Custom Events in Objective-C

Raphael — Mon, 02 Mar 2009 19:59:38 +0000

If you are living in the ActionScript world and you are looking for something like an event handling system, you might won’t find any. I googled events and custom events, callbacks and finally figured out that they call it “Notifications”.

Events or in this case Notifications are used for having loosely coupled relationships between instances in your application. So it is possible to let other instances listen to specific events without knowing each other.

If you want to listen to a custom event in ActionScript it would look something like this:

instance.addEventListener("eventType", eventHandler);

function eventHandler(e:Event)
{
trace("event triggered");
}

In your instance you would trigger the event like this:

dispatchEvent( new Event("eventType") );

The Objective-C approach to listen to a Notification looks like this:

[[NSNotificationCenter defaultCenter]
addObserver:self
selector:@selector(eventHandler:)
name:@"eventType"
object:nil ];

-(void)eventHandler: (NSNotification *) notification
{
NSLog(@"event triggered");
}

The funny thing is that you don’t need to put this Event Listener on an instance. It just listens to the eventtype and as far as I understood you can trigger this event wherever you want. This makes it even more loosely coupled than the instance.addEventListener(); approach.

The “NSNotificationCenter” is something like a Singleton which is one unique Class that manages your events in your application.

And this is how you dispatch or trigger the event:

[[NSNotificationCenter defaultCenter]
postNotificationName:@"eventType"
object:nil ];

That’s it. Actually I like the approach in Objective-C even if it is more to write it is pretty easy to read.

Raphael Wichmann » notifications

Custom Events in Objective-C