AppController

...

- (void)loadEverything 
{ 
   loginController = [[LoginWindowController alloc] init...]; 
   [loginController setDelegate:self]; 
}

- (void)someFunction
{
   // do something cool
}

@end

LoginWindowController

@implementation LoginWindowController... 
{ 
   id delegate @accessors; // this creates the delegate and setDelegate accessors! 
}

…

- (void)callSomeFunction
{
   [delegate someFunction];
}

@end