网络数据下载优化：

  网络数据下载优化：
  
//method1
  NSURL
     |scheme+host+path+parameters
     |http://(1.2章)
  NSURLRequest
  NSURLRequest *request = [NSURLRequest requestWithURL:url]; 
  NSURLResquest *response = nil;
  NSError *error = nil;
  NSURLConnection（异步下载sendAsynchronousRequest:）
//method2
  //同步
  NSData *data = [NSURLConnection sendSynchronousRequest:request returningResponse:&response error:&error];


  NSHTTPURLResponse
  http状态码
//method3,常用方法
  <NSURLConnectionDataDelegate>
  NSURL *url;
  NSURLRequest *request =[NSURLRequest requestWithURL:url];
  NSURLConnection *connection = [[NSURLConnection alloc] initWithRequest:request delegate:self];
  //NSURLConnection *connection = [NSURLConnection connectionWithRequest:request delegate:self];
  [connection start];
  一般会实现如下方法：
  didReceiveResponse://得到请求
  didReceiveData://得到数据
  connectionDidFinishLoading://完成下载


  NSHTTPURLResponse继承自NSURLResponse(expectedContentLength,statusCode)
  NSURLResponse
  NSOperationQueue(mainQueue)