MICROSOFT_PRESS_EBOOK_PROGRAMMING_WINDOWS_8_APPS_WITH_HTML_CSS_AND_JAVASCRIPT_PDF

There is also a variation of createDownload that takes a second StorageFile argument whose
contents provide the body of the HTTP GET or FTP RETR request that will be sent to the server URI
before the download is started. This accommodates some web sites that require you to fill out a form to
start the download.
Sidebar: Where Is Cancel?
You might have already noticed that neither DownloadOperation nor UploadOperation has a
cancellation method. So how is this accomplished? You cancel the transfer by canceling the
startAsync operation—that is, call the cancel method of the promise returned by startAsync.
This means that you need to hold onto the promises for each transfers you initiate.
Basic Uploads
Scenario 2 of the Background transfer sample (js/uploadFile.js) exercises the background upload
capability, specifically sending some file (chosen through the file picker) to a URI that can receive it. By
default the URI points to http://localhost/BackgroundTransferSample/upload.aspx, a page installed with
the PowerShell script that sets up the server. As with Scenario 1, the URI entry control is disabled
because the sample performs no validation, as you would again always want to do if you accepted any
URI from an untrusted source (user input in this case). For testing purposes, of course, you can remove
disabled="disabled" from the serverAddressField element in html/uploadFile.html and enter other URIs
that will exercise your own upload services. This is especially handy if you run the server part of the
sample in Visual Studio 2012 Express for Web where the URI will need a localhost port number as
assigned by the debugger.
In addition to a button to start an upload and to cancel it, the sample provides another button to
start a multipart upload; we’ll talk more of this in the “Multipart Uploads” section below.
In code, an upload happens very much like a download. Assuming you have a StorageFile with the
contents to upload, create an UploadOperation object for the transfer with
BackgroundUploader.createUpload. If, on the other hand, you have data in a stream (IInputStream),
create the operation object with BackgroundUploader.createUploadFromStreamAsync instead. This
can also be used to break up a large file into discrete chunks, if the server can accommodate it; see
“Breaking Up Large Files” below.
With the operation object in hand you can customize a few properties of the transfer, overriding the
defaults provided by the BackgroundUploader. These are method (HTTP POST or PUT, or FTP STOR),
costPolicy, and group. For the latter, again see “Setting Cost Policy” and “Grouping Multiple Transfers”
below.
Once you’re ready, then, calling the operation’s startAsync will proceed with the upload: 73
73 As with downloads, the code in the sample has more structure than shown here and again defines its own
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