Question

here is my code, it tries to download a webpage using NSURLDownload. but it does not work, its a command line program.

- (void)startDownloadingURL
{
    NSURLRequest *theRequest = [NSURLRequest requestWithURL:[NSURL URLWithString:@"http://www.apple.com/index.html"]
                                             cachePolicy:NSURLRequestUseProtocolCachePolicy
                                         timeoutInterval:60.0];

    // Create the download with the request and start loading the data.
    NSURLDownload  *theDownload = [[NSURLDownload alloc] initWithRequest:theRequest delegate:self];

    if (theDownload) {
        // Set the destination file.
        [theDownload setDestination:@"/saleh" allowOverwrite:YES];
    } else {
        // inform the user that the download failed.
        NSLog(@"download has failed!");
    }

}

Answer 1

Ensure you implement the downloadDidFinish and doanload:didFailWithError: callbacks.

Here’s an overview that covers how to use NSURLDownload:

http://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/URLLoadingSystem/Tasks/UsingNSURLDownload.html#//apple_ref/doc/uid/20001839-BAJEAIEE

Specifically, this callback will give you more details on why it failed:

- (void)download:(NSURLDownload *)download didFailWithError:(NSError *)error
{
    // Release the connection.
    [download release];

    // Inform the user.
    NSLog(@"Download failed! Error - %@ %@",
          [error localizedDescription],
          [[error userInfo] objectForKey:NSURLErrorFailingURLStringErrorKey]);
}

EDIT:

Below you said it’s a command line. async callbacks require an NSRunLoop. See:

Cocoa: NSURLConnection not attempting an HTTP Request

Per the docs:

initWithRequest:delegate:
Returns an initialized URL download for a URL request and begins to download the data for the request.

- (id)initWithRequest:(NSURLRequest *)request delegate:(id)delegate

delegate

The delegate for the download. This object will receive delegate
messages as the download progresses. Delegate messages will be sent on
the thread which calls this method. For the download to work correctly
the calling thread’s run loop must be operating in the default run
loop mode.

NSURLConnection has a synchronous way to download data.

http://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/URLLoadingSystem/Tasks/UsingNSURLConnection.html#//apple_ref/doc/uid/20001836-BAJEAIEE

NSURLConnection provides support for downloading the contents of an
NSURLRequest in a synchronous manner using the class method
sendSynchronousRequest:returningResponse:error:. Using this method is
not recommended, because it has severe limitations:

The main limitation is the client blocks but that’s a concern in a command line app.