oc中检测网络状态

    // 监测网络情况
    [[NSNotificationCenter defaultCenter] addObserver:self
                                             selector:@selector(reachabilityChanged:)
                                                 name: kReachabilityChangedNotification
                                               object: nil];
    hostReach = [[Reachability reachabilityWithHostName:@"www.google.com"] retain];

    [hostReach startNotifier];

- (void)reachabilityChanged:(NSNotification *)note {
    Reachability* curReach = [note object];
    NSParameterAssert([curReach isKindOfClass: [Reachability class]]);
    NetworkStatus status = [curReach currentReachabilityStatus];

    if (status == NotReachable) {
        UIAlertView *alert = [[UIAlertView alloc] initWithTitle:@"AppName"
                              message:@"NotReachable"
                              delegate:nil
                              cancelButtonTitle:@"YES" otherButtonTitles:nil];
                              [alert show];
                              [alert release];
    }
}