a promise that delivers its results to your completed
handler (in this case processPosts) and will even call a progress handler if provided. With the former,
the result contains a responseXML property, which is a DomParser object. With the latter, the event object
contains the current XML in its response property, which we can easily use to display a download count:
function showProgress(e) {
var bytes = Math.floor(e.response.length / 1024);
document.getElementById("status").innerText = "Downloaded " + bytes + " KB";
}
The rest of the app just chews on the response text looking for item elements and displaying the
title, pubDate, and link fields. With a little styling (see default.css), and utilizing the WinJS typography
style classes of win-type-x-large (for title), win-type-medium (for pubDate), and win-type-small (for
link), we get a quick app that looks like Figure 3-9. You can look at the code to see the details. 22
22 WinRT has a specific API for dealing with RSS feeds in Windows.Web.Syndication, which we’ll see in Chapter 14. You can
use this if you want a more structured means of dealing with such data sources. As it is, JavaScript has intrinsic APIs to
work with XML, so it’s really your choice. In a case like this, the syndication API along with Windows.Web.AtomPub and
Windows.Data.Xml are very much needed by Windows 8 apps written in other languages that don’t have the same built-in
features as JavaScript.
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