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XmlHttpRequest
As we’ve seen a number of times already in this book, transferring data to and from web services with
XmlHttpRequest is a common activity for Windows Store apps, especially those written in JavaScript for
which handling XML and/or JSON is simple and straightforward. This is especially true when using the
WinJS.xhr wrapper that turns the whole process into a simple promise.
To build on what we already covered in Chapter 3, in the section “Data from Services and WinJS.xhr,”
there are a few other points to make where such requests are concerned, most of which come from the
section in the documentation entitled Connecting to a web service.
First, Downloading different types of content provides the details of the different content types
supported by XHR for Windows Store apps. These are summarized here:
Type Use responseText responseXML
arraybuffer Binary content as an array of Int8 or Int64, or another integer or float undefined
undefined
type.
Blob Binary content represented as a single entity. undefined undefined
document An XML DOM object representing XML content (MIME type of undefined
The XML content
text/XML).
json JSON strings. The JSON string undefined
ms-stream Streaming data; see XmlHttpRequest enhancements. undefined undefined
Text Text (the default). The text string undefined
Second, know that XHR responses can be automatically cached, meaning that later requests to the
same URI might return old data. To resend the request despite the cache, add an If-Modified-Since HTTP
header as shown on How to ensure that WinJS.xhr resends requests.
Along similar lines, you can wrap a WinJS.xhr operation in another promise to encapsulate
automatic retries if there is an error in any given request. That is, build your retry logic around the core
XHR operation, with the result stored in some variable. Then place that whole block of code within
WinJS.Promise.wrap (or a new WinJS.Promise) and use that elsewhere in the app.
In each XHR attempt, remember that you can also use WinJS.Promise.timeout in conjunction with
WinJS.Xhr as described on Setting timeout values with WinJS.xhr., because WinJS.xhr doesn’t have a
timeout notion directly. You can, of course, set a timeout in the raw request, but that would mean
rebuilding everything that WinJS.xhr already does.
Generally speaking, XHR headers are accessible to the app with the exception of cookies (the
set-cookie and set-cookie2 headers), as these are filtered out by design for XHR done from a local
context. They are not filtered for XHR from the web context, so if you need cookies, try acquiring them
in a web context iframe and pass them to a local context using postMessage.
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