Reachability用于在任何地方判断网络是否可用

 #import <UIKit/UIKit.h>
 #import "Reachability.h"

 @interface AppDelegate : UIResponder <UIApplicationDelegate>

 @property (strong, nonatomic) UIWindow *window;
 @property (strong, nonatomic) Reachability *reach;

 @end

 #import "AppDelegate.h"
 #import "MainViewController.h"

 @implementation AppDelegate

 - (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
 {
     self.window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];
     // Override point for customization after application launch.
     self.window.backgroundColor = [UIColor whiteColor];

     self.window.rootViewController = [[MainViewController alloc]init];

     ///开启网络状况的监听
     [[NSNotificationCenter defaultCenter] addObserver:self
                                              selector:@selector(reachabilityChanged:)
                                                  name:kReachabilityChangedNotification
                                                object:nil];

     self.reach = [Reachability reachabilityWithHostName:@"www.baidu.com"];

     [self.reach startNotifier]; //开始监听，会启动一个run loop


     [self.window makeKeyAndVisible];
     return YES;
 }

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

 //    //用于检测是否是WIFI
 //    NSLog(@"%d",([[Reachability reachabilityForLocalWiFi] currentReachabilityStatus] != NotReachable));
 //    //用于检查是否是3G
 //    NSLog(@"%d",([[Reachability reachabilityForInternetConnection] currentReachabilityStatus] != NotReachable));

     if (status == NotReachable) {
         UIAlertView *alertView = [[UIAlertView alloc] initWithTitle:nil message:@"网络已断开" delegate:nil cancelButtonTitle:@"确定" otherButtonTitles:nil];
         [alertView show];
         NSLog(@"Notification Says Unreachable");
     }else if(status == ReachableViaWWAN){


         UIAlertView *alertView = [[UIAlertView alloc] initWithTitle:nil message:@"移动网络" delegate:nil cancelButtonTitle:@"确定" otherButtonTitles:nil];
         [alertView show];
         NSLog(@"Notification Says mobilenet");
     }else if(status == ReachableViaWiFi){

         UIAlertView *alertView = [[UIAlertView alloc] initWithTitle:nil message:@"WIfi网络" delegate:nil cancelButtonTitle:@"确定" otherButtonTitles:nil];
         [alertView show];
         NSLog(@"Notification Says wifinet");
     }

 }

 - (void)dealloc {
     // 删除通知对象
     [[NSNotificationCenter defaultCenter] removeObserver:self];

 }


 - (void)applicationWillResignActive:(UIApplication *)application
 {
     // Sent when the application is about to move from active to inactive state. This can occur for certain types of temporary interruptions (such as an incoming phone call or SMS message) or when the user quits the application and it begins the transition to the background state.
     // Use this method to pause ongoing tasks, disable timers, and throttle down OpenGL ES frame rates. Games should use this method to pause the game.
 }

 - (void)applicationDidEnterBackground:(UIApplication *)application
 {
     // Use this method to release shared resources, save user data, invalidate timers, and store enough application state information to restore your application to its current state in case it is terminated later.
     // If your application supports background execution, this method is called instead of applicationWillTerminate: when the user quits.
 }

 - (void)applicationWillEnterForeground:(UIApplication *)application
 {
     // Called as part of the transition from the background to the inactive state; here you can undo many of the changes made on entering the background.
 }

 - (void)applicationDidBecomeActive:(UIApplication *)application
 {
     // Restart any tasks that were paused (or not yet started) while the application was inactive. If the application was previously in the background, optionally refresh the user interface.
 }

 - (void)applicationWillTerminate:(UIApplication *)application
 {
     // Called when the application is about to terminate. Save data if appropriate. See also applicationDidEnterBackground:.
 }

在要用的地方：

 - (void)viewDidLoad
 {
     [super viewDidLoad];
     UIButton *btn = [UIButton buttonWithType:UIButtonTypeContactAdd];
     btn.center = CGPointMake(100, 200);
     [btn addTarget:self action:@selector(btnClick) forControlEvents:UIControlEventTouchUpInside];
     [self.view addSubview:btn];

 }

 -(void)btnClick
 {
     AppDelegate *appDlg = (AppDelegate *)[[UIApplication sharedApplication] delegate];
     NSParameterAssert([appDlg.reach isKindOfClass: [Reachability class]]);
     NetworkStatus status = [appDlg.reach currentReachabilityStatus];

     if (status == NotReachable) {
 //        UIAlertView *alertView = [[UIAlertView alloc] initWithTitle:nil message:@"网络已断开" delegate:nil cancelButtonTitle:@"确定" otherButtonTitles:nil];
 //        [alertView show];
         NSLog(@"----Notification Says Unreachable");
     }else if(status == ReachableViaWWAN){
 //        UIAlertView *alertView = [[UIAlertView alloc] initWithTitle:nil message:@"移动网络" delegate:nil cancelButtonTitle:@"确定" otherButtonTitles:nil];
 //        [alertView show];
         NSLog(@"----Notification Says mobilenet");
     }else if(status == ReachableViaWiFi){
 //        UIAlertView *alertView = [[UIAlertView alloc] initWithTitle:nil message:@"WIfi网络" delegate:nil cancelButtonTitle:@"确定" otherButtonTitles:nil];
 //        [alertView show];
         NSLog(@"----Notification Says wifinet");
     }
 }