网络编程基础知识（一）

一：确认网络环境3G/WIFI1. 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。1.2.添加framework：将SystemConfiguration.framework 添加进工程。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:nilcancelButtonTitle:@"YES" otherButtonTitles:nil];[alert show];[alert release];}}- (void)applicationDidFinishLaunching:(UIApplication *)application {// ...// 监测网络情况[[NSNotificationCenter defaultCenter] addObserver:selfselector:@selector(reachabilityChanged:)name: kReachabilityChangedNotificationobject: 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 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 *)elementNamenamespaceURI:(NSString *)namespaceURIqualifiedName:(NSString *)qNameattributes:(NSDictionary *)attributeDict;- (void)parser:(NSXMLParser *)parser didEndElement:(NSString *)elementNamenamespaceURI:(NSString *)namespaceURIqualifiedName:(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 *)namespaceURIqualifiedName:(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 *)namespaceURIqualifiedName:(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];四 :使用NSOperation和NSOperationQueue启动多线程在app store中的很多应用程序非常的笨重，他们有好的界面，但操作性很差，比如说当程序从网上或本地载入数据的时候，界面被冻结了，用户只能等程序完全载入数据之后才能进行操作。当打开一个应用程序时，iphone会产生一个包含main方法的线程，所用程序中的界面都是运行在这个线程之中的（table views, tab bars, alerts…）,有时候我们会用数据填充这些view，现在问 题是如何有效的载入数据，并且用户还能自如的操作程序。方法是启动新的线程，专门用于数据的下载，而主线程不会因为下载数据被阻塞。不管使用任何编程语言，在实现多线程时都是一件很麻烦的事情。更糟糕的是，一旦出错，这种错误通常相当糟糕。然而，幸运的是apple从os x10.5在这方面做了很多的改进，NSThread的引入，使得开发多线程应用程序容易多了。除此之外，它们还引入了两个全新的类，NSOperation和NSOperationQueue。接下来我们通过一个实例来剖析如何使用这两个类实现多线程。这里指示展示这两个类的基本用法，当然这不是使用他们的唯一办法。如果你熟悉java或者它的别的变种语言的话 ，你会发现NSOperation对象很像java.lang.Runnable接口，就像java.lang.Runnable接口那样，NSOperation类也被设计为可扩展的，而且只有一个需要重写的方法。它就是－(void)main。使用NSOperation的最简单的方式就是把一个NSOperation对象加入到NSOperationQueue队列中，一旦这个对象被加入到队列，队列就开始处理这个对象，直到这个对象的所有操作完成。然后它被队列释放。下面的例子中，使用一个获取网页，并对其解析程NSXMLDocument，最后将解析得到的NSXMLDocument返回给主线程。PageLoadOperation.h@interface PageLoadOperation : NSOperation {NSURL *targetURL;}@property(retain) NSURL *targetURL;- (id)initWithURL:(NSURL*)url;@endPageLoadOperation.m#import "PageLoadOperation.h"#import "AppDelegate.h"@implementation PageLoadOperation@synthesize targetURL;- (id)initWithURL:(NSURL*)url;{if (![super init]) return nil;[self setTargetURL:url];return self;}- (void)dealloc {[targetURL release], targetURL = nil;[super dealloc];}- (void)main{NSString *webpageString = [[[NSString alloc]initWithContentsOfURL:[self targetURL]] autorelease];NSError *error = nil;NSXMLDocument *document = [[NSXMLDocument alloc]initWithXMLString:webpageStringoptions:NSXMLDocumentTidyHTML error:&error];if (!document) {NSLog(@"%s Error loading document (%@): %@",_cmd, [[self targetURL] absoluteString], error);return;}[[AppDelegate shared]performSelectorOnMainThread:@selector(pageLoaded:)withObject:document waitUntilDone:YES];[document release];}@end正如我们所看到的那样，这个类相当的简单，在它的init方法中接受一个url并保存起来，当main函数被调用的时候，它使用这个保存的url创建一个字符串，并将这个字符串传递给NSXMLDocumentinit方法。如果加载的xml数据没有出错，数据会被传递给AppDelegate，它处于主线程中。到此，这个线程的任务就完成了。在主线程中注销操作队列的时候，会将这个NSOperation对象释放。AppDelegate.h@interface AppDelegate : NSObject {NSOperationQueue *queue;}+ (id)shared;- (void)pageLoaded:(NSXMLDocument*)document;@endAppDelegate.m #import "AppDelegate.h"#import "PageLoadOperation.h"@implementation AppDelegatestatic AppDelegate *shared;static NSArray *urlArray;- (id)init{if (shared){[self autorelease];return shared;}if (![super init]) return nil; NSMutableArray *array = [[NSMutableArray alloc] init];[array addObject:@"http://www.google.com"];[array addObject:@"http://www.apple.com"];[array addObject:@"http://www.yahoo.com"];[array addObject:@"http://www.zarrastudios.com"];[array addObject:@"http://www.macosxhints.com"];urlArray = array; queue = [[NSOperationQueue alloc] init];shared = self;return self;}• (void)applicationDidFinishLaunching:(NSNotification *)aNotification{for (NSString *urlString in urlArray){NSURL *url =[NSURL URLWithString:urlString]; PageLoadOperation *plo =[[PageLoadOperation alloc] initWithURL:url];[queue addOperation:plo];[plo release];}}- (void)dealloc{[queue release], queue = nil;[super dealloc];}+ (id)shared;{if (!shared) {[[AppDelegate alloc] init];}return shared;}- (void)pageLoaded:(NSXMLDocument*)document;{NSLog(@"%s Do something with the XMLDocument: %@",_cmd, document);}@endNSOperationQueue的并行控制（NSOperationQueue Concurrency）在上面这个简单的例子中，我们很难看出这些操作是并行运行的，然而，如果你你的操作花费的时间远远比这里的要长，你将会发现，队列是同时执行这些操作的。幸运的是，如果你想要为队列限制同时只能运行几个操作，你可以使用NSOperationQueue的setMaxConcurrentOperationCount:方法。例如，[queue setMaxConcurrentOperationCount:2];