确认网络环境3G/WIFI Reachability的使用

1. 添加源文件和framework
    开发Web等网络应用程序的时候，需要确认网络环境，连接情况等信息。如果没有处理它们，是不会通过Apple的审(我们的)查的。
    Apple 的 例程 Reachability 中介绍了取得/检测网络状态的方法。要在应用程序程序中使用Reachability，首先要完成如下两部：
    1.1. 添加源文件：
    在你的程序中使用 Reachability 只须将该例程中的 Reachability.h 和 Reachability.m 拷贝到你的工程中。如下图：
    1.2.添加framework：
    将SystemConfiguration.framework 添加进工程。如下图：
    2. 网络状态
    Reachability.h中定义了三种网络状态：
    typedef enum {
        NotReachable = 0,            //无连接
        ReachableViaWiFi,            //使用3G/GPRS网络
        ReachableViaWWAN            //使用WiFi网络
    } NetworkStatus;
    因此可以这样检查网络状态：
    Reachability *r = [Reachability reachabilityWithHostName:@“www.apple.com”];
    switch ([r currentReachabilityStatus]) {
            case NotReachable:
                    // 没有网络连接
                    break;
            case ReachableViaWWAN:
                    // 使用3G网络
                    break;
            case ReachableViaWiFi:
                    // 使用WiFi网络
                    break;
    }
    3.检查当前网络环境
    程序启动时，如果想检测可用的网络环境，可以像这样
    // 是否wifi
    + (BOOL) IsEnableWIFI {
        return ([[Reachability reachabilityForLocalWiFi] currentReachabilityStatus] != NotReachable);
    }
    // 是否3G
    + (BOOL) IsEnable3G {
        return ([[Reachability reachabilityForInternetConnection] currentReachabilityStatus] != NotReachable);
    }
    例子：
    - (void)viewWillAppear:(BOOL)animated {    
    if (([Reachability reachabilityForInternetConnection].currentReachabilityStatus == NotReachable) && 
            ([Reachability reachabilityForLocalWiFi].currentReachabilityStatus == NotReachable)) {
            self.navigationItem.hidesBackButton = YES;
            [self.navigationItem setLeftBarButtonItem:nil animated:NO];
        }
    }
    4. 链接状态的实时通知
    网络连接状态的实时检查，通知在网络应用中也是十分必要的。接续状态发生变化时，需要及时地通知用户：
    Reachability 1.5版本
    // My.AppDelegate.h
    #import "Reachability.h"
    @interface MyAppDelegate : NSObject <UIApplicationDelegate> {
        NetworkStatus remoteHostStatus;
    }
    @property NetworkStatus remoteHostStatus;
    @end
    // My.AppDelegate.m
    #import "MyAppDelegate.h"
    @implementation MyAppDelegate
    @synthesize remoteHostStatus;
    // 更新网络状态
    - (void)updateStatus {
        self.remoteHostStatus = [[Reachability sharedReachability] remoteHostStatus];
    }
    // 通知网络状态
    - (void)reachabilityChanged:(NSNotification *)note {
        [self updateStatus];
        if (self.remoteHostStatus == NotReachable) {
            UIAlertView *alert = [[UIAlertView alloc] initWithTitle:NSLocalizedString(@"AppName", nil)
                         message:NSLocalizedString (@"NotReachable", nil)
                        delegate:nil cancelButtonTitle:@"OK" otherButtonTitles: nil];
            [alert show];
            [alert release];
        }
    }
    // 程序启动器，启动网络监视
    - (void)applicationDidFinishLaunching:(UIApplication *)application {
        // 设置网络检测的站点
        [[Reachability sharedReachability] setHostName:@"www.apple.com"];
        [[Reachability sharedReachability] setNetworkStatusNotificationsEnabled:YES];
        // 设置网络状态变化时的通知函数
        [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(reachabilityChanged:)
                                                 name:@"kNetworkReachabilityChangedNotification" object:nil];
        [self updateStatus];
    }
    - (void)dealloc {
        // 删除通知对象
        [[NSNotificationCenter defaultCenter] removeObserver:self];
        [window release];
        [super dealloc];
    } 
    
    Reachability 2.0版本
    // MyAppDelegate.h
    @class Reachability;

        @interface MyAppDelegate : NSObject <UIApplicationDelegate> {
            Reachability  *hostReach;
        }

    @end

    // MyAppDelegate.m
    - (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];
        }
    }
                              
    - (void)applicationDidFinishLaunching:(UIApplication *)application {
        // ...
                  
        // 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 startNotifer];
        // ...
    }