MICROSOFT_PRESS_EBOOK_PROGRAMMING_WINDOWS_8_APPS_WITH_HTML_CSS_AND_JAVASCRIPT_PDF

Note Working with the lock screen is not supported in the Visual Studio simulator. To debug lock
screen apps and background tasks, you’ll need to use the Local Machine or Remote Machine
debugging options.
Background tasks for these triggers are created and registered as we’ve already seen. A TimeTrigger,
for example, is created with its freshnessTime interval (in minutes) and a oneShot flag, as seen in
Scenario 5 of the Background tasks sample (js/time-trigger-background-task.js):
BackgroundTaskSample.registerBackgroundTask(
BackgroundTaskSample.sampleBackgroundTaskEntryPoint,
BackgroundTaskSample.timeTriggerTaskName,
new Windows.ApplicationModel.Background.TimeTrigger(15, false), null);
A TimeTrigger is also used in Scenario 3 of the Geolocation sample to allow the user to add a
navigation app to the lock screen for more continuous tracking. Generally speaking, though, a
navigation app isn’t particularly useful on the lock screen in the first place, since it wouldn’t be able to
show a map! Better, then, to again use the Windows.System.Display.DisplayRequest API to prevent
going to the lock screen at all.
Creating a PushNotificationTrigger is even simpler as there are no parameters. This can be seen in
the Raw notifications sample, Scenario 1 (js/scenario1.js):
function registerBackgroundTask() {
// Register the background task for raw notifications
var taskBuilder = new background.BackgroundTaskBuilder();
var trigger = new background.PushNotificationTrigger();
taskBuilder.setTrigger(trigger);
taskBuilder.taskEntryPoint = sampleTaskEntryPoint;
taskBuilder.name = sampleTaskName;
}
var task = taskBuilder.register();
task.addEventListener("completed", backgroundTaskComplete);
Although the call to BackgroundTaskBuilder.register might succeed, the task itself will not execute
until the user add the app to the lock screen, as we saw earlier in Figure 13-10. This latter action is never
under the app’s control—all it can do is make sure it’s available for the user to select on that section of
PC Settings, which is what asking for access is all about.
The request is made through the method Windows.ApplicationModel.Background.-
BackgroundExecutionManager.requestAccessAsync; this call should be made prior to registering the
background task (see Scenario 5 of the Background task sample again):
Windows.ApplicationModel.Background.BackgroundExecutionManager.requestAccessAsync();
When this is called the first time in an app, it will generate a user consent prompt, as shown in Figure
13-19. If the user chooses Allow, the app will appear in PC Settings as an option for the lock screen,
otherwise it won’t. As with other permissions, users can change their minds later on through the
Permissions settings, as shown in Figure 13-20.
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