NSURLConnection和NSURLRequest

最新推荐文章于 2016-08-08 11:41:25 发布

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本文详细介绍了iOS环境下如何使用NSURLRequest和NSURLConnection构建HTTP请求，包括URL配置、缓存策略、数据接收及错误处理等关键步骤，并通过实例展示了同步加载URL请求的方法。

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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. 无错误,就为NULL