获取要下载文件的大小信息

-(void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)response{    //This method is called when the server has determined that it     //has enough information to create the NSURLResponse.        //It can be called multiple times, for example in the case of a     //redirect, so each time we reset the data.        //receivedData is an instance variable declared elsewhere.        NSLog(@"要下载文件大小为 %lld",response.expectedContentLength);    [self.receivedData setLength:0];}