Apple's background execution example for ios - ios

Question

Apple gives this of background execution:
- (void)applicationDidEnterBackground:(UIApplication *)application
{
bgTask = [application beginBackgroundTaskWithName:#"MyTask"
expirationHandler:^{
// Clean up any unfinished task business by marking where you
// stopped or ending the task outright.
[application endBackgroundTask:bgTask];
bgTask = UIBackgroundTaskInvalid;
}];
// Start the long-running task and return immediately.
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
// Do the work associated with the task, preferably in chunks.
[application endBackgroundTask:bgTask];
bgTask = UIBackgroundTaskInvalid;
});
}
This example has never made much sense to me and I've seen it copied to numerous background application examples.
The first thing that doesn't make sense are these two lines in the expirationHandler:
[application endBackgroundTask:bgTask];
bgTask = UIBackgroundTaskInvalid;
It seems like bgTask won't have a value when captured in the block. The compiler complains as such. Then below in the dispatch_async the sample shows the same two lines. I would expect it in the dispatch_async but not in the block. Can anyone explain why we have these lines in the block?
Also the documentation per beginBackgroundTaskWithName says "Marks the beginning of a new long-running background task." How exactly is it doing this? What defines the task? Is it any code that follows in the block scope?

Answer 1

bgTask = [application beginBackgroundTaskWithName:#"MyTask" expirationHandler:]
tells iOS that your application is starting a new background task. iOS doesn't care which code comprises the task, it just knows that it needs to give your app more time to execute in the background.
After this line executes, bgTask will contain the new background task identifier.
When your background task is complete, you then call [application endBackgroundTask:bgTask]; and iOS knows that your app has finished the specified background task and may not need any more background execution time (You may still have other background tasks initiated by beginBackgroundTaskWithName:expirationHandler outstanding).
The line:
bgTask = UIBackgroundTaskInvalid;
is just housekeeping; If you omit this line nothing bad will happen, but bgTask will contain an invalid identifier.
If you don't call endBackgroundTask before your app's background time expires, then then expiration handler block will be invoked.
In the expiration handler bgTask will have the value that was assigned when you called beginBackgroundTaskWithName:expirationHandler, so this is what is passed to endBackgroundTask and again assigning UIBackgroundTaskInvalid is just housekeeping