使用NSConnection下载数据

1.创建NSConnection对象，设置委托对象
    
    NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:[NSURL URLWithString:[self urlString]]];
    [NSURLConnection connectionWithRequest:request delegate:self];
    
    2. NSURLConnection delegate委托方法
        - (void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)response;  
        - (void)connection:(NSURLConnection *)connection didFailWithError:(NSError *)error;  
        - (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data;  
        - (void)connectionDidFinishLoading:(NSURLConnection *)connection;  

    3. 实现委托方法
    - (void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)response {
        // store data
        [self.receivedData setLength:0];            //通常在这里先清空接受数据的缓存
    }
    
    - (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data {
           /* appends the new data to the received data */
        [self.receivedData appendData:data];        //可能多次收到数据，把新的数据添加在现有数据最后
    }

    - (void)connection:(NSURLConnection *)connection didFailWithError:(NSError *)error {
        // 错误处理
    }

    - (void)connectionDidFinishLoading:(NSURLConnection *)connection {
        // disconnect
        [UIApplication sharedApplication].networkActivityIndicatorVisible = NO;   
        NSString *returnString = [[NSString alloc] initWithData:self.receivedData encoding:NSUTF8StringEncoding];
        NSLog(returnString);
        [self urlLoaded:[self urlString] data:self.receivedData];
        firstTimeDownloaded = YES;
    }