GrabDuck

setKeepAliveTimeout and BackgroundTasks

:

I've a big headache with the topic. I'm working on an application that needs to poll a webserver regularly, in order to check for new data. Based on the returned information, I wish to push a local notification to the user.

I know that this approach is slightly different from the one depicted by Apple, in which a remote server makes the work, pushing a remote notification, based on APNS. However, there are many reasons for which i cannot take this approach in consideration. One for all, is the user authentication mechanism. The remote server, for security reasons, cannot take into account the user credentials. All that i can do is to move the login and fetching core, to the client (iPhone).

I noticed that Apple offers an opportunity for applications to wake-up and keep opened a Socket connection (ie. a VoIP application).

So, I started investigate in this way. Added the required information in the plist, I'm able to "wake" my application, using something like this in my appDelegate:

[[UIApplication sharedApplication] setKeepAliveTimeout:1200 handler:^{ 
    NSLog(@"startingKeepAliveTimeout");
    [self contentViewLog:@"startingKeepAliveTimeout"];
    MyPushOperation *op = [[MyPushOperation alloc] initWithNotificationFlag:0 andDataSource:nil];
    [queue addOperation:op];
    [op release];
}];

The NSOperation, then starts a background task using the following block code:

#pragma mark SyncRequests
-(void) main {
    NSLog(@"startSyncRequest");
    [self contentViewLog:@"startSyncRequest"];
    bgTask = [app beginBackgroundTaskWithExpirationHandler:^{ 
        NSLog(@"exipiration handler triggered");
        [app endBackgroundTask:bgTask];
        bgTask = UIBackgroundTaskInvalid;
        [self cancel];
    }];


        dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
            NSMutableURLRequest *anURLRequest;
            NSURLResponse *outResponse;
            NSError *exitError;
            NSString *username;
            NSString *password;

            NSLog(@"FirstLogin");
            anURLRequest = [NSMutableURLRequest requestWithURL:[NSURL URLWithString:[NSString stringWithFormat:webserverLogin, username, password]]];
            [anURLRequest setHTTPMethod:@"GET"];
            [anURLRequest setTimeoutInterval:120.00];
            [anURLRequest setCachePolicy:NSURLRequestReloadIgnoringCacheData];

            exitError = nil;
            NSData *tmpData = [NSURLConnection sendSynchronousRequest:anURLRequest returningResponse:&outResponse error:&exitError];
            [anURLRequest setTimeoutInterval:120.00];
            if(exitError != nil) { //somethings goes wrong
                NSLog(@"somethings goes wrong");
                [app endBackgroundTask:bgTask];
                bgTask = UIBackgroundTaskInvalid;
                [self cancel];
                return;
            }

            //do some stuff with NSData and prompt the user with a UILocalNotification

            NSLog(@"AlltasksCompleted");
            [app endBackgroundTask:bgTask];
            bgTask = UIBackgroundTaskInvalid;
            [self cancel];
        });
    }
}

The above code seems to work (sometimes), but many other it crashes my application, with the following log information:

Exception Type:  00000020
Exception Codes: 0x8badf00d
Highlighted Thread:  3

Application Specific Information:
DemoBackApp[5977] has active assertions beyond permitted time: 
{(
    <SBProcessAssertion: 0xa9da0b0> identifier: UIKitBackgroundCompletionTask process: DemoBackApp[5977] permittedBackgroundDuration: 600.000000 reason: finishTask owner pid:5977 preventSuspend  preventIdleSleep 
)}

Elapsed total CPU time (seconds): 0.010 (user 0.010, system 0.000), 100% CPU 
Elapsed application CPU time (seconds): 0.000, 0% CPU

For ones who ask, yes. I've tried the Async NSURLConnection approach, too. No matter. It crash the same, even if I use an async approach with timeout handler and didFinishLoading:WithError.

I'm stuck. Any hints are high appreciated.