添加的调用的 startNotifier 方法只会执行一次，不知为何 ？
代码如下：
- (void)viewDidLoad
{
[super viewDidLoad];
// Do any additional setup after loading the view, typically from a nib.

Reachability    *reach = [Reachability reachabilityForInternetConnection];

self.reach = reach;

// NSLog(@"%d",[reach currentReachabilityStatus]);

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

[self.reach startNotifier];

}

• (void)networkChanged:(NSNotification *)sender
  {
  NSLog(@"%@",sender.object);
  }