监听网络

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    self.window = [[[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]] autorelease];
    
    //开启网络状况的监听
    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(reachabilityChanged:) name:kReachabilityChangedNotification object:nil];
    
    self.hostReach = [Reachability reachabilityWithHostName:@"www.baidu.com"] ;
    [self.hostReach startNotifier];  //开始监听，会启动一个run loop

    self.window.rootViewController = self.tabBarController;
    [self.window makeKeyAndVisible];
    return YES;
}

//网络链接改变时会调用的方法
-(void)reachabilityChanged:(NSNotification *)note
{
    Reachability *currReach = [note object];
    NSParameterAssert([currReach isKindOfClass:[Reachability class]]);
    
    //对连接改变做出响应处理动作
    NetworkStatus status = [currReach currentReachabilityStatus];
    //如果没有连接到网络就弹出提醒实况
    self.isReachable = YES;
    if(status == NotReachable)
    {
        UIAlertView *alert = [[UIAlertView alloc] initWithTitle:@"网络连接异常" message:@"暂无法访问书城信息" delegate:nil cancelButtonTitle:@"确定" otherButtonTitles:nil];
        [alert show];
        [alert release];
        self.isReachable = NO;
    }
    else
    {
        UIAlertView *alert = [[UIAlertView alloc] initWithTitle:@"网络连接信息" message:@"网络连接正常" delegate:nil cancelButtonTitle:@"确定" otherButtonTitles:nil];
        [alert show];
        [alert release];
        self.isReachable = YES;
    }
}

    AppDelegate *appDlg = (AppDelegate *)[[UIApplication sharedApplication] delegate];
    if(appDlg.isReachable)
    {
        NSLog(@"网络已连接");//执行网络正常时的代码
    }
    else
    {
        NSLog(@"网络连接异常");//执行网络异常时的代码
    }