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triggers—subject to specific quotas on CPU time and network I/O. As you might expect, an app declares
such background tasks in its manifest.
Triggers include a change in network connectivity, a time zone change, an update of an app, the
expiration of a timer (with a 15-minute resolution), or the arrival of a push notification from an online
source (that is, a notification sent in response to a condition that’s completely external to the device
itself). Each trigger can also be configured with conditions such as whether there is Internet connectivity
or not. Whatever the case, the whole purpose of background tasks is not to launch an app—in fact,
background tasks cannot display arbitrary UI. It is rather to allow them to update their internal state
and, when needed, issue tile updates or toast notifications through which the user can make the choice
to activate the app for further action.
One additional aspect of background tasks is that Windows also places a limit on the total number of
apps that can handle certain kinds of triggers: timers, receipt of push notifications, and receipt of
network traffic on a control channel as used by real-time communications apps. The limit is imposed by
the fact that such apps must be added to the lock screen for their tasks to run at all.
The lock screen, as you certainly know by now and as shown in Figure 13-9, is what’s displayed
anytime the user must log into the device. A device will be locked directly by the user or after a period
of inactivity. An exception is made when an app has disabled auto-locking through the Windows.-
System.Display.DisplayRequest API, as discussed in Chapter 10 in the “Disabling Screen Savers and the
Lock Screen During Playback” section.
FIGURE 13-9 A typical lock screen. Up to seven apps can display badges along the bottom of the screen; one app
can display text next to the clock.
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