如何使用NSURL Connection下载文件

我知道这很愚蠢,但我不知道我错在哪里.我是NSURLConnections的新手.我必须使用NSURLConnection下载文件.这是我的代码:

NSURL *url = [NSURL URLWithString:@"http://www.comicbookresources.com/feed.php?feed=previews"];

NSURLRequest *request = [NSURLRequest requestWithURL:url cachePolicy:NSURLRequestReloadIgnoringCacheData timeoutInterval:30];
NSURLConnection *connection = [[NSURLConnection alloc]initWithRequest:request delegate:self startImmediately:YES ];
[self.data appendData:self.data];
NSString  *filePath = [NSString stringWithFormat:@"%u/%@", NSDocumentDirectory,@"usman.pdf"];
[self.data writeToFile:filePath atomically:YES];

它看起来很荒谬,但我仍在问

编辑:对不起.我忘了添加单词:异步连接