做iOS开发时，我们需要监控/监听网络状况，苹果提供了Reachability.h, Reachability.m。

导入Reachability.h

我们可以在 MainViewController的viewDidLoad方法内部写上：

[self checkReachability];

#pragma mark
#pragma mark Reachability Methods
#pragma mark
- (void)checkReachability
{
    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(reachabilityChanged:) name:kReachabilityChangedNotification object:nil];
    self.reachability = [Reachability reachabilityForInternetConnection];
    [self.reachability startNotifier];
    [self updateInterfaceWithReachability:self.reachability];
}

/*!
 * Called by Reachability whenever status changes.
 */
- (void) reachabilityChanged:(NSNotification *)note
{
    Reachability* curReach = [note object];
    NSParameterAssert([curReach isKindOfClass:[Reachability class]]);
    [self updateInterfaceWithReachability:curReach];
}

- (void)updateInterfaceWithReachability:(Reachability *)reachability
{
    NetworkStatus status = [reachability currentReachabilityStatus];
    AppDelegate *appDelegate = ((AppDelegate *) [[UIApplication sharedApplication] delegate]);

    if(status == NotReachable)
    {
        //No internet
        NSLog(@"No Internet");
        appDelegate.isNetworkReachable = NO;
        [_reachabilityImage setImage:[UIImage imageNamed:@"stop-32.png"]];
    }
    else if (status == ReachableViaWiFi)
    {
        //WiFi
        NSLog(@"Reachable WIFI");
        appDelegate.isNetworkReachable = YES;
        [_reachabilityImage setImage:[UIImage imageNamed:@"Airport.png"]];
    }
    else if (status == ReachableViaWWAN)
    {
        //3G
        NSLog(@"Reachable 3G");
        appDelegate.isNetworkReachable = YES;
        [_reachabilityImage setImage:[UIImage imageNamed:@"WWAN5.png"]];
    }
}

iOS开发之监听网络连接，改变，断开