NSURLConnection和NSURLRequest

iOS中我们可以通过NSURLRequest和NSURLConnection来建立一些HTTP请求下载数据等等

先看一个例子：

NSURLRequest *theRequest＝［NSURLRequest requestWithURL：    
                  ［NSURL URLWithString：@“http://www.baidu.com/”］    
                 cachePolicy：NSURLRequestUseProtocolCachePolicy    
                 timeoutInterval：60.0];    
NSURLConnection *theConnection[[NSURLConnection alloc]         
                   initWithRequest:theRequest delegate:self];    
if(theConnection)    
{    
//创建NSMutableData    
  receivedData＝[[NSMutableData data］ retain］；    
}else // 创建失败

先创建一个NSURLRequest，设置好URL和cachePolicy,timeoutInterval。

cachePolicy有以下几种：

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；

然后通过NSURLRequest来构建NSURLConnection，设置delegate，通过返回的theConnection 来判断是否创建成功。

其中delegate要实现一下几个方法：供NSURLConnection来回调

- (void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)response
{
    // This method is called when the server has determined that it
    // has enough information to create the NSURLResponse.
 
    // It can be called multiple times, for example in the case of a
    // redirect, so each time we reset the data.
 
    // receivedData is an instance variable declared elsewhere.
    [receivedData setLength:0];
}

- (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data
{
    // Append the new data to receivedData.
    // receivedData is an instance variable declared elsewhere.
    [receivedData appendData:data];
}

- (void)connection:(NSURLConnection *)connection
  didFailWithError:(NSError *)error
{
    // release the connection, and the data object
    [connection release];
    // receivedData is declared as a method instance elsewhere
    [receivedData release];
 
    // inform the user
    NSLog(@"Connection failed! Error - %@ %@",
          [error localizedDescription],
          [[error userInfo] objectForKey:NSURLErrorFailingURLStringErrorKey]);
}

- (void)connectionDidFinishLoading:(NSURLConnection *)connection
{
    // do something with the data
    // receivedData is declared as a method instance elsewhere
    NSLog(@"Succeeded! Received %d bytes of data",[receivedData length]);
 
    // release the connection, and the data object
    [connection release];
    [receivedData release];
}

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.