12.2. Completing a Long-Running Task in the Background
Problem
You want to borrow some time from iOS to complete a long-running task when your application is being sent to the background.
Solution
Use the beginBackgroundTaskWithExpirationHandler:
instance method of UIApplication
.
After you have finished the task, call the endBackgroundTask:
instance method of UIApplication
.
Discussion
When an iOS application is sent to the background, its main
thread is paused. The threads you create
within your application using the detachNewThreadSelector:to
Target:withObject:
class method of NSThread
are also
suspended. If you are attempting to finish a long-running task when
your application is being sent to the background, you must call the
beginBackgroundTaskWithExpirationHandler:
instance method of UIApplication
to
borrow some time from iOS. The backgroundTimeRemaining
property of UIApplication
contains the number of seconds
the application has to finish its job. If the application doesn’t
finish the long-running task before this time expires, iOS will
terminate the application. Every call to
the beginBackgroundTaskWithExpiration
Handler:
method must
have a corresponding call to endBackgroundTask:
(another instance method of UIApplication
). In other words, if you ask for more time from iOS to complete a task, you must tell iOS when you are done with that task. Once this is done and no more tasks are requested to be running in the background, your application will be fully put into the background ...
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