NSURLRequest详解

URLRequest 的一个实例

// Create the request.//所构建的NSURLRequest具有一个依赖于缓存响应的特定策略，cachePolicy取得策略，timeoutInterval取得超时值NSURLRequest *theRequest=[NSURLRequest requestWithURL:[NSURL URLWithString:@"http://www.apple.com/"]                        cachePolicy:NSURLRequestUseProtocolCachePolicy                    timeoutInterval:60.0];// create the connection with the request// and start loading the dataNSURLConnection *theConnection=[[NSURLConnection alloc] initWithRequest:theRequest delegate:self];if (theConnection) {    // Create the NSMutableData to hold the received data.    // receivedData is an instance variable declared elsewhere.    receivedData = [[NSMutableData data] retain];} else {    // Inform the user that the connection failed.}

其中：

NSURLRequest默认的cache policy是NSURLRequestUseProtocolCachePolicy， 是最能保持一致性的协议。

NSURLRequestReloadIgnoringCacheData 忽略缓存直接从原始地址下载

NSURLRequestReturnCacheDataElseLoad 只有在cache中不存在data时才从原始地址下载

NSURLRequestReturnCacheDataDontLoad 允许app确定是否要返回cache数据，如果使用这种协议当本地不存在response的时候，创建NSURLConnection or NSURLDownload实例时将会马上返回nil；这类似于离线模式，没有建立网络连接；

你只需要实现以下delegate方法来处理数据响应

- (void)connection:(NSURLConnection*)connection didReceiveResponse:(NSHTTPURLResponse*)response

- (void)connection:(NSURLConnection*)connection didReceiveData:(NSData*)data

- (void)connection:(NSURLConnection*)connection didFailWithError:(NSError*)error

- (void)connectionDidFinishLoading:(NSURLConnection *)connection

NSURLConnect还提供了一个方便的类方法(class method) : sendSynchronousRequest:returningResponse:error:可用来 同步地加载一个URL请求

+ (NSData *)sendSynchronousRequest:    (NSURLRequest *)request      returningResponse:   (NSURLResponse **)response    error:  (NSError **)error

• request 要装载的URL请求. 这个request 对象 作为初始化进程的一部分,被深度复制(deep-copied). 在这个方法返回之后, 再修改request, 将不会影响用在装载的过程中的request
• reponse 输出参数, 由服务器返回的URL响应
• error   输出参数, 如果在处理请求的过程中发生错误,就会使用.  无错误,就为NULL

一个实现异步get请求的例子：

NSString *url = [NSString stringWithFormat:@"http://localhost/chat/messages.php?past=%ld&t=%ld", lastId, time(0) ];NSMutableURLRequest *request = [[[NSMutableURLRequest alloc] init] autorelease];[request setURL:[NSURL URLWithString:url]];[request setHTTPMethod:@"GET"];    NSURLConnection *conn=[[NSURLConnection alloc] initWithRequest:request delegate:self];      if (conn)    {          receivedData = [[NSMutableData data] retain];      }       else       {      } - (void)timerCallback {//[timer release];[self getNewMessages];}- (void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)response  {      [receivedData setLength:0];  }  - (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data  {      [receivedData appendData:data];  }  - (void)connectionDidFinishLoading:(NSURLConnection *)connection  {  if (chatParser)        [chatParser release];if ( messages == nil )messages = [[NSMutableArray alloc] init];chatParser = [[NSXMLParser alloc] initWithData:receivedData];[chatParser setDelegate:self];//set the delegate[chatParser parse];//start parse[receivedData release];  [messageList reloadData];NSInvocation *invocation = [NSInvocation invocationWithMethodSignature:                                    [self methodSignatureForSelector: @selector(timerCallback)]];[invocation setTarget:self];[invocation setSelector:@selector(timerCallback)];//timer = [NSTimer scheduledTimerWithTimeInterval:5.0 invocation:invocation repeats:NO];    [NSTimer scheduledTimerWithTimeInterval:5.0 invocation:invocation repeats:NO];//if set yes,then very 5 seconds updata the table} 

一个实现同步Get请求的例子：

    // 初始化请求    NSMutableURLRequest *request = [[NSMutableURLRequest alloc] init];             // 设置URL    [request setURL:[NSURL URLWithString:urlStr]];    // 设置HTTP方法    [request setHTTPMethod:@"GET"];    // 发 送同步请求, 这里得returnData就是返回得数据了    NSData *returnData = [NSURLConnection sendSynchronousRequest:request                                                returningResponse:nil error:nil];     // 释放对象    [request release];

更多详细信息请参见苹果开发文档：

http://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/URLLoadingSystem/Tasks/UsingNSURLConnection.html