iPhone - Backgrounding to poll for events
For quite a while I'd been looking into a way in my iPhone app to poll every X minutes to check the data counters. After much reading of the Background Execution documentation and a few trial apps I'd dismissed this as impossible without abusing the background APIs.
Last week I found this application which does exactly that. http://itunes.apple.com/us/app/dataman-real-time-data-usage/id393282873?mt=8
It runs in the background and keeps track of the count of Cellular/WiFi data you've used. I suspect that the developer is registering his app as tracking location changes but the location services icon isn't visible while the app is running, which I thought was a requirement.
Does anyone have any clues as to how this can be accomplished?
Answer
I have seen this behavior, too. After trying a lot I discovered two things, which could help. But I am still uncertain how this may influence the reviewing process.
If you use one of the backgrounding features, the app will be launched by iOS in background again once it was quit (by the system). This we will abuse later.
In my case I used VoIP backgrounding enabled in my plist. All the code here is done in your AppDelegate:
// if the iOS device allows background execution,
// this Handler will be called
- (void)backgroundHandler {
NSLog(@"### -->VOIP backgrounding callback");
// try to do sth. According to Apple we have ONLY 30 seconds to perform this Task!
// Else the Application will be terminated!
UIApplication* app = [UIApplication sharedApplication];
NSArray* oldNotifications = [app scheduledLocalNotifications];
// Clear out the old notification before scheduling a new one.
if ([oldNotifications count] > 0) [app cancelAllLocalNotifications];
// Create a new notification
UILocalNotification* alarm = [[[UILocalNotification alloc] init] autorelease];
if (alarm)
{
alarm.fireDate = [NSDate date];
alarm.timeZone = [NSTimeZone defaultTimeZone];
alarm.repeatInterval = 0;
alarm.soundName = @"alarmsound.caf";
alarm.alertBody = @"Don't Panic! This is just a Push-Notification Test.";
[app scheduleLocalNotification:alarm];
}
}
and the registration is done in
- (void)applicationDidEnterBackground:(UIApplication *)application {
// This is where you can do your X Minutes, if >= 10Minutes is okay.
BOOL backgroundAccepted = [[UIApplication sharedApplication] setKeepAliveTimeout:600 handler:^{ [self backgroundHandler]; }];
if (backgroundAccepted)
{
NSLog(@"VOIP backgrounding accepted");
}
}
Now the magic happens: I don't even use VoIP-Sockets. But this 10 Minutes callback provides a nice side effect: After 10 Minutes (sometimes earlier) I discovered that my timers and previous running treads are being executed for a short while. You can see this, if you place some NSLog(..) into your code. This means, that this short "wakeup" executes the code for a while. According to Apple we have 30 seconds execution time left. I assume, that background code like threads are being executed for nearly 30 seconds. This is useful code, if you must "sometimes" check something.
The doc says that all background tasks (VoIP, audio, location updates) will be automatically restarted in background if the app was terminated. VoIP apps will be started in background automatically after bootup!
With abusing this behavior, you can make your app be looking like running "forever". Register for one background process (i.e. VoIP). This will cause your app to be restarted after termination.
Now write some "Task has to be finished" code. According to Apple you have some time (5 seconds?) left to finish tasks. I discovered, that this must be CPU time. So that means: if you do nothing, your app is still being executed! Apple suggest to call an expirationhandler, if you are finished with your work. In the code below you can see, that i have a comment at the expirationHandler. This will cause your app running as long as the system allows your app to be running. All timers and threads stay running until iOS terminates your app.
- (void)applicationDidEnterBackground:(UIApplication *)application {
UIApplication* app = [UIApplication sharedApplication];
bgTask = [app beginBackgroundTaskWithExpirationHandler:^{
[app endBackgroundTask:bgTask];
bgTask = UIBackgroundTaskInvalid;
}];
// Start the long-running task and return immediately.
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
// you can do sth. here, or simply do nothing!
