Asynchronous, scheduled outbox thread. This snippet was used for a bespoke project where they required phones to be syncing up constantly with their API. This allows for all requests to be queued on the device and synced up at a later date when there is connectivity.

Outbox.m

// This method acts as an entry point for the outbox service thread, it will create an additional thread in a continuous loop for the outbox services
- (void)startAndObserveOutboxThread {

    // Only carry out the rest of the method if this isn't the main thread and there isn't a locationServiceObserverThread already running
    if ([NSThread isMainThread] || outboxServiceObserverThread != nil) {
        return;
    }

    outboxServiceObserverThread = [NSThread currentThread];

    // 'Get permission' to run in the background indefinitely by passing expiration handler that 'ends' and restarts background task
    // iOS calls this when our background time is up, we have to play nice and end the thread, before creating another
    dispatch_block_t __block expirationHandler = ^{
        if ([outboxServiceObserverThread isExecuting]) {
            // Inform observer thread that it is time to cancel
            [outboxServiceObserverThread cancel];
            // Wait for thread to finish
            while (![outboxServiceObserverThread isCancelled]) {
                [NSThread sleepForTimeInterval: 0.1];
            }
            // Start it again
            [self performSelectorInBackground:@selector(startAndObserveOutboxThread) withObject:nil];
        }
    };
    outboxServiceIdentifier = [[UIApplication sharedApplication] beginBackgroundTaskWithExpirationHandler: expirationHandler];

    // Run loop while observer thread hasn't been cancelled
    while(![outboxServiceObserverThread isCancelled]) {
        // If we don't have an active service work thread, create one
        if (outboxServiceWorkThread == nil || [outboxServiceWorkThread isFinished] || [outboxServiceWorkThread isCancelled]) {
            NSLog(@"outboxServiceObserverThread: Starting work thread.");
            outboxServiceWorkThread = [[NSThread alloc] initWithTarget:[WpDatabase sharedDatabase] selector:@selector(schedule) object:nil];
            [outboxServiceWorkThread start];
        } else {
            NSLog(@"outboxServiceObserverThread: Work thread is still running.");
        }

        // Sleep observer thread for 60 seconds, so we aren't over checking
        [NSThread sleepForTimeInterval: 15];
        NSLog(@"outboxServiceObserverThread: Checking work thread's state.");
    }
    // Mark thread as finished
    [[UIApplication sharedApplication] endBackgroundTask: outboxServiceIdentifier];
    // Set thread ref to nil
    outboxServiceObserverThread = nil;
}