Typically since it is your code that is doing the dismissal, there is no need for an event. if you require one for some reason (decoupling, etc) then you can easily use the NSNotificationCenter.
// listen for an event
[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(onDialogDimissed:) name:@"myapp_dialog_dismissed" object:nil];
- (void) onDialogDismissed:(NSNotification*)n
{
NSLog(@"yay");
}
// raise an event
[[NSNotificationCenter defaultCenter] postNotificationName:@"myapp_dialog_dismissed" object:nil userInfo:nil];
It's probably worth mentioning that you can pass a NSDictionary* into userInfo and if all goes well will show up on the other end, so you don't have to create your own sub-classes or special event wrappers just to pass around data between events.