Background Task

- (void)applicationDidEnterBackground:(UIApplication *)application
{
    // register bgTask while enter background mode
    __block UIBackgroundTaskIdentifier bgTask = [application beginBackgroundTaskWithExpirationHandler:^{
        [application endBackgroundTask:bgTask];
        bgTask = UIBackgroundTaskInvalid;
    }];
}

Enable background task in AppDelegate.m for CrystalExpress™ SDK to have extra time to fetch ad creatives when app entering background.