检测网络状态的代码，使用block的形式：

Reachability * reach = [Reachability reachabilityWithHostname:@"www.google.com"];
    reach.reachableBlock = ^(Reachability * reachability)
    {
        dispatch_async(dispatch_get_main_queue(), ^{
            blockLabel.text = @"Block Says Reachable";
        });
    };
    reach.unreachableBlock = ^(Reachability * reachability)
    {
        dispatch_async(dispatch_get_main_queue(), ^{
            blockLabel.text = @"Block Says Unreachable";
        });
    };
    [reach startNotifier];

使用通知的形式：

- (void)viewDidLoad
{

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



-(void)reachabilityChanged:(NSNotification*)note
{
    Reachability * reach = [note object];
    
    if([reach isReachable])
    {
        notificationLabel.text = @"Notification Says Reachable";
    }
    else
    {
        notificationLabel.text = @"Notification Says Unreachable";
    }
}