Question

is it possible to add a custom event for a property value change?. I need to trigger a event when the value of a property changes.

Answer 1

I assume you want to trigger an event in an object (called “observer”) when a property changes in another object (“observed”).

You can do this in two ways:

1. using the notification center, you can override the setter by posting a notification using any of NSNotificationCenter “postNotification:” methods, then all interested observers will be notified provided they registered themselves for this kind of notification.

2. using KVO (Key-Value Observing) you must register the observer explicitly with the instance that has the property; e.g. if self wants to register to “instanceToObserve” property “myProperty”:

[instanceToObserve addObserver:self forKeyPath:@"myProperty" option:NSKeyValueObservingOptionNew context:NULL];

and then implement in the “self”‘s class the “observation” function:

- (void)observeValueForKeyPath:(NSString *)keyPath
                      ofObject:(id)object
                        change:(NSDictionary *)change
                       context:(void *)context {
// your implementation goes here
}

What to use of the two is up to you. Consider that KVO is automatically supported if you follow basic Cocoa convention rules, so no extra effort is required and no change is needed to the synthesized setter.