关于在iOS设备上探测WIFI，3G，GPRS使用情况的细节

由于设计的游戏需要有一些联网请求，但有时候在设备未连接网络的情况下，如果对网络情况不加以判断，则可能造成游戏为了等游戏超时，浪费不必要的时间。

所以在游戏启动时检测一下网络状况是很必要的，而且当玩家的设备连接上网络以后，有一个回调函数也是非常必要的一件事儿，这样可能更方便我们在后台下载数据库等信息。

apple为我们提供了一套范例代码，下面我就来分析一下这段代码。
范例代码地址在：https://developer.apple.com/library/ios/#samplecode/Reachability/Listings/Classes_ReachabilityAppDelegate_m.html#//apple_ref/doc/uid/DTS40007324-Classes_ReachabilityAppDelegate_m-DontLinkElementID_4

范例中包含两部分，一部分是appdelegate,一部分是reachability

 #import <sys/socket.h>
 #import <netinet/in.h>
 #import <netinet6/in6.h>
 #import <arpa/inet.h>
 #import <ifaddrs.h>
 #import <netdb.h>
 
 #import <CoreFoundation/CoreFoundation.h>
 
 #import "Reachability.h"
 
 #define kShouldPrintReachabilityFlags 1
 
 static void PrintReachabilityFlags(SCNetworkReachabilityFlags    flags, const char* comment)
 {
 #if kShouldPrintReachabilityFlags
 
     NSLog(@"Reachability Flag Status: %c%c %c%c%c%c%c%c%c %s\n",
             (flags & kSCNetworkReachabilityFlagsIsWWAN)               ? 'W' : '-',//当前网络为蜂窝网络，即3G或者GPRS
             (flags & kSCNetworkReachabilityFlagsReachable)            ? 'R' : '-',//网络请求地址可达
 
             (flags & kSCNetworkReachabilityFlagsTransientConnection)  ? 't' : '-',
             (flags & kSCNetworkReachabilityFlagsConnectionRequired)   ? 'c' : '-',
             (flags & kSCNetworkReachabilityFlagsConnectionOnTraffic)  ? 'C' : '-',
             (flags & kSCNetworkReachabilityFlagsInterventionRequired) ? 'i' : '-',
             (flags & kSCNetworkReachabilityFlagsConnectionOnDemand)   ? 'D' : '-',
             (flags & kSCNetworkReachabilityFlagsIsLocalAddress)       ? 'l' : '-',//该值为一个本地地址
             (flags & kSCNetworkReachabilityFlagsIsDirect)             ? 'd' : '-',
             comment
             );
 #endif
 }
 
 @implementation Reachability
 static void ReachabilityCallback(SCNetworkReachabilityRef target, SCNetworkReachabilityFlags flags, void* info)
 {
     #pragma unused (target, flags)
     NSCAssert(info != NULL, @"info was NULL in ReachabilityCallback");
     NSCAssert([(NSObject*) info isKindOfClass: [Reachability class]], @"info was wrong class in ReachabilityCallback");
 
     //We're on the main RunLoop, so an NSAutoreleasePool is not necessary, but is added defensively
     // in case someon uses the Reachablity object in a different thread.
     NSAutoreleasePool* myPool = [[NSAutoreleasePool alloc] init];
 
     Reachability* noteObject = (Reachability*) info;
     // Post a notification to notify the client that the network reachability changed.
     [[NSNotificationCenter defaultCenter] postNotificationName: kReachabilityChangedNotification object: noteObject];
 
