Question

My application has to poll a server for a maximum of 10 minutes (using RestKit), even if the application is sent to the background. (polling always starts while the application is in the foreground)

I have a View Controller (not the RootViewController) that listens to applicationDidEnterBackground.

Also, there’s a class “Order” that has a method “poll” which is used to send a request to the server, and several other callback methods for “timeout”, “request cancel”, “handle response”, etc.

- (void)poll
{
    RKRequest* request = [[RKClient sharedClient] requestWithResourcePath:@"/foo.php" delegate:self];
request.backgroundPolicy = RKRequestBackgroundPolicyContinue;    
[request send];    

NSLog(@"I am your RKClient singleton : %@", [RKClient sharedClient]);        
}
- (void)requestDidStartLoad:(RKRequest *)request {
NSLog(@"requestDidStartLoad");
}
- (void)requestDidTimeout:(RKRequest *)request {
NSLog(@"requestDidTimeout");
}
- (void)request:(RKRequest *)request didFailLoadWithError:(NSError *)error {
NSLog(@"didFailLoadWithError");
}
- (void)request:(RKRequest*)request didLoadResponse:(RKResponse*)response 
{  
}

While the app is in the foreground, everything works fine and the callbacks are triggered.

When my application enters the background i want to continue polling the server. I use this method, “poll” is called, but no callbacks are triggered..

- (void)applicationDidEnterBackground:(NSNotification *) notification 
{
Order *order = [[Order alloc] init];

UIApplication *app = [UIApplication sharedApplication];

__block UIBackgroundTaskIdentifier taskId;

taskId = [app beginBackgroundTaskWithExpirationHandler:^{    
    [app endBackgroundTask:taskId]; 
}];

if (taskId == UIBackgroundTaskInvalid) { 
    return;
}

dispatch_async(dispatch_get_global_queue(0, 0), ^{ 

    while(YES)
    {
        sleep(1);

        [order poll];
    }

    [app endBackgroundTask:taskId]; 
});

[order release];

}

What am I doing wrong?

Answer 1

I don’t know this RKClient you’re using but probably it’s based on NSURLConnection API. This asynchronous API calls delegates only if it’s running inside a run-loop; from NSURLConnection documentation:

Messages to the delegate will be sent on the thread that calls this method. For the connection to work correctly the calling thread’s run loop must be operating in the default run loop mode.

Unfortunately GCD doesn’t guarantee you to run a block inside a thread which executes a run-loop. The suggestion in such case is that you run your “poll” inside a NSOperation which is optimized for this kind of situations.