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] setNetworkStatusNotificati*****Enabled: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]; 
        // ... 
    } 
 
 
二：使用NSConnection下载数据 
     
    1.创建NSConnection对象，设置委托对象 
     
    NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:[NSURL URLWithString:[self urlString]]]; 
    [NSURLConnection connectionWithRequest:request delegate:self]; 
     
    2. NSURLConnection delegate委托方法 
        - (void)connection:(NSURLConnection *)connection didReceiveResp*****e:(NSURLResp*****e *)resp*****e;   
        - (void)connection:(NSURLConnection *)connection didFailWithError:(NSError *)error;   
        - (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data;   
        - (void)connectionDidFinishLoading:(NSURLConnection *)connection;   
 
    3. 实现委托方法 
    - (void)connection:(NSURLConnection *)connection didReceiveResp*****e:(NSURLResp*****e *)resp*****e { 
        // store data 
        [self.receivedData setLength:0];            //通常在这里先清空接受数据的缓存 
    } 
     
    - (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data { 
           /* appends the new data to the received data */ 
        [self.receivedData appendData:data];        //可能多次收到数据，把新的数据添加在现有数据最后 
    } 
 
    - (void)connection:(NSURLConnection *)connection didFailWithError:(NSError *)error { 
        // 错误处理 
    } 
 
    - (void)connectionDidFinishLoading:(NSURLConnection *)connection { 
        // disconnect 
        [UIApplication sharedApplication].networkActivityIndicatorVisible = NO;    
        NSString *returnString = [[NSString alloc] initWithData:self.receivedData encoding:NSUTF8StringEncoding]; 
        NSLog(returnString); 
        [self urlLoaded:[self urlString] data:self.receivedData]; 
        firstTimeDownloaded = YES; 
    } 
 
三：使用NSXMLParser解析xml文件 
 
    1. 设置委托对象，开始解析 
    NSXMLParser *parser = [[NSXMLParser alloc] initWithData:data];   //或者也可以使用initWithContentsOfURL直接下载文件，但是有一个原因不这么做： 
    // It‘s also possible to have NSXMLParser download the data, by passing it a URL, but this is not desirable 
    // because it gives less control over the network, particularly in responding to connection errors. 
    [parser setDelegate:self]; 
    [parser parse]; 
 
    2. 常用的委托方法 
    - (void)parser:(NSXMLParser *)parser didStartElement:(NSString *)elementName  
                                namespaceURI:(NSString *)namespaceURI 
                                qualifiedName:(NSString *)qName  
                                attributes:(NSDictionary *)attributeDict; 
    - (void)parser:(NSXMLParser *)parser didEndElement:(NSString *)elementName  
                                namespaceURI:(NSString *)namespaceURI  
                                qualifiedName:(NSString *)qName; 
    - (void)parser:(NSXMLParser *)parser foundCharacters:(NSString *)string; 
    - (void)parser:(NSXMLParser *)parser parseErrorOccurred:(NSError *)parseError; 
 
    static NSString *feedURLString = @"http://www.yifeiyang.net/test/test.xml"; 
 
    3.  应用举例 
    - (void)parseXMLFileAtURL:(NSURL *)URL parseError:(NSError **)error 
    { 
        NSXMLParser *parser = [[NSXMLParser alloc] initWithContentsOfURL:URL]; 
        [parser setDelegate:self]; 
        [parser setShouldProcessNamespaces:NO]; 
        [parser setShouldReportNamespacePrefixes:NO]; 
        [parser setShouldResolveExternalEntities:NO]; 
        [parser parse]; 
        NSError *parseError = [parser parserError]; 
        if (parseError && error) { 
            *error = parseError; 
        } 
        [parser release]; 
    } 
 
    - (void)parser:(NSXMLParser *)parser didStartElement:(NSString *)elementName namespaceURI:(NSString *)namespaceURI  
                                        qualifiedName:(NSString*)qName attributes:(NSDictionary *)attributeDict{ 
        // 元素开始句柄 
        if (qName) { 
            elementName = qName; 
        } 
        if ([elementName isEqualToString:@"user"]) { 
            // 输出属性值 
            NSLog(@"Name is %@ , Age is %@", [attributeDict objectForKey:@"name"], [attributeDict objectForKey:@"age"]); 
        } 
    } 
 
    - (void)parser:(NSXMLParser *)parser didEndElement:(NSString *)elementName namespaceURI:(NSString *)namespaceURI  
                                        qualifiedName:(NSString *)qName 
    { 
        // 元素终了句柄 
        if (qName) { 
               elementName = qName; 
        } 
    } 
 
    - (void)parser:(NSXMLParser *)parser foundCharacters:(NSString *)string 
    { 
        // 取得元素的text 
    } 
 
    NSError *parseError = nil; 
    [self parseXMLFileAtURL:[NSURL URLWithString:feedURLString] parseError:&parseError];