iOS-何时加载外部数据

在我的应用程序委派中,我有以下内容:

Reachability* reach = [Reachability reachabilityWithHostname:@"www.google.com"];

[[NSNotificationCenter defaultCenter] addObserver:self 
                                         selector:@selector(reachabilityChanged:) 
                                             name:kReachabilityChangedNotification 
                                           object:nil];

[reach startNotifier];

HomeViewController_iPhone *homeViewController = [[HomeViewController_iPhone alloc] initWithNibName:@"HomeViewController_iPhone" bundle:nil];
homeViewController.managedObjectContext = self.managedObjectContext;
UINavigationController *homeNavController = [[UINavigationController alloc] initWithRootViewController: homeViewController];
homeNavController.navigationBar.barStyle = UIBarStyleBlackTranslucent;

self.tabBarController.viewControllers = [NSArray arrayWithObjects:homeNavController, nil];

...

-(void)reachabilityChanged:(NSNotification*)note
{
Reachability * reach = [note object];

if([reach isReachable])
{
    NSLog(@"Notification Says Reachable");
    self.isConnected = YES;
}
else
{
    NSLog(@"Notification Says UN-Reachable");
    self.isConnected = NO;
}
}

问题是,在我的HomeViewController(viewDidLoad)中,我这样做:

AppDelegate *appDelegate = (AppDelegate*)[[UIApplication sharedApplication] delegate];

if (appDelegate.isConnected)
{
    dispatch_async(kBgQueue, ^{
        NSData* data = [NSData dataWithContentsOfURL: kFeedURL];
        [self performSelectorOnMainThread:@selector(fetchedData:) 
                               withObject:data waitUntilDone:YES];
    });
}

但appDelegate.isConnected始终为NO,即使我有连接。我认为检查是在Reachability类建立连接之前进行的。但是我在哪里打电话来获取数据呢?我已经尝试过viewDidLoad和viewWillAppear,但isConnected在这两个点上仍然是NO。