网络监听主要是基于ASIHTTPRequest内的Reachability的调用

可以在AppDelegate内写，也可以自己定义单例来写这个操作。

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    // Override point for customization after application launch.
    
    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(reachabilityChanged:) name:kReachabilityChangedNotification object:nil];
    self.hostReach = [Reachability reachabilityWithHostName:@"www.apple.com"] ;
    [self.hostReach startNotifier]; //开始监听，会启动一个run loop
    
   
    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:@"请检测网络连接" delegate:nil cancelButtonTitle:@"确定" otherButtonTitles:nil];
        [alert show];
        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];
        self.isReachable = YES; 
    }
}

//调用方法
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   
} 

不用于appdelegate里面的方法：

#import

@interface CheckNetwork : NSObject {
   
}
+(BOOL)isExistenceNetwork;
@end
#import "CheckNetwork.h"
#import "Reachability.h"
@implementation CheckNetwork
+(BOOL)isExistenceNetwork
{
    BOOL isExistenceNetwork = FALSE;
    Reachability *r = [Reachability reachabilityWithHostName:@"www.apple.com"];
    switch ([r currentReachabilityStatus]) {
        case NotReachable:
            isExistenceNetwork=FALSE;
         //   NSLog(@"没有网络");
            break;
        case ReachableViaWWAN:
            isExistenceNetwork=TRUE;
         //   NSLog(@"正在使用3G网络");
            break;
        case ReachableViaWiFi:
            isExistenceNetwork=TRUE;
          //  NSLog(@"正在使用wifi网络");       
            break;
    }
//    if (!isExistenceNetwork) {
//        UIAlertView *myalert = [[UIAlertView alloc] initWithTitle:@"警告" message:@"网络不存在" delegate:self cancelButtonTitle:@"确认" otherButtonTitles:nil,nil];
//        [myalert show];
//        [myalert release];
//    }
    return isExistenceNetwork;
}
@end