MICROSOFT_PRESS_EBOOK_PROGRAMMING_WINDOWS_8_APPS_WITH_HTML_CSS_AND_JAVASCRIPT_PDF

var internetCondition = new
background.SystemCondition(background.SystemConditionType.internetAvailable);
taskBuilder.addCondition(internetCondition);
taskBuilder.register();
Because the expiration period for channel URIs is 30 days, the sample creates a trigger on a recurring
10-day interval (10 days * 24 hours/day * 60 minutes/hour). It also wisely adds the internetAvailable
condition because it’s again pointless to attempt to renew channel URIs when there’s no connectivity.
The task itself can be found in the js/backgroundTask.js file of the sample, as indicated in the
taskEntryPoint property:
(function () {
// Import the Notifier helper object
importScripts("//Microsoft.WinJS.1.0/js/base.js");
importScripts("notifications.js");
var closeFunction = function () {
close();
};
var notifier = new SampleNotifications.Notifier();
notifier.renewAllAsync().done(closeFunction, closeFunction);
})();
This task code pulls in a couple of other script files using importScripts, the second of which,
notifications.js, is the sample’s set of helper functions for notifications where renewAllAsync refreshes
the app’s list of previously saved channel URIs.
Important You’ll also notice that the completed and error handlers given to the promise from
renewAllAsync both go to closeFunction, which makes this mysterious call to close. What close is
this? Well, it’s not window.close (as you might guess) but rather WorkerGlobalScope.-close.
Background tasks in an app written in JavaScript run within the scope of a web worker, so the global
scope within the code is WorkerGlobalScope rather than window. Calling this makes sure the
independently running background task is shut down and guarantees that the resources that were
allocated for the task are properly released.
Sidebar: The Task Instance and Background Task Deferrals
Within a JavaScript background task, the
Windows.UI.WebUI.WebUIBackgroundTask-Instance.current contains a
WebUIBackgroundTaskInstanceRuntimeClass object that provides additional details about the
running task: its instanceId, its associated Background-TaskRegistration object in the task
property, a progress property in which the task can store a percentage value, a succeeded flag to
indicate that the task has completed, a suspended count (when the task is suspended due to the
resource quota being exceeded), and a canceled event that informs the task that the app as a
whole has been terminated.
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