// All your background treads and timers are still being executed
while (background)
[self doSomething];
// This is where you can do your "X minutes" in seconds (here 10)
sleep(10);
}
// And never call the expirationHandler, so your App runs
// until the system terminates our process
//[app endBackgroundTask:bgTask];
//bgTask = UIBackgroundTaskInvalid;
});
}
Be very spare with CPU-Time here, and your app runs longer! But one thing is for sure: your app will be terminated after a while. But because you registered your app as VoIP or one of the others, the system restarts the app in background, which will restart your background process ;-) With this PingPong I can do a lot of backgrounding. but remember be very spare with CPU time. And save all data, to restore your views - your app will be terminated some time later. To make it appear still running, you must jump back into your last "state" after wakeup.
I don't know if this is the approach of the apps you mentioned before, but it works for me.
Hope I could help
Update:
After measuring the time of the BG task, there was a surprise. The BG Task is limited to 600 seconds. This is the exact minimum time of the VoIP minimumtime (setKeepAliveTimeout:600).
So THIS code leads into "infinite" execution in background:
Header:
UIBackgroundTaskIdentifier bgTask;
Code:
// if the iOS device allows background execution,
// this Handler will be called
- (void)backgroundHandler {
NSLog(@"### -->VOIP backgrounding callback");
UIApplication* app = [UIApplication sharedApplication];
bgTask = [app beginBackgroundTaskWithExpirationHandler:^{
[app endBackgroundTask:bgTask];
bgTask = UIBackgroundTaskInvalid;
}];
// Start the long-running task
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
while (1) {
NSLog(@"BGTime left: %f", [UIApplication sharedApplication].backgroundTimeRemaining);
[self doSomething];
sleep(1);
}
});
- (void)applicationDidEnterBackground:(UIApplication *)application {
BOOL backgroundAccepted = [[UIApplication sharedApplication] setKeepAliveTimeout:600 handler:^{ [self backgroundHandler]; }];
if (backgroundAccepted)
{
NSLog(@"VOIP backgrounding accepted");
}
UIApplication* app = [UIApplication sharedApplication];
bgTask = [app beginBackgroundTaskWithExpirationHandler:^{
[app endBackgroundTask:bgTask];
bgTask = UIBackgroundTaskInvalid;
}];
// Start the long-running task
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
while (1) {
NSLog(@"BGTime left: %f", [UIApplication sharedApplication].backgroundTimeRemaining);
[self doSomething];
sleep(1);
}
});
}
After your app has timed out, the VoIP expirationHandler will be called, where you simply restart a long running task. This task will be terminated after 600 seconds. But there will be again a call to the expiration handler, which starts another long running task, etc. Now you only have to check weather the App is getting back to foreground. Then close the bgTask, and you're done. Maybe one can do sth. like this inside the expirationHandler from the long running task. Just try it out. Use your Console, to see what happens... Have Fun!
Update 2:
Sometimes simplifying things helps. My new approach is this one:
- (void)applicationDidEnterBackground:(UIApplication *)application {
UIApplication* app = [UIApplication sharedApplication];
// it's better to move "dispatch_block_t expirationHandler"
// into your headerfile and initialize the code somewhere else
// i.e.
// - (void)applicationDidFinishLaunching:(UIApplication *)application {
//
// expirationHandler = ^{ ... } }
// because your app may crash if you initialize expirationHandler twice.
dispatch_block_t expirationHandler;
expirationHandler = ^{
[app endBackgroundTask:bgTask];
bgTask = UIBackgroundTaskInvalid;
bgTask = [app beginBackgroundTaskWithExpirationHandler:expirationHandler];
};
bgTask = [app beginBackgroundTaskWithExpirationHandler:expirationHandler];
// Start the long-running task and return immediately.
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
// inform others to stop tasks, if you like
[[NSNotificationCenter defaultCenter] postNotificationName:@"MyApplicationEntersBackground" object:self];
// do your background work here
});
}
This is working without the VoIP hack. According to the documentation, the expiration handler (in this case my 'expirationHandler' block) will be executed if execution time is over. By defining the block into a block variable, one can recursively start the long running task again within the expiration handler. This leads into endless execution, too.
Be aware to terminate the task, if your application enters foreground again. And terminate the task if you don't need it anymore.
For my own experience I measured something. Using the location callbacks with having the GPS radio on is sucking my battery down very quickly. Using the approach which I posted in Update 2 is taking nearly no energy. According to the "userexperience" this is a better approach. Maybe other Apps work like this, hiding its behavior behind GPS functionality ...