1、     同步GET请求//第一步，创建URL
 NSURL *url = [NSURL URLWithString:@"http://api.hudong.com/iphonexml.do?type=focus-c"];//第二步，通过URL创建网络请求
 NSURLRequest *request = [[NSURLRequest alloc]initWithURL:url cachePolicy:NSURLRequestUseProtocolCachePolicy timeoutInterval:10];//NSURLRequest初始化方法第一个参数：请求访问路径，第二个参数：缓存协议，第三个参数：网络请求超时时间（秒）
 其中缓存协议是个枚举类型包含：NSURLRequestUseProtocolCachePolicy（基础策略）NSURLRequestReloadIgnoringLocalCacheData（忽略本地缓存）NSURLRequestReturnCacheDataElseLoad（首先使用缓存，如果没有本地缓存，才从原地址下载）NSURLRequestReturnCacheDataDontLoad（使用本地缓存，从不下载，如果本地没有缓存，则请求失败，此策略多用于离线操作）NSURLRequestReloadIgnoringLocalAndRemoteCacheData（无视任何缓存策略，无论是本地的还是远程的，总是从原地址重新下载）NSURLRequestReloadRevalidatingCacheData（如果本地缓存是有效的则不下载，其他任何情况都从原地址重新下载）//第三步，连接服务器
 NSData *received = [NSURLConnection sendSynchronousRequest:request returningResponse:nil error:nil];NSString *str = [[NSString alloc]initWithData:received encoding:NSUTF8StringEncoding];NSLog(@"%@",str);2、同步POST请求//第一步，创建URL
 NSURL *url = [NSURL URLWithString:@"http://api.hudong.com/iphonexml.do"];//第二步，创建请求
 NSMutableURLRequest *request = [[NSMutableURLRequest alloc]initWithURL:url cachePolicy:NSURLRequestUseProtocolCachePolicy timeoutInterval:10];[request setHTTPMethod:@"POST"];//设置请求方式为POST，默认为GET
 NSString *str = @"type=focus-c";//设置参数
 NSData *data = [str dataUsingEncoding:NSUTF8StringEncoding];[request setHTTPBody:data];//第三步，连接服务器
 NSData *received = [NSURLConnection sendSynchronousRequest:request returningResponse:nil error:nil];NSString *str1 = [[NSString alloc]initWithData:received encoding:NSUTF8StringEncoding];NSLog(@"%@",str1);
3、异步GET请求//第一步，创建url
 NSURL *url = [NSURL URLWithString:@"http://api.hudong.com/iphonexml.do?type=focus-c"];//第二步，创建请求
 NSURLRequest *request = [[NSURLRequest alloc]initWithURL:url cachePolicy:NSURLRequestUseProtocolCachePolicy timeoutInterval:10];//第三步，连接服务器
 NSURLConnection *connection = [[NSURLConnection alloc]initWithRequest:request delegate:self];
4、异步POST请求//第一步，创建url
 NSURL *url = [NSURL URLWithString:@"http://api.hudong.com/iphonexml.do"];//第二步，创建请求
 NSMutableURLRequest *request = [[NSMutableURLRequest alloc]initWithURL:url cachePolicy:NSURLRequestUseProtocolCachePolicy timeoutInterval:10];[request setHTTPMethod:@"POST"];NSString *str = @"type=focus-c";NSData *data = [str dataUsingEncoding:NSUTF8StringEncoding];[request setHTTPBody:data];//第三步，连接服务器
 NSURLConnection *connection = [[NSURLConnection alloc]initWithRequest:request delegate:self];5、异步请求的代理方法//接收到服务器回应的时候调用此方法- (void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)response{NSHTTPURLResponse *res = (NSHTTPURLResponse *)response;NSLog(@"%@",[res allHeaderFields]);self.receiveData = [NSMutableData data];}//接收到服务器传输数据的时候调用，此方法根据数据大小执行若干次-(void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data{[self.receiveData appendData:data];}//数据传完之后调用此方法-(void)connectionDidFinishLoading:(NSURLConnection *)connection{NSString *receiveStr = [[NSString alloc]initWithData:self.receiveData encoding:NSUTF8StringEncoding];NSLog(@"%@",receiveStr);}//网络请求过程中，出现任何错误（断网，连接超时等）会进入此方法-(void)connection:(NSURLConnection *)connectiondidFailWithError:(NSError *)error{NSLog(@"%@",[error localizedDescription]);