iOS网络编程

一、检测网络状态

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. 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：
2. 添加framework：
   将SystemConfiguration.framework 添加进工程。如下图：

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;
}

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);
}


- (void)viewWillAppear:(BOOL)animated {    
if (([Reachability reachabilityForInternetConnection].currentReachabilityStatus == NotReachable) && 
        ([Reachability reachabilityForLocalWiFi].currentReachabilityStatus == NotReachable)) {
        self.navigationItem.hidesBackButton = YES;
        [self.navigationItem setLeftBarButtonItem:nil animated:NO];
    }
}

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

// MyAppDelegate.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
#import "Reachability.h"
@class Reachability;
@interface MyAppDelegate : NSObject <UIApplicationDelegate> {
        Reachability  *hostReach;
}
@end

// MyAppDelegate.m
#import "MyAppDelegate.h"
@implementation MyAppDelegate
- (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];
    // ...
}
@end

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 didReceiveResponse:(NSURLResponse *)response;  
- (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 didReceiveResponse:(NSURLResponse *)response {
       // 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];