MICROSOFT_PRESS_EBOOK_PROGRAMMING_WINDOWS_8_APPS_WITH_HTML_CSS_AND_JAVASCRIPT_PDF

Your app should go through this short process to obtain the necessary channel URIs whenever it’s
launched as well as when it’s resumed (especially if any channel’s expirationTime has passed). It’s
unlikely that an app would stay suspended for that long, but it’s still possible! Furthermore, if you’re
concerned that your app might not run for more than 30 days, you can implement a background task
on a maintenance trigger for this purpose. See “Tasks for Maintenance Triggers” later in this chapter and
Scenario 2 of the sample.
Again note that you may have more than one channel URI if you’re also using push notifications for
secondary tiles as well as your app tile. In this case you’ll be managing a separate channel URIs for each
tile.
Each time through the process, save the channel URI for each tile in your local app state. This is so
that you can check on subsequent runs if the URI is the same as one you’ve already obtained and sent to
your web service, in which case you can avoid unnecessary network traffic.
Sending the URI to your web service can be done with a simple call to WinJS.xhr, as in the sample
(inside channelOperation.done). Here we also see the checks for whether the URI is the same as before:
channelOperation.done(function (newChannel) {
// _urls[] is an array of channel ids for primary and secondary tiles
var tileData = that._urls[itemId];
// Upload the channel URI if the client hasn't recorded sending the same
// uri to the server
if (tileData && newChannel.uri === tileData.channelUri) {
// This saves the URI to local app data
that._updateUrl(url, newChannel.uri, itemId, isPrimaryTile);
completed(newChannel);
} else {
WinJS.xhr({
type: "POST",
url: url,
headers: { "Content-Type": "application/x-www-form-urlencoded" },
data: "channelUri=" + encodeURIComponent(newChannel.uri) +
"&itemId=" + encodeURIComponent(itemId)
}).done(function (request) {
// Only update the data on the client if uploading the channel URI succeeds.
// If it fails, you may considered setting another background task, trying
// again, etc. (An exception will be thrown if it fails, ending up in the
// error hander instead.)
that._updateUrl(url, newChannel.uri, itemId, isPrimaryTile);
completed(newChannel);
}, failed);
}
}, failed);
609