     [myPool release];
 }
 
 //启动探测网络
 - (BOOL) startNotifier
 {
     BOOL retVal = NO;
     SCNetworkReachabilityContext    context = {, self, NULL, NULL, NULL};
     //SCNetworkReachabilitySetCallback函数为指定一个target（此处为reachabilityRef，即www.apple.com,在reachabilityWithHostName里设置的）
     //当设备对于这个target链接状态发生改变时(比如断开链接，或者重新连上)，则回调reachabilityCallback函数，
     if(SCNetworkReachabilitySetCallback(reachabilityRef, ReachabilityCallback, &context))
     {
         //指定一个runloop给指定的target
         if(SCNetworkReachabilityScheduleWithRunLoop(reachabilityRef, CFRunLoopGetCurrent(), kCFRunLoopDefaultMode))
         {
             retVal = YES;
         }
     }
     return retVal;
 }
  //停止网络探测
 - (void) stopNotifier
 {
     if(reachabilityRef!= NULL)
     {
         SCNetworkReachabilityUnscheduleFromRunLoop(reachabilityRef, CFRunLoopGetCurrent(), kCFRunLoopDefaultMode);
     }
 }
 
 - (void) dealloc
 {
     [self stopNotifier];
     if(reachabilityRef!= NULL)
     {
         CFRelease(reachabilityRef);
     }
     [super dealloc];
 }
  //通过域名设定一个目标地址比如www.apple.com
 + (Reachability*) reachabilityWithHostName: (NSString*) hostName;
 {
     Reachability* retVal = NULL;
     SCNetworkReachabilityRef reachability = SCNetworkReachabilityCreateWithName(NULL, [hostName UTF8String]);
     if(reachability!= NULL)
     {
         retVal= [[[self alloc] init] autorelease];
         if(retVal!= NULL)
         {
             retVal->reachabilityRef = reachability;
             retVal->localWiFiRef = NO;
         }
     }
     return retVal;
 }
   //通过ip地址设定一个目标的地址，可以加端口号，貌似我实验的不太成功
 + (Reachability*) reachabilityWithAddress: (const struct sockaddr_in*) hostAddress;
 {
     SCNetworkReachabilityRef reachability = SCNetworkReachabilityCreateWithAddress(kCFAllocatorDefault, (const struct sockaddr*)hostAddress);
     Reachability* retVal = NULL;
     if(reachability!= NULL)
     {
         retVal= [[[self alloc] init] autorelease];
         if(retVal!= NULL)
         {
             retVal->reachabilityRef = reachability;
             retVal->localWiFiRef = NO;
         }
     }
     return retVal;
 }
  //检测当前网络能否连上internet
 + (Reachability*) reachabilityForInternetConnection;
 {
     struct sockaddr_in zeroAddress;
     bzero(&zeroAddress, sizeof(zeroAddress));
     zeroAddress.sin_len = sizeof(zeroAddress);
     zeroAddress.sin_family = AF_INET;
     return [self reachabilityWithAddress: &zeroAddress];
 }
  //检测当前网络是否能够连上本地wifi
 + (Reachability*) reachabilityForLocalWiFi;
 {
     struct sockaddr_in localWifiAddress;
     bzero(&localWifiAddress, sizeof(localWifiAddress));
     localWifiAddress.sin_len = sizeof(localWifiAddress);
     localWifiAddress.sin_family = AF_INET;
     // IN_LINKLOCALNETNUM is defined in <netinet/in.h> as 169.254.0.0
     localWifiAddress.sin_addr.s_addr = htonl(IN_LINKLOCALNETNUM);
     Reachability* retVal = [self reachabilityWithAddress: &localWifiAddress];
     if(retVal!= NULL)
     {
         retVal->localWiFiRef = YES;
     }
     return retVal;
 }
 
 #pragma mark Network Flag Handling
 
 - (NetworkStatus) localWiFiStatusForFlags: (SCNetworkReachabilityFlags) flags
 {
     PrintReachabilityFlags(flags, "localWiFiStatusForFlags");
 
     BOOL retVal = NotReachable;
     if((flags & kSCNetworkReachabilityFlagsReachable) && (flags & kSCNetworkReachabilityFlagsIsDirect))
     {
         retVal = ReachableViaWiFi;
     }
     return retVal;
 }
  //通过flags返回当前网络状态
 - (NetworkStatus) networkStatusForFlags: (SCNetworkReachabilityFlags) flags
 {
     PrintReachabilityFlags(flags, "networkStatusForFlags");
     if ((flags & kSCNetworkReachabilityFlagsReachable) == )
     {
         // if target host is not reachable
         return NotReachable;
     }
 
