nsurldownload未启动

我正在尝试使用nsurldownload下载一个URL,但它没有开始下载。在继续之前,必须说我正在为此使用gnustep。

我的项目大纲如下:

MyClass:H:

@interface MyClass : Object {

}

-(void)downloadDidBegin:(NSURLDownload*)download;
-(void)downloadDidFinish:(NSURLDownload*)download;

@end

主m

int main()
{
   NSAutoreleasePool* pool = [[NSAutoreleasePool alloc] init];

   NSLog(@"creating url");
   NSURL* url = [[[NSURL alloc] initWithString:@"http://developer.apple.com/mac/library/documentation/Cocoa/Reference/Foundation/Classes/NSURLRequest_Class/NSURLRequest_Class.pdf"] autorelease];

   NSLog(@"creating url request");
   NSURLRequest* url_request = [[[NSURLRequest alloc] initWithURL:url] autorelease];

   NSLog(@"creating MyClass instance");
   MyClass* my_class = [[MyClass alloc] init];

   NSLog(@"creating url download");
   NSURLDownload* url_download = [[[NSURLDownload alloc] initWithRequest:url_request
                                                                delegate:my_class] autorelease];

   [pool drain];
}

我在MyClass的两个函数上都有nslog,但都没有命中。我需要做什么才能开始下载?或者这是Gnustep的问题?