iOS: how to perform a HTTP POST request?

You can use NSURLConnection as follows:

1. Set your NSURLRequest: Use requestWithURL:(NSURL *)theURL to initialise the request.

   If you need to specify a POST request and/or HTTP headers, use NSMutableURLRequest with

   • (void)setHTTPMethod:(NSString *)method
   • (void)setHTTPBody:(NSData *)data
   • (void)setValue:(NSString *)value forHTTPHeaderField:(NSString *)field

2. Send your request in 2 ways using NSURLConnection:

   • Synchronously: (NSData *)sendSynchronousRequest:(NSURLRequest *)request returningResponse:(NSURLResponse **)response error:(NSError **)error

     This returns a NSData variable that you can process.

     IMPORTANT: Remember to kick off the synchronous request in a separate thread to avoid blocking the UI.

   • Asynchronously: (void)start

Don't forget to set your NSURLConnection's delegate to handle the connection as follows:

- (void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)response {
    [self.data setLength:0];
}

- (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)d {
    [self.data appendData:d];
}

- (void)connection:(NSURLConnection *)connection didFailWithError:(NSError *)error {
    [[[[UIAlertView alloc] initWithTitle:NSLocalizedString(@"Error", @"")
                                 message:[error localizedDescription]
                                delegate:nil
                       cancelButtonTitle:NSLocalizedString(@"OK", @"") 
                       otherButtonTitles:nil] autorelease] show];
}

- (void)connectionDidFinishLoading:(NSURLConnection *)connection {
    NSString *responseText = [[NSString alloc] initWithData:self.data encoding:NSUTF8StringEncoding];

    // Do anything you want with it 

    [responseText release];
}

// Handle basic authentication challenge if needed
- (void)connection:(NSURLConnection *)connection didReceiveAuthenticationChallenge:(NSURLAuthenticationChallenge *)challenge {
    NSString *username = @"username";
    NSString *password = @"password";

    NSURLCredential *credential = [NSURLCredential credentialWithUser:username
                                                             password:password
                                                          persistence:NSURLCredentialPersistenceForSession];
    [[challenge sender] useCredential:credential forAuthenticationChallenge:challenge];
}

EDIT: ASIHTTPRequest has been abandoned by the developer. It's still really good IMO, but you should probably look elsewhere now.

I'd highly recommend using the ASIHTTPRequest library if you are handling HTTPS. Even without https it provides a really nice wrapper for stuff like this and whilst it's not hard to do yourself over plain http, I just think the library is nice and a great way to get started.

The HTTPS complications are far from trivial in various scenarios, and if you want to be robust in handling all the variations, you'll find the ASI library a real help.