MICROSOFT_PRESS_EBOOK_PROGRAMMING_WINDOWS_8_APPS_WITH_HTML_CSS_AND_JAVASCRIPT_PDF

Windows.Networking.BackgroundTransfer.BackgroundDownloader.getCurrentDownloadsAsync()
.done(function (downloads) {
for (var i = 0; i < downloads.size; i++) {
var download = downloads[i];
}
});
Windows.Networking.BackgroundTransfer.BackgroundUploader.getCurrentUploadsAsync()
.done(function (uploads) {
for (var i = 0; i < uploads.size; i++) {
var upload = uploads[i];
}
});
In each case, the progress property of each operation will tell you how far the transfer has come
along. The progress.status property is especially important. Again, status is a
Background-TransferStatus value and will be one of idle, running, pausedByApplication,
pausedCosted-Network, pausedNoNetwork, canceled, error, and completed). These are clearly necessary to
inform the user, as appropriate, and to give her the ability to restart transfers that are paused or
experienced an error, to pause running transfers, and to act on completed transfers.
Speaking of which, when using the background transfer API, an app should always give the user
control over pending transfers. Downloads can be paused through the DownloadOperation.pause
method and resumed through DownloadOperation.resume. (There are no equivalents for uploads.)
Download and upload operations are canceled by canceling the promises returned from startAsync.
This brings up an interesting situation: if your app has been terminated and later restarted, how do
you restart transfers that were paused? The answer is quite simple. By enumerating transfers through
getCurrentDownloadsAsync and getCurrentUploadsAsync, incomplete transfers are automatically
restarted. But then how do you get back to the promises originally returned by the startAsync
methods? Those are not values that you can save in your app state and reload on startup, and yet you
need them to be able to cancel those operations, if necessary, and also to attach your completed, error,
and progress handlers.
For this reason, both DownloadOperation and UploadOperation provide a method called attachAsync,
which returns a promise for the operation just like startAsync did originally. You can then call the
promise’s then or done methods to provide your handlers:
promise = download.attachAsync().then(complete, error, progress);
and call promise.cancel() if needed. In short, when Windows restarts a background transfer and
essentially calls startAsync on your app’s behalf, it holds that promise internally. The attachAsync
methods simply return that new promise.
A final question is whether a suspended app can be notified when a transfer is complete, perhaps to
issue a toast to inform the user. Such a feature isn’t supported in Windows 8 as there is no background
task available for this purpose. At present, then, the user needs to switch back to the app to check on
transfer progress.
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