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];

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