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 data
 NSURLConnection *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];