IOS学习篇之NSURLConnection, 异步同步

 移动互联网时代，网络通信已是手机终端必不可少的功能。我们的应用中也必不可少的使用了网络通信，增强客户端与服务器交互。这一篇提供了使用NSURLConnection实现http通信的方式。

          NSURLConnection提供了异步请求、同步请求两种通信方式。

1、异步请求

       iOS5.0 SDK NSURLConnection类新增的sendAsynchronousRequest:queue:completionHandler:方法，从而使iOS5支持两种异步请求方式。我们先从新增类开始。

1）sendAsynchronousRequest

iOS5.0开始支持sendAsynchronousReques方法，方法使用如下：

 - (void)httpAsynchronousRequest{

     NSURL *url = [NSURL URLWithString:@"http://url"];

     NSString *post=@"postData";

     NSData *postData = [post dataUsingEncoding:NSASCIIStringEncoding allowLossyConversion:YES];

     NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:url];
     [request setHTTPMethod:@"POST"];
     [request setHTTPBody:postData];
     [request setTimeoutInterval:10.0];

     NSOperationQueue *queue = [[NSOperationQueue alloc]init];
     [NSURLConnection sendAsynchronousRequest:request
                                        queue:queue
                            completionHandler:^(NSURLResponse *response, NSData *data, NSError *error){
                                if (error) {
                                    NSLog(@"Httperror:%@%d", error.localizedDescription,error.code);
                                }else{

                                    NSInteger responseCode = [(NSHTTPURLResponse *)response statusCode];

                                    NSString *responseString = [[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding];

                                    NSLog(@"HttpResponseCode:%d", responseCode);
                                    NSLog(@"HttpResponseBody %@",responseString);
                                }
                            }];


 }

      sendAsynchronousReques可以很容易地使用NSURLRequest接收回调，完成http通信。

2）connectionWithRequest

iOS2.0就开始支持connectionWithRequest方法，使用如下：

 - (void)httpConnectionWithRequest{

     NSString *URLPath = [NSString stringWithFormat:@"http://url"];
     NSURL *URL = [NSURL URLWithString:URLPath];
     NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:URL];
     [NSURLConnection connectionWithRequest:request delegate:self];

 }

 - (void)connection:(NSURLConnection *)theConnection didReceiveResponse:(NSURLResponse *)response
 {

     NSInteger responseCode = [(NSHTTPURLResponse *)response statusCode];
     NSLog(@"response length=%lld  statecode%d", [response expectedContentLength],responseCode);
 }


 // A delegate method called by the NSURLConnection as data arrives.  The
 // response data for a POST is only for useful for debugging purposes,
 // so we just drop it on the floor.
 - (void)connection:(NSURLConnection *)theConnection didReceiveData:(NSData *)data
 {
     if (mData == nil) {
         mData = [[NSMutableData alloc] initWithData:data];
     } else {
         [mData appendData:data];
     }
     NSLog(@"response connection");
 }

 // A delegate method called by the NSURLConnection if the connection fails.
 // We shut down the connection and display the failure.  Production quality code
 // would either display or log the actual error.
 - (void)connection:(NSURLConnection *)theConnection didFailWithError:(NSError *)error
 {

     NSLog(@"response error%@", [error localizedFailureReason]);
 }

 // A delegate method called by the NSURLConnection when the connection has been
 // done successfully.  We shut down the connection with a nil status, which
 // causes the image to be displayed.
 - (void)connectionDidFinishLoading:(NSURLConnection *)theConnection
 {
     NSString *responseString = [[NSString alloc] initWithData:mData encoding:NSUTF8StringEncoding];
      NSLog(@"response body%@", responseString);
 }

   connectionWithRequest需要delegate参数，通过一个delegate来做数据的下载以及Request的接受以及连接状态，此处delegate:self，所以需要本类实现一些方法，并且定义mData做数据的接受。

需要实现的方法：

1、获取返回状态、包头信息。

 - (void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)response;

2、连接失败，包含失败。

 - (void)connection:(NSURLConnection *)connection didFailWithError:(NSError *)error;

3、接收数据

 - (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data;

4、数据接收完毕

- (void)connectionDidFinishLoading:(NSURLConnection *)connection;

    connectionWithRequest使用起来比较繁琐，而iOS5.0之前用不支持sendAsynchronousRequest。有网友提出了AEURLConnection解决方案。

 AEURLConnection is a simple reimplementation of the API for use on iOS 4. Used properly, it is also guaranteed to be safe against The Deallocation Problem, a thorny threading issue that affects most other networking libraries.

2、同步请求

同步请求数据方法如下：

 - (void)httpSynchronousRequest{

     NSURLRequest * urlRequest = [NSURLRequest requestWithURL:[NSURL URLWithString:@"http://google.com"]];
     NSURLResponse * response = nil;
     NSError * error = nil;
     NSData * data = [NSURLConnection sendSynchronousRequest:urlRequest
                                           returningResponse:&response
                                                       error:&error];

     if (error == nil)
     {
         // 处理数据
     }
 }

同步请求数据会造成主线程阻塞，通常在请求大数据或网络不畅时不建议使用。

        从上面的代码可以看出，不管同步请求还是异步请求，建立通信的步骤基本是一样的：

         1、创建NSURL

         2、创建Request对象

         3、创建NSURLConnection连接。

         NSURLConnection创建成功后，就创建了一个http连接。异步请求和同步请求的区别是：创建了异步请求，用户可以做其他的操作，请求会在另一个线程执行，通信结果及过程会在回调函数中执行。同步请求则不同，需要请求结束用户才能做其他的操作。

参考：

http://developer.apple.com/library/ios/#documentation/Cocoa/Conceptual/URLLoadingSystem/Tasks/UsingNSURLConnection.html#//apple_ref/doc/uid/20001836-BAJEAIEE

http://codewithchris.com/tutorial-how-to-use-ios-nsurlconnection-by-example/

http://kelp.phate.org/2011/06/ios-stringwithcontentsofurlnsurlconnect.html