Question

Is there a way onto the iPhone for an object to send a message without a specific receiver object, and into another object, listen to such messages, that could come with objects (parameters), and do what is needed ?

I searched around NSNotification but I don’t see what I should do.

Answer 1

Objects that want to be notified need to register to receive notifications with the notification center. Thereafter, when a notification is posted to the notification center, the notification center will check it against all the registered filters, and the corresponding action will be taken for each matching filter.

A “filter” in this case is the pair of (notification name, notification object). A nil object in the filter is equivalent to any object (the notification object is ignored in matching). The name is required.

Example:

/* Subscribe to be sent -noteThis:
 * whenever a notification named @"NotificationName" is posted to the center
 * with any (or no) object. */
NSNotificationCenter *nc = [NSNotificationCenter defaultCenter];
[nc addObserver:self selector:@selector(noteThis:)
           name:@"NotificationName"
         object:nil];

/* Post a notification. */
[nc postNotificationName:@"NotificationName" object:self userInfo:someDict];

/* Handle a notification. */
- (void)noteThis:(NSNotification *)note
{
   id object = [note object];
   NSDictionary *userInfo = [note userInfo];
   /* take some action */
}

There is a more modern API using queues and blocks, but I find the old API easier to illustrate and explain.