如何使用 AFNetworking 下载文件并将其保存到文档目录？

我正在使用 AFNetworking 库。我不知道如何下载文件并将其保存到文档目录。

NSURLRequest *request = [NSURLRequest requestWithURL:[NSURL URLWithString:@"..."]];
AFHTTPRequestOperation *operation = [[[AFHTTPRequestOperation alloc] initWithRequest:request] autorelease];

NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
NSString *path = [[paths objectAtIndex:0] stringByAppendingPathComponent:@"filename"];
operation.outputStream = [NSOutputStream outputStreamToFileAtPath:path append:NO];

[operation setCompletionBlockWithSuccess:^(AFHTTPRequestOperation *operation, id responseObject) {
    NSLog(@"Successfully downloaded file to %@", path);
} failure:^(AFHTTPRequestOperation *operation, NSError *error) {
    NSLog(@"Error: %@", error);
}];

[operation start];