     BOOL retVal = NotReachable;
 
     if ((flags & kSCNetworkReachabilityFlagsConnectionRequired) == )
     {
         // if target host is reachable and no connection is required
         //  then we'll assume (for now) that your on Wi-Fi
         retVal = ReachableViaWiFi;
     }
 
     if ((((flags & kSCNetworkReachabilityFlagsConnectionOnDemand ) != ) ||
         (flags & kSCNetworkReachabilityFlagsConnectionOnTraffic) != ))
     {
             // ... and the connection is on-demand (or on-traffic) if the
             //     calling application is using the CFSocketStream or higher APIs
 
             if ((flags & kSCNetworkReachabilityFlagsInterventionRequired) == )
             {
                 // ... and no [user] intervention is needed
                 retVal = ReachableViaWiFi;
             }
         }
 
     if ((flags & kSCNetworkReachabilityFlagsIsWWAN) == kSCNetworkReachabilityFlagsIsWWAN)
     {
         // ... but WWAN connections are OK if the calling application
         //     is using the CFNetwork (CFSocketStream?) APIs.
         retVal = ReachableViaWWAN;
     }
     return retVal;
 }
 
 - (BOOL) connectionRequired;
 {
     NSAssert(reachabilityRef != NULL, @"connectionRequired called with NULL reachabilityRef");
     SCNetworkReachabilityFlags flags;
     if (SCNetworkReachabilityGetFlags(reachabilityRef, &flags))
     {
         return (flags & kSCNetworkReachabilityFlagsConnectionRequired);
     }
     return NO;
 }
  //获得当前网络状态
 - (NetworkStatus) currentReachabilityStatus
 {
     NSAssert(reachabilityRef != NULL, @"currentNetworkStatus called with NULL reachabilityRef");
     NetworkStatus retVal = NotReachable;
     SCNetworkReachabilityFlags flags;
     if (SCNetworkReachabilityGetFlags(reachabilityRef, &flags))
     {
         if(localWiFiRef)
         {
             retVal = [self localWiFiStatusForFlags: flags];
         }
         else
         {
             retVal = [self networkStatusForFlags: flags];
         }
     }
     return retVal;
 }
 @end

以上的部分为具体实现的部分，下面我们再来看在appdelegate里面调用的部分

 - (void) applicationDidFinishLaunching: (UIApplication* )application
 {
     #pragma unused(application)
     contentView.backgroundColor = [UIColor groupTableViewBackgroundColor];
 
     summaryLabel.hidden = YES;        
 
     //这句是设定一个回调函数reachability，当网络条件发生变化时则去调用reachabilityChanged
     [[NSNotificationCenter defaultCenter] addObserver: self selector: @selector(reachabilityChanged:) name: kReachabilityChangedNotification object: nil];
 
     //Change the host name here to change the server your monitoring
     remoteHostLabel.text = [NSString stringWithFormat: @"Remote Host: %@", @"www.apple.com"];
     //设定target地址，这里是www.apple.com
     hostReach = [[Reachability reachabilityWithHostName: @"www.apple.com"] retain];
     //启动网络条件检测
     [hostReach startNotifier];
     //更新UI的网络状态显示
     [self updateInterfaceWithReachability: hostReach];
 
     //探测是否能连上internet网
     internetReach = [[Reachability reachabilityForInternetConnection] retain];
     [internetReach startNotifier];
     [self updateInterfaceWithReachability: internetReach];
 
     //探测wifi是否开启
     wifiReach = [[Reachability reachabilityForLocalWiFi] retain];
     [wifiReach startNotifier];
     [self updateInterfaceWithReachability: wifiReach];
 
     [window makeKeyAndVisible];
 
 }
 
 //如需要查看网络状态，如下调用即可
 NetworkStatus netStatus = [curReach currentReachabilityStatus];
 netStatus有ReachableViaWWAN（蜂窝网络3G之类的）,ReachableViaWiFi（wifi连接）,NotReachable(无法连接)