NSURLConnection

1:初始化 NSURLConnection

[NSURLConnection connectionWithRequest:[NSURLRequest requestWithURL:url] delegate:self];

2: 在 didReceiveResponse 委托中获取服务器返回过来的信息

-(void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)response
{
    NSHTTPURLResponse *httpResponse = (NSHTTPURLResponse *)response;
    NSDictionary *responseHeaders = [httpResponse allHeaderFields];
    NSLog(@"didReceiveResponseHeaders = %@",responseHeaders);
}

3:在任何时候都可以将connection 撤销对网络的请求和访问

[connection cancel];

4:网络连接成功后会开始下载, 会定时调用这个委托. 里面提供每一段的下载数据

-(void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data
{
    NSLog(@"定时会有新的数据进入这个委托");
}