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pounding the network, flaring up the CPU, and so forth. I, for one, have dug through Task Manager and
the Resource Monitor to figure out—and kill off—whatever process is pouring molasses on my system,
let come what will. This is the kind of user experience that Windows 8 is trying to avoid.
“OK,” you say (assuming that I’ve actually convinced you to some small degree), “does that mean
that there isn’t any way to do some background work like indexing data, creating picture thumbnails,
processing video, and so on?”
Actually, there are ways to do this. For one, when an app is in the foreground, it can do however
much it wants of all these things because it’s ultimately responsible for its own user experience. (An app
written in JavaScript can, for such purposes, employ web workers to move such work off the UI thread,
as well as delegate tasks to WinRT components that do their work on other threads and return results
asynchronously. We’ll look at this in Chapter 16.)
Second, when a device is on AC power instead of battery, Windows allows apps to run background
tasks in response to maintenance triggers on 15-minute or longer intervals (whatever is appropriate for
the app). These are still limited in the total amount of CPU time they can consume, but tasks that don’t
involve UI—which background tasks are not allowed—can burn through a few billion instructions in one
or two seconds on a gigahertz CPU!
What we have in this whole story, then, are three distinct classes of background tasks and their
associated triggers:
• Tasks for maintenance triggers that run on AC power only
• Tasks for potentially conditioned system triggers that run on AC or battery and don’t require
being on the lock screen
• Tasks for those privileged apps that the user had added to the lock screen
We’ll look at each of these in detail in the sections that follow, but first there are a few aspects that
are shared by them all: declaring background tasks in the manifest, the general process of building the
task with the WinRT API, and conditions.
Background Tasks in the Manifest
All background tasks for an app are declared in the manifest, where each declaration indicates the type
of task as well as the code to execute for that task, as shown in Figure 13-18. We’ve seen this section of
the manifest before in Chapter 10 where we checked Audio for a background audio app. As for the
other options, System Event is used for the first two classes of background tasks in the list above, and
Control Channel, Timer, and Push Notification are specifically for tasks reserved for lock screen apps